explanation blue bibcodes open ADS page with paths to full text
Author name code: keller
ADS astronomy entries on 2022-09-14
author:"Keller, Christoph U."
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Title: Upgrading the high contrast imaging facility SPHERE: science
drivers and instrument choices
Authors: Boccaletti, A.; Chauvin, G.; Wildi, F.; Milli, J.; Stadler,
E.; Diolaiti, E.; Gratton, R.; Vidal, F.; Loupias, M.; Langlois, M.;
Cantalloube, F.; N'Diaye, M.; Gratadour, D.; Ferreira, F.; Tallon, M.;
Mazoyer, J.; Segransan, D.; Mouillet, D.; Beuzit, J. -L.; Bonnefoy,
M.; Galicher, R.; Vigan, A.; Snellen, I.; Feldt, M.; Desidera, S.;
Rousseau, S.; Baruffolo, A.; Goulas, C.; Baudoz, P.; Bechet, C.;
Benisty, M.; Bianco, A.; Carry, B.; Cascone, E.; Charnay, B.; Choquet,
E.; Christiaens, V.; Cortecchia, F.; de Caprio, V.; De Rosa, A.;
Desgrange, C.; D'Orazi, V.; Douté, S.; Frangiamore, M.; Gendron, E.;
Ginski, C.; Huby, E.; Keller, C.; Kulcsár, C.; Landman, R.; Lagarde,
S.; Lagadec, E.; Lagrange, A. -M.; Kasper, M. Lombini M.; Ménard,
F.; Magnard, Y.; Malaguti, G.; Maurel, D.; Mesa, D.; Morgante, G.;
Pantin, E.; Pichon, T.; Potier, A.; Rabou, P.; Rochat, S.; Terenzi,
L.; Thiébaut, E.; Tallon-Bosc, I.; Raynaud, H. -F.; Rouan, D.; Sevin,
A.; Schiavone, F.; Schreiber, L.; Zanutta, A.
2022arXiv220902092B Altcode:
SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT,
which is intended to boost the current performances of detection and
characterization for exoplanets and disks. SPHERE+ will also serve as a
demonstrator for the future planet finder (PCS) of the European ELT. The
main science drivers for SPHERE+ are 1/ to access the bulk of the young
giant planet population down to the snow line ($3-10$ au), to bridge
the gap with complementary techniques (radial velocity, astrometry); 2/
to observe fainter and redder targets in the youngest ($1-10$\,Myr)
associations compared to those observed with SPHERE to directly
study the formation of giant planets in their birth environment; 3/
to improve the level of characterization of exoplanetary atmospheres
by increasing the spectral resolution in order to break degeneracies
in giant planet atmosphere models. Achieving these objectives requires
to increase the bandwidth of the xAO system (from $\sim$1 to 3\,kHz)
as well as the sensitivity in the infrared (2 to 3\,mag). These
features will be brought by a second stage AO system optimized in the
infrared with a pyramid wavefront sensor. As a new science instrument,
a medium resolution integral field spectrograph will provide a spectral
resolution from 1000 to 5000 in the J and H bands. This paper gives
an overview of the science drivers, requirements and key instrumental
trade-off that were done for SPHERE+ to reach the final selected
baseline concept.
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Title: Joint optimization of wavefront sensing and reconstruction
with automatic differentiation
Authors: Landman, Rico; Keller, Christoph; Por, Emiel H.; Haffert,
Sebastiaan; Doelman, David; Stockmans, Thijs
2022arXiv220905904L Altcode:
High-contrast imaging instruments need extreme wavefront control to
directly image exoplanets. This requires highly sensitive wavefront
sensors which optimally make use of the available photons to sense the
wavefront. Here, we propose to numerically optimize Fourier-filtering
wavefront sensors using automatic differentiation. First, we optimize
the sensitivity of the wavefront sensor for different apertures and
wavefront distributions. We find sensors that are more sensitive than
currently used sensors and close to the theoretical limit, under the
assumption of monochromatic light. Subsequently, we directly minimize
the residual wavefront error by jointly optimizing the sensing and
reconstruction. This is done by connecting differentiable models of
the wavefront sensor and reconstructor and alternatingly improving them
using a gradient-based optimizer. We also allow for nonlinearities in
the wavefront reconstruction using Convolutional Neural Networks, which
extends the design space of the wavefront sensor. Our results show
that optimization can lead to wavefront sensors that have improved
performance over currently used wavefront sensors. The proposed
approach is flexible, and can in principle be used for any wavefront
sensor architecture with free design parameters.
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Title: Spectropolarimetry of life: airborne measurements from a hot
air balloon
Authors: Mulder, Willeke; Patty, C. H. Lucas; Spadaccia, Stefano;
Pommerol, Antoine; Demory, Brice-Olivier; Keller, Christoph U.; Kühn,
Jonas G.; Snik, Frans; Stam, Daphne M.
2022arXiv220802317M Altcode:
Does life exist outside our Solar System? A first step towards
searching for life outside our Solar System is detecting life
on Earth by using remote sensing applications. One powerful and
unambiguous biosignature is the circular polarization resulting
from the homochirality of biotic molecules and systems. We aim to
investigate the possibility of identifying and characterizing life on
Earth by using airborne spectropolarimetric observations from a hot
air balloon during our field campaign in Switzerland, May 2022. In
this work we present the optical-setup and the data obtained from
aerial circular spectropolarimetric measurements of farmland, forests,
lakes and urban sites. We make use of the well-calibrated FlyPol
instrument that measures the fractionally induced circular polarization
($V/I$) of (reflected) light with a sensitivity of $<10^{-4}$. The
instrument operates in the visible spectrum, ranging from 400 to
900 nm. We demonstrate the possibility to distinguish biotic from
abiotic features using circular polarization spectra and additional
broadband linear polarization information. We review the performance
of our optical-setup and discuss potential improvements. This sets
the requirements on how to perform future airborne spectropolarimetric
measurements of the Earth's surface features from several elevations.
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Title: Detecting life outside our solar system with a large
high-contrast-imaging mission
Authors: Snellen, Ignas A. G.; Snik, F.; Kenworthy, M.; Albrecht, S.;
Anglada-Escudé, G.; Baraffe, I.; Baudoz, P.; Benz, W.; Beuzit, J. -L.;
Biller, B.; Birkby, J. L.; Boccaletti, A.; van Boekel, R.; de Boer,
J.; Brogi, Matteo; Buchhave, L.; Carone, L.; Claire, M.; Claudi, R.;
Demory, B. -O.; Désert, J. -M.; Desidera, S.; Gaudi, B. S.; Gratton,
R.; Gillon, M.; Grenfell, J. L.; Guyon, O.; Henning, T.; Hinkley,
S.; Huby, E.; Janson, M.; Helling, C.; Heng, K.; Kasper, M.; Keller,
C. U.; Krause, O.; Kreidberg, L.; Madhusudhan, N.; Lagrange, A. -M.;
Launhardt, R.; Lenton, T. M.; Lopez-Puertas, M.; Maire, A. -L.; Mayne,
N.; Meadows, V.; Mennesson, B.; Micela, G.; Miguel, Y.; Milli, J.;
Min, M.; de Mooij, E.; Mouillet, D.; N'Diaye, M.; D'Orazi, V.; Palle,
E.; Pagano, I.; Piotto, G.; Queloz, D.; Rauer, H.; Ribas, I.; Ruane,
G.; Selsis, F.; Sozzetti, A.; Stam, D.; Stark, C. C.; Vigan, A.;
de Visser, Pieter
2021ExA...tmp..124S Altcode:
In this White Paper, which was submitted in response to the European
Space Agency (ESA) Voyage 2050 Call, we recommend the ESA plays a
proactive role in developing a global collaborative effort to construct
a large high-contrast imaging space telescope, e.g. as currently
under study by NASA. Such a mission will be needed to characterize a
sizable sample of temperate Earth-like planets in the habitable zones
of nearby Sun-like stars and to search for extraterrestrial biological
activity. We provide an overview of relevant European expertise,
and advocate ESA to start a technology development program towards
detecting life outside the Solar System.
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Title: LOUPE: Observing the Earth from the Moon to prepare for
detecting life on Earth-like exoplanets
Authors: Klindžić, Dora; Stam, Daphne; Snik, Frans; Keller,
Christoph; Pallichadath, Vidhya; van Dijk, Chris; Esposito, Marco;
van Dam, Dirk
2021EPSC...15..657K Altcode:
LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth,
is a small, robust spectro-polarimeter for observing the Earth as if
it were an exoplanet, designed to accompany any landing, roving or
orbiting mission to the Moon. Detecting Earth-like planets in stellar
habitable zones is one of the key challenges of modern exoplanetary
science. Characterizing such planets and searching for traces of life
requires the direct detection of their signals. LOUPE provides unique
spectral flux and polarization data of sunlight reflected by Earth,
the only planet known to harbour life. These data will be used to
test numerical codes to predict signals of Earth-like exoplanets, to
test algorithms that retrieve planet properties, and to fine-tune the
design and observational strategies of future space observatories.We
present a novel spectropolarimetric instrument design: LOUPE, the
Lunar Observatory for Unresolved Polarimetry of the Earth (Klindžić,
2020), which aims to observe the Earth from the Moon as if it were
an exoplanet and perform spectropolarimetric measurements spanning
the full range of phase angles. Various reasons make observing the
Earth from the Moon or from a Lunar orbit, rather than a low Earth
orbit, crucial to the experiment:The Moon is sufficiently far away
to allow a spatially unresolved view of the whole Earth. For a lander
on the Lunar surface, the Earth is always visible in a confined area
in the sky. From the Moon, the Earth can be observed at all phase
angles during a month. From the Moon, the Earth's daily rotation
can be captured. LOUPE's science requirements include:Perform
near-instantaneous (snapshot) spectropolarimetry of the entire
Earth. Detect the presence of liquid water oceans and clouds. Derive
and monitor atmospheric properties, e.g. via Rayleigh scattering,
for potential climate research applications. Detect the O₂A band in
flux and polarization and its variance with cloud cover, altitude and
phase angle. Detect the Chlorophyll Green Bump and Vegetation Red Edge,
the spectroscopic signature of plant life. Derive a map of continents
from the disk-integrated signal and identify notable features,
such as rainforests, deserts and ice caps. LOUPE shall perform its
science goals by recording and demodulating the disk-integrated
Stokes vector of sunlight reflected from the Earth. The leading
instrument design principle adopted for LOUPE is to create a compact,
low-mass, low-volume, space-ready hyperspectropolarimeter with no
moving parts. These constraints require creative solutions from the
cutting edge of hyperspectral and polarimetric instrument design,
where polarimeters traditionally used active rotating optics (temporal
modulation) or beam-splitting (spatial modulation).The latest LOUPE
concept (Fig. 1.) utilizes Patterned Liquid Crystal (PLC) plates for
encoding polarization information as a modulation orthogonal to the
spectral flux measurement, enabling the linear-Stokes vector of a target
to be recorded in one single "snapshot", as shown in Fig. 2. Unlike a
traditional rotating-retarder polarimeter, polarization is modulated in
the cross-spectral direction, meaning polarimetry can be performed at
full spectral resolution, which is not possible in the case of channeled
spectropolarimetry with spectral modulation. This pioneering use of
Patterned Liquid Crystals makes it possible to forgo the use of moving
elements, resulting in a compact, space-ready instrument with versatile
options of installation on a range of landing, roving and orbiting
missions.Here we discuss our detailed design process and the challenges
involved in creating a unique space-qualified spectropolarimeter with
no moving parts, whilst maintaining flexibility for different usage
scenarios: rovers, landers, orbiters, and more. We present a performance
trade-off, optical design informed by ray tracing with polarization
effects, and the development of methods for spectral and polarimetric
demodulation of simulated Earth observation data.Figure 1: Tentative
design of LOUPE. Figure 2: Simulated LOUPE measurement. Wavelength
filtering is applied in the y-direction, and polarization modulation in
the x-direction. Each dot represents an unresolved image of the Earth.
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Title: Pale polarized dots: spectropolarimetry of the Earth as an
exoplanet with LOUPE
Authors: Klindžić, Dora; Snik, Frans; Stam, Daphne M.; Keller,
Christoph U.; Stockmans, Thijs; Hoeijmakers, H. Jens; van Dam, Dirk
M.; Willebrands, Michele; Karalidi, Theodora; Pallichadath, Vidhya;
van Dijk, Chris N.; Esposito, M.
2021SPIE11833E..06K Altcode:
We present LOUPE, the Lunar Observatory for Unresolved Polarimetry of
the Earth, a compact snapshot spectropolarimeter designed to observe
the Earth from the Moon as if it were an exoplanet. Viewing the Earth
as it would be seen by a faraway observer will offer novel insight into
the spectropolarimetric signatures of planets harboring life, as well
as a chance to refine algorithms for the retrieval of exoplanetary
properties such as the presence of liquid water, clouds, vegetation,
and more. LOUPE boasts a novel solid-state design based on patterned
liquid crystal optics built atop the cosine HyperScout<SUP>®</SUP>,
a flight-proven hyperspectral imager. Uniquely to LOUPE, a microlens
array creates a two- dimensional grid of unresolved Earth-images on
the detector, resulting in an array of "pale (blue) dots" filtered
spectrally along one direction, with polarization modulation applied
in the perpendicular direction. The clever use of custom-patterned
liquid crystals as a passive modulator thus replaces the need for
classical dispersion elements and polarization modulation optics. This
pioneering approach enables LOUPE to simultaneously obtain spectral and
Stokes measurements for the entire Earth, whilst the position of the
Earth-dots also has the benefit of providing input for angle-dependent
spectral and polarization calibration. Here we discuss our detailed
design process and the challenges involved in creating a unique,
space-qualified spectropolarimeter with no moving parts and no bulky
optics, whilst maintaining flexibility for different usage scenarios:
rovers, landers, orbiters, and more. We present a performance trade-off
and optical design informed by ray tracing with polarization effects,
to prepare for the demodulation of simulated Earth observation data.
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Title: Spatial polarization modulators: distinguishing diffraction
effects from spatial polarization modulation
Authors: Mulder, Willeke; Doelman, David S.; Keller, Christoph U.;
Patty, C. H. Lucas; Snik, Frans
2021SPIE11833E..0MM Altcode: 2021arXiv210802538M
Are we alone? In our quest to find life beyond Earth, we use our own
planet to develop and verify new methods and techniques to remotely
detect life. Our Life Signature Detection polarimeter (LSDpol),
a snapshot full-Stokes spectropolarimeter to be deployed in the
field and in space, looks for signals of life on Earth by sensing the
linear and circular polarization states of reflected light. Examples
of these biosignatures are linear polarization resulting from O2-A
band and vegetation, e.g. the Red edge and the Green bump, as well
as circular polarization resulting from the homochirality of biotic
molecules. LSDpol is optimized for sensing circular polarization. To
this end, LSDpol employs a spatial light modulator in the entrance slit
of the spectrograph, a liquid-crystal quarter-wave retarder where the
fast axis rotates as a function of slit position. The original design
of LSDpol implemented a dual-beam spectropolarimeter by combining a
quarter-wave plate with a polarization grating. Unfortunately, this
design causes significant linear-to-circular cross-talk. In addition,
it revealed spurious polarization modulation effects. Here, we present
numerical simulations that illustrate how Fresnel diffraction effects
can create these spurious modulations. We verified the simulations
with accurate polarization state measurements in the lab using 100%
linearly and circularly polarized light.
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Title: Spectropolarimetry
Authors: Keller, Christoph U.; Snik, Frans
2021hai3.book..239K Altcode:
No abstract at ADS
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Title: Vector-apodizing phase plate coronagraph: design, current
performance, and future development [Invited]
Authors: Doelman, D. S.; Snik, F.; Por, E. H.; Bos, S. P.; Otten,
G. P. P. L.; Kenworthy, M.; Haffert, S. Y.; Wilby, M.; Bohn, A. J.;
Sutlieff, B. J.; Miller, K.; Ouellet, M.; de Boer, J.; Keller, C. U.;
Escuti, M. J.; Shi, S.; Warriner, N. Z.; Hornburg, K.; Birkby, J. L.;
Males, J.; Morzinski, K. M.; Close, L. M.; Codona, J.; Long, J.;
Schatz, L.; Lumbres, J.; Rodack, A.; Van Gorkom, K.; Hedglen, A.;
Guyon, O.; Lozi, J.; Groff, T.; Chilcote, J.; Jovanovic, N.; Thibault,
S.; de Jonge, C.; Allain, G.; Vallée, C.; Patel, D.; Côté, O.;
Marois, C.; Hinz, P.; Stone, J.; Skemer, A.; Briesemeister, Z.;
Boehle, A.; Glauser, A. M.; Taylor, W.; Baudoz, P.; Huby, E.; Absil,
O.; Carlomagno, B.; Delacroix, C.
2021ApOpt..60D..52D Altcode: 2021arXiv210411211D
Over the last decade, the vector-apodizing phase plate (vAPP)
coronagraph has been developed from concept to on-sky application in
many high-contrast imaging systems on 8-m class telescopes. The vAPP is
an geometric-phase patterned coronagraph that is inherently broadband,
and its manufacturing is enabled only by direct-write technology for
liquid-crystal patterns. The vAPP generates two coronagraphic PSFs
that cancel starlight on opposite sides of the point spread function
(PSF) and have opposite circular polarization states. The efficiency,
that is the amount of light in these PSFs, depends on the retardance
offset from half-wave of the liquid-crystal retarder. Using different
liquid-crystal recipes to tune the retardance, different vAPPs operate
with high efficiencies ($>96\%$) in the visible and thermal infrared
(0.55 $\mu$m to 5 $\mu$m). Since 2015, seven vAPPs have been installed
in a total of six different instruments, including Magellan/MagAO,
Magellan/MagAO-X, Subaru/SCExAO, and LBT/LMIRcam. Using two integral
field spectrographs installed on the latter two instruments, these
vAPPs can provide low-resolution spectra (R$\sim$30) between 1 $\mu$m
and 5 $\mu$m. We review the design process, development, commissioning,
on-sky performance, and first scientific results of all commissioned
vAPPs. We report on the lessons learned and conclude with perspectives
for future developments and applications.
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Title: Self-optimizing adaptive optics control with reinforcement
learning for high-contrast imaging
Authors: Landman, Rico; Haffert, Sebastiaan Y.; Radhakrishnan, Vikram
M.; Keller, Christoph U.
2021JATIS...7c9002L Altcode: 2021arXiv210811332L
Current and future high-contrast imaging instruments require extreme
adaptive optics systems to reach contrasts necessary to directly imaged
exoplanets. Telescope vibrations and the temporal error induced by the
latency of the control loop limit the performance of these systems. One
way to reduce these effects is to use predictive control. We describe
how model-free reinforcement learning can be used to optimize
a recurrent neural network controller for closed-loop predictive
control. First, we verify our proposed approach for tip-tilt control in
simulations and a lab setup. The results show that this algorithm can
effectively learn to mitigate vibrations and reduce the residuals for
power-law input turbulence as compared to an optimal gain integrator. We
also show that the controller can learn to minimize random vibrations
without requiring online updating of the control law. Next, we show
in simulations that our algorithm can also be applied to the control
of a high-order deformable mirror. We demonstrate that our controller
can provide two orders of magnitude improvement in contrast at small
separations under stationary turbulence. Furthermore, we show more
than an order of magnitude improvement in contrast for different wind
velocities and directions without requiring online updating of the
control law.
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Title: Biosignatures of the Earth. I. Airborne spectropolarimetric
detection of photosynthetic life
Authors: Patty, C. H. Lucas; Kühn, Jonas G.; Lambrev, Petar H.;
Spadaccia, Stefano; Jens Hoeijmakers, H.; Keller, Christoph; Mulder,
Willeke; Pallichadath, Vidhya; Poch, Olivier; Snik, Frans; Stam,
Daphne M.; Pommerol, Antoine; Demory, Brice-Olivier
2021A&A...651A..68P Altcode: 2021arXiv210600493P
Context. Homochirality is a generic and unique property of life on Earth
and is considered a universal and agnostic biosignature. Homochirality
induces fractional circular polarization in the incident light
that it reflects. Because this circularly polarized light can be
sensed remotely, it can be one of the most compelling candidate
biosignatures in life detection missions. While there are also other
sources of circular polarization, these result in spectrally flat
signals with lower magnitude. Additionally, circular polarization
can be a valuable tool in Earth remote sensing because the circular
polarization signal directly relates to vegetation physiology. <BR
/> Aims: While high-quality circular polarization measurements can
be obtained in the laboratory and under semi-static conditions in
the field, there has been a significant gap to more realistic remote
sensing conditions. <BR /> Methods: In this study, we present sensitive
circular spectropolarimetric measurements of various landscape elements
taken from a fast-moving helicopter. <BR /> Results: We demonstrate
that during flight, within mere seconds of measurements, we can
differentiate (S∕N > 5) between grass fields, forests, and abiotic
urban areas. Importantly, we show that with only nonzero circular
polarization as a discriminant, photosynthetic organisms can even be
measured in lakes. <BR /> Conclusions: Circular spectropolarimetry can
be a powerful technique to detect life beyond Earth, and we emphasize
the potential of utilizing circular spectropolarimetry as a remote
sensing tool to characterize and monitor in detail the vegetation
physiology and terrain features of Earth itself.
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Title: A MUSE view of the asymmetric jet from HD 163296
Authors: Xie, C.; Haffert, S. Y.; de Boer, J.; Kenworthy, M. A.;
Brinchmann, J.; Girard, J.; Snellen, I. A. G.; Keller, C. U.
2021A&A...650L...6X Altcode: 2021arXiv210601661X
Context. Jets and outflows are thought to play important roles in
regulating star formation and disk evolution. An important question
is how the jets are launched. HD 163296 is a well-studied Herbig Ae
star that hosts proto-planet candidates, a protoplanetary disk, a
protostellar jet, and a molecular outflow, which makes it an excellent
laboratory for studying jets. <BR /> Aims: We aim to characterize
the jet at the inner regions and check if there are large differences
with the features at large separations. A secondary objective is to
demonstrate the performance of Multi Unit Spectroscopic Explorer
(MUSE) in high-contrast imaging of extended line emission. <BR />
Methods: MUSE in the narrow field mode (NFM) can provide observations
at optical wavelengths with high spatial (∼75 mas) and medium
spectral (R ∼ 2500) resolution. With the high-resolution spectral
differential imaging technique, we can characterize the kinematic
structures and physical conditions of jets down to 100 mas. <BR />
Results: We detect multiple atomic lines in two new knots, B3 and
A4, at distances of < 4″ from the host star with MUSE. The
derived \Mdot;<SUB>jet</SUB>/\Mdot;<SUB>acc</SUB> is about 0.08 and
0.06 for knots B3 and A4, respectively. The observed [Ca II]/[S II]
ratios indicate that there is no sign of dust grains at distances
of < 4″. Assuming the A4 knot traced the streamline, we can
estimate a jet radius at the origin by fitting the half width half
maximum of the jet, which sets an upper limit of 2.2 au on the size
of the launching region. Although MUSE has the ability to detect the
velocity shifts caused by high- and low-velocity components, we found
no significant evidence of velocity decrease transverse to the jet
direction in our 500 s MUSE observation. <BR /> Conclusions: Our work
demonstrates the capability of using MUSE NFM observations for the
detailed study of stellar jets in the optical down to 100 mas. The
derived \Mdot;<SUB>jet</SUB>/\Mdot;<SUB>acc</SUB>, no dust grain,
and jet radius at the star support the magneto-centrifugal models as
a launching mechanism for the jet.
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Title: First light of a holographic aperture mask: Observation at
the Keck OSIRIS Imager
Authors: Doelman, David S.; Wardenier, Joost P.; Tuthill, Peter;
Fitzgerald, Michael P.; Lyke, Jim; Sallum, Steph; Norris, Barnaby;
Warriner, N. Zane; Keller, Christoph; Escuti, Michael J.; Snik, Frans
2021A&A...649A.168D Altcode: 2021arXiv210411210D
Context. As an interferometric technique, sparse aperture masking
(SAM) is capable of imaging beyond the diffraction limit of single
telescopes. This makes SAM an important technique for studying processes
such as planet formation at Solar System scales. However, it comes at
the cost of a reduction in throughput, typically by 80-90%. <BR /> Aims:
We report on the design, construction, and commissioning of a prototype
aperture masking technology implemented at the Keck OH-Suppressing
Infrared Integral Field Spectrograph (OSIRIS) Imager: the holographic
aperture mask. Holographic aperture masking (HAM) aims at (i) increasing
the throughput of SAM by selectively combining all subapertures across
a telescope pupil in multiple interferograms using a phase mask, and
(ii) adding low-resolution spectroscopic capabilities. <BR /> Methods:
Using liquid-crystal geometric phase patterns, we manufacture a HAM
mask that uses an 11-hole SAM design as the central component and a
holographic component comprising 19 different subapertures. Thanks
to a multilayer liquid-crystal implementation, the mask has a
diffraction efficiency higher than 96% from 1.1 to 2.5 micron. We
create a pipeline that extracts monochromatic closure phases from the
central component as well as multiwavelength closure phases from the
holographic component. We test the performance of the HAM mask in the
laboratory and on-sky. <BR /> Results: The holographic component yields
26 closure phases with spectral resolutions between R ∼ 6.5 and R
∼ 15, depending on the interferogram positions. On April 19, 2019,
we observed the binary star HDS 1507 in the Hbb filter (λ<SUB>0</SUB>
= 1638 nm and Δλ = 330 nm) and retrieved a constant separation of
120.9 ± 0.5 mas for the independent wavelength bins, which is in
excellent agreement with literature values. For both the laboratory
measurements and the observations of unresolved reference stars, we
recorded nonzero closure phases - a potential source of systematic
error that we traced to polarization leakage of the HAM optic. We
propose a future upgrade that improves the performance, reducing this
effect to an acceptable level. <BR /> Conclusions: Holographic aperture
masking is a simple upgrade of SAM with increased throughput and a new
capability of simultaneous low-resolution spectroscopy that provides new
differential observables (e.g., differential phases with wavelength).
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Title: PCS — A Roadmap for Exoearth Imaging with the ELT
Authors: Kasper, M.; Cerpa Urra, N.; Pathak, P.; Bonse, M.; Nousiainen,
J.; Engler, B.; Heritier, C. T.; Kammerer, J.; Leveratto, S.;
Rajani, C.; Bristow, P.; Le Louarn, M.; Madec, P. -Y.; Ströbele,
S.; Verinaud, C.; Glauser, A.; Quanz, S. P.; Helin, T.; Keller, C.;
Snik, F.; Boccaletti, A.; Chauvin, G.; Mouillet, D.; Kulcsár, C.;
Raynaud, H. -F.
2021Msngr.182...38K Altcode: 2021arXiv210311196K
The Planetary Camera and Spectrograph (PCS) for the Extremely Large
Telescope (ELT) will be dedicated to detecting and characterising
nearby exoplanets with sizes from sub-Neptune to Earth-size in the
neighbourhood of the Sun. This goal is achieved by a combination of
eXtreme Adaptive Optics (XAO), coronagraphy and spectroscopy. PCS will
allow us not only to take images, but also to look for biosignatures
such as molecular oxygen in the exoplanets' atmospheres. This article
describes the PCS primary science goals, the instrument concept and
the research and development activities that will be carried out over
the coming years.
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Title: A survey of the linear polarization of directly imaged
exoplanets and brown dwarf companions with SPHERE-IRDIS. First
polarimetric detections revealing disks around DH Tau B and GSC
6214-210 B
Authors: van Holstein, R. G.; Stolker, T.; Jensen-Clem, R.; Ginski,
C.; Milli, J.; de Boer, J.; Girard, J. H.; Wahhaj, Z.; Bohn, A. J.;
Millar-Blanchaer, M. A.; Benisty, M.; Bonnefoy, M.; Chauvin, G.;
Dominik, C.; Hinkley, S.; Keller, C. U.; Keppler, M.; Langlois, M.;
Marino, S.; Ménard, F.; Perrot, C.; Schmidt, T. O. B.; Vigan, A.;
Zurlo, A.; Snik, F.
2021A&A...647A..21V Altcode: 2021arXiv210104033V
Context. Young giant planets and brown dwarf companions emit
near-infrared radiation that can be linearly polarized up to several
percent. This polarization can reveal the presence of an (unresolved)
circumsubstellar accretion disk, rotation-induced oblateness of the
atmosphere, or an inhomogeneous distribution of atmospheric dust
clouds. <BR /> Aims: We aim to measure the near-infrared linear
polarization of 20 known directly imaged exoplanets and brown dwarf
companions. <BR /> Methods: We observed the companions with the
high-contrast imaging polarimeter SPHERE-IRDIS at the Very Large
Telescope. We reduced the data using the IRDAP pipeline to correct
for the instrumental polarization and crosstalk of the optical system
with an absolute polarimetric accuracy <0.1% in the degree of
polarization. We employed aperture photometry, angular differential
imaging, and point-spread-function fitting to retrieve the polarization
of the companions. <BR /> Results: We report the first detection of
polarization originating from substellar companions, with a polarization
of several tenths of a percent for DH Tau B and GSC 6214-210 B in
H-band. By comparing the measured polarization with that of nearby
stars, we find that the polarization is unlikely to be caused by
interstellar dust. Because the companions have previously measured
hydrogen emission lines and red colors, the polarization most likely
originates from circumsubstellar disks. Through radiative transfer
modeling, we constrain the position angles of the disks and find that
the disks must have high inclinations. For the 18 other companions,
we do not detect significant polarization and place subpercent upper
limits on their degree of polarization. We also present images of
the circumstellar disks of DH Tau, GQ Lup, PDS 70, β Pic, and HD
106906. We detect a highly asymmetric disk around GQ Lup and find
evidence for multiple scattering in the disk of PDS 70. Both disks
show spiral-like features that are potentially induced by GQ Lup B
and PDS 70 b, respectively. <BR /> Conclusions: The presence of the
disks around DH Tau B and GSC 6214-210 B as well as the misalignment of
the disk of DH Tau B with the disk around its primary star suggest in
situ formation of the companions. The non-detections of polarization
for the other companions may indicate the absence of circumsubstellar
disks, a slow rotation rate of young companions, the upper atmospheres
containing primarily submicron-sized dust grains, and/or limited cloud
inhomogeneity. <P />Based on observations collected at the European
Southern Observatory under ESO programs 098.C-0790, 0101.C-0502,
0101.C-0635, 0101.C-0855, 0102.C-0453, 0102.C-0466, 0102.C-0871,
0102.C-0916, and 0104.C-0265.
---------------------------------------------------------
Title: LOUPE: observing Earth from the Moon to prepare for detecting
life on Earth-like exoplanets
Authors: Klindžić, D.; Stam, D. M.; Snik, F.; Keller, C. U.;
Hoeijmakers, H. J.; van Dam, D. M.; Willebrands, M.; Karalidi, T.;
Pallichadath, V.; van Dijk, C. N.; Esposito, M.
2021RSPTA.37990577K Altcode: 2020arXiv200716078K
LOUPE, the Lunar Observatory for Unresolved Polarimetry of the
Earth, is a small, robust spectro-polarimeter for observing the
Earth as an exoplanet. Detecting Earth-like planets in stellar
habitable zones is one of the key challenges of modern exoplanetary
science. Characterizing such planets and searching for traces of life
requires the direct detection of their signals. LOUPE provides unique
spectral flux and polarization data of sunlight reflected by Earth,
the only planet known to harbour life. These data will be used to
test numerical codes to predict signals of Earth-like exoplanets, to
test algorithms that retrieve planet properties, and to fine-tune the
design and observational strategies of future space observatories. From
the Moon, LOUPE will continuously see the entire Earth, enabling it to
monitor the signal changes due to the planet's daily rotation, weather
patterns and seasons, across all phase angles. Here, we present both
the science case and the technology behind LOUPE's instrumental and
mission design. <P />This article is part of a discussion meeting issue
`Astronomy from the Moon: the next decades'.
---------------------------------------------------------
Title: Using the generalised-optical differentiation wavefront sensor
for laser guide star wavefront sensing
Authors: Haffert, Sebastiaan Y.; Keller, Christoph U.; Dekany, Richard
2020SPIE11448E..1TH Altcode: 2020arXiv201202336H
Laser guide stars (LGS) are used in many adaptive optics systems
to extend sky coverage. The most common wavefront sensor used in
combination with a LGS is a Shack-Hartmann wavefront sensor (SHWFS). The
ShackHartmann has a major disadvantage for extended source wavefront
sensing because it directly samples the image. In this proceeding we
propose to use the generalized-Optical Differentation Wavefront Sensor
(g-ODWFS) a wavefront sensor for wavefront sensing of LGS. The g-ODWFS
uses only 4 pixels per sub-aperture, has little to no aliasing noise
and therefore no spurious low-order errors and has no need for centroid
gain calibrations. In this proceeding we show the results of simulations
that compare the g-ODWFS with the SHWFS.
---------------------------------------------------------
Title: Prediction of the planet yield of the MaxProtoPlanetS
high-contrast survey for H-alpha protoplanets with MagAO-X based on
first light contrasts
Authors: Close, Laird M.; Males, Jared; Long, Joseph D.; Van Gorkom,
Kyle; Hedglen, Alexander D.; Kautz, Maggie; Lumbres, Jennifer; Haffert,
Sebastiaan Y.; Follette, Katherine; Wagner, Kevin; Miller, Kelsey;
Apai, Daniel; Wu, Ya-Lin; Guyon, Olivier; Schatz, Lauren; Rodack,
Alex; Doelman, David; Snik, Frans; Knight, Justin M.; Morzinski,
Katie; Gasho, Victor; Keller, Christoph; Pearce, Logan; Weinberger,
Alycia; Pérez, Laura; Doyon, René
2020SPIE11448E..0UC Altcode:
Our past GAPplanetS survey over the last 5 years with the MagAO visible
AO system discovered the first examples of accreting protoplanets
(by direct observation of H-alpha emission). Examples include LkCa15
b (Sallum et al. 2015) and PDS70 b (Wagner et al. 2018). In this
paper we review the science performance of the newly (Dec. 2019)
commissioned MagAO-X extreme AO system. In particular, we use the vAPP
coronagraphic contrasts measured during MagAO-X first light. We use the
Massive Accreting Gap (MAG) protoplanet model of Close 2020 to predict
the H-alpha contrasts of 19 of the best transitional disk systems
(ages 1-5 Myr) for the direct detection of H-alpha from accretion of
hydrogen onto these protoplanets. The MAG protoplanet model applied
to the observed first light MagAO-X contrasts predict a maximum yield
of 46+/-7 planets from 19 stars (42 of these planets would be new
discoveries). This suggests that there is a large, yet, unexplored
reservoir of protoplanets that can be discovered with an extreme AO
coronagraphic survey of 19 of the best transitional disk systems. Based
on our first light contrasts we predict a healthy yield of protoplanets
from our MaxProtoPlanetS survey of 19 transitional disks with MagAO-X.
---------------------------------------------------------
Title: Focal plane wavefront sensing on SUBARU/SCExAO
Authors: Vievard, S.; Bos, S. P.; Cassaing, F.; Currie, T.; Deo, V.;
Guyon, O.; Jovanovic, N.; Keller, C. U.; Lamb, M.; Lopez, C.; Lozi,
J.; Martinache, F.; Miller, K.; Montmerle-Bonnefois, A.; Mugnier,
L. M.; N'Diaye, M.; Norris, B.; Sahoo, A.; Sauvage, J. -F.; Skaf,
N.; Snik, F.; Wilby, M. J.; Wong, A.
2020SPIE11448E..6DV Altcode: 2020arXiv201212417V
Focal plane wavefront sensing is an elegant solution for wavefront
sensing since near-focal images of any source taken by a detector show
distortions in the presence of aberrations. Non-Common Path Aberrations
and the Low Wind Effect both have the ability to limit the achievable
contrast of the finest coronagraphs coupled with the best extreme
adaptive optics systems. To correct for these aberrations, the Subaru
Coronagraphic Extreme Adaptive Optics instrument hosts many focal plane
wavefront sensors using detectors as close to the science detector as
possible. We present seven of them and compare their implementation
and efficiency on SCExAO. This work will be critical for wavefront
sensing on next generation of extremely large telescopes that might
present similar limitations.
---------------------------------------------------------
Title: MagAO-X first light
Authors: Males, Jared R.; Close, Laird M.; Guyon, Olivier; Hedglen,
Alexander D.; Van Gorkom, Kyle; Long, Joseph D.; Kautz, Maggie;
Lumbres, Jennifer; Schatz, Lauren; Rodack, Alexander; Miller,
Kelsey; Doelman, David; Snik, Frans; Bos, Steven; Knight, Justin
M.; Morzinski, Katie; Gasho, Victor; Keller, Christoph; Haffert,
Sebastiaan; Pearce, Logan
2020SPIE11448E..4LM Altcode:
MagAO-X is a new "extreme" adaptive optics system for the Magellan Clay
6.5 m telescope which began commissioning in December, 2019. MagAO-X is
based around a 2040 actuator deformable mirror, controlled by a pyramid
wavefront sensor operating at up to 3.6 kHz. When fully optimized,
MagAO-X will deliver high Strehls (< 70%), high resolution (19 mas),
and high contrast (< 1 × 10<SUP>-4</SUP>) at Hα (656 nm). We
present a brief review of the instrument design and operations, and
then report on the results of the first-light run.
---------------------------------------------------------
Title: Self-optimizing adaptive optics control with reinforcement
learning
Authors: Landman, R.; Haffert, S. Y.; Radhakrishnan, V. M.; Keller,
C. U.
2020SPIE11448E..49L Altcode: 2020arXiv201201997L
Current and future high-contrast imaging instruments require extreme
Adaptive Optics (XAO) systems to reach contrasts necessary to directly
image exoplanets. Telescope vibrations and the temporal error induced
by the latency of the control loop limit the performance of these
systems. Optimization of the (predictive) control algorithm is crucial
in reducing these effects. We describe how model-free Reinforcement
Learning can be used to optimize a Recurrent Neural Network controller
for closed-loop adaptive optics control. We verify our proposed
approach for tip-tilt control in simulations and a lab setup. The
results show that this algorithm can effectively learn to suppress a
combination of tip-tilt vibrations. Furthermore, we report decreased
residuals for power-law input turbulence compared to an optimal gain
integrator. Finally, we demonstrate that the controller can learn
to identify the parameters of a varying vibration without requiring
online updating of the control law. We conclude that Reinforcement
Learning is a promising approach towards data-driven predictive control;
future research will apply this approach to the control of high-order
deformable mirrors.
---------------------------------------------------------
Title: Planet formation with all flavors of adaptive optics:
VLT/MUSE's laser tomography adaptive optics to directly image young
accreting exoplanets
Authors: Girard, Julien H.; Haffert, Sebastiaan Y.; Bae, Jaehan;
Zeidler, Peter; de Boer, Jozua; Bohn, Alexander; van Holstein, Rob G.;
Brinchmann, Jarle; Snellen, Ignas; Bacon, Rolan; Keller, Christoph
2020SPIE11448E..08G Altcode:
We present recent results obtained with the VLT/MUSE Integral Field
Spectrograph fed by the 4LGSF and its laser tomography adaptive
optics module GALACSI. While this so-called narrow-field mode of
MUSE was not designed to perform directly imaging of exoplanets
and outflows, we show that it can be a game changer to detect and
characterize young exoplanets with a prominent emission lines (i.e Hα,
tracer of accretion), at moderate contrasts. These performances are
achieved thanks to the combo of a near-diffraction limited PSF and
a medium resolution spectrograph and a cross-correlation approach in
post-processing . We discuss this in the context of ground and space,
infrared and visible wavelengths, preparing for missions like JWST
and WFIRST in great synergy and as pathfinder for future ELT/GSMT
(Extremely Large and/or Giant Segmented Mirror Telescopes) instruments.
---------------------------------------------------------
Title: Multi-core fibre-fed integral-field unit (MCIFU): overview
and first-light
Authors: Haffert, Sebastiaan Y.; Harris, Robert J.; Zanutta, Alessio;
Pike, Fraser A.; Bianco, Andrea; Redaelli, Edoardo; Benoît,
Aurélien; MacLachlan, David G.; Ross, Calum A.; Gris-Sánchez,
Itandehui; Trappen, Mareike D.; Xu, Yilin; Blaicher, Matthias; Maier,
Pascal; Riva, Giulio; Sinquin, Baptiste; Kulcsár, Caroline; Bharmal,
Nazim Ali; Gendron, Eric; Staykov, Lazar; Morris, Tim J.; Barboza,
Santiago; Muench, Norbert; Bardour, Lisa; Prengère, Léonard; Raynaud,
Henri-François; Hottinger, Philipp; Anagnos, Theodoros; Osborn,
James; Koos, Christian; Thompson, Robert R.; Birks, Tim A.; Snellen,
Ignas A. G.; Keller, Christoph U.; Close, Laird; Males, Jared R.
2020SPIE11448E..4MH Altcode: 2021arXiv210109766A
The Multi-Core Integral-Field Unit (MCIFU) is a new diffraction-limited
near-infrared integral-field unit for exoplanet atmosphere
characterization with extreme adaptive optics (xAO) instruments. It has
been developed as an experimental pathfinder for spectroscopic upgrades
for SPHERE+/VLT and other xAO systems. The wavelength range covers 1.0
um to 1.6um at a resolving power around 5000 for 73 points on-sky. The
MCIFU uses novel astrophotonic components to make this very compact
and robust spectrograph. We performed the first successful on-sky test
with CANARY at the 4.2 meter William Herschel Telescope in July 2019,
where observed standard stars and several stellar binaries. An improved
version of the MCIFU will be used with MagAO-X, the new extreme adaptive
optics system at the 6.5 meter Magellan Clay telescope in Chile. We
will show and discuss the first-light performance and operations of the
MCIFU at CANARY and discuss the integration of the MCIFU with MagAO-X.
---------------------------------------------------------
Title: Design of the life signature detection polarimeter LSDpol
Authors: Keller, Christoph U.; Snik, Frans; Patty, C. H. Lucas;
Klindžic, Dora; Krasteva, Mariya; Doelman, David S.; Wijnen, Thomas;
Pallichadath, Vidhya; Stam, Daphne M.; Demory, Brice-Olivier; Kühn,
Jonas G.; Hoeijmakers, H. Jens; Pommerol, Antoine; Poch, Olivier
2020SPIE11443E..3RK Altcode: 2020arXiv201209105K
Many biologically produced chiral molecules such as amino
acids and sugars show a preference for left or right handedness
(homochirality). Light reflected by biological materials such as algae
and leaves therefore exhibits a small amount of circular polarization
that strongly depends on wavelength. Our Life Signature Detection
polarimeter (LSDpol) is optimized to measure these signatures of
life. LSDpol is a compact spectropolarimeter concept with no moving
parts that instantaneously measures linear and circular polarization
averaged over the field of view with a sensitivity of better than
10<SUP>-4</SUP>. We expect to launch the instrument into orbit after
validating its performance on the ground and from aircraft. LSDpol is
based on a spatially varying quarter-wave retarder that is implemented
with a patterned liquid-crystal. It is the first optical element to
maximize the polarimetric sensitivity. Since this pattern as well as the
entrance slit of the spectrograph have to be imaged onto the detector,
the slit serves as the aperture, and an internal field stop limits
the field of view. The retarder's fast axis angle varies linearly
along one spatial dimension. A fixed quarter-wave retarder combined
with a polarization grating act as the disperser and the polarizing
beam-splitter. Circular and linear polarization are thereby encoded at
incompatible modulation frequencies across the spectrum, which minimizes
the potential cross-talk from linear into circular polarization.
---------------------------------------------------------
Title: Searching for proto-planets with MUSE
Authors: Xie, C.; Haffert, S. Y.; de Boer, J.; Kenworthy, M. A.;
Brinchmann, J.; Girard, J.; Snellen, I. A. G.; Keller, C. U.
2020A&A...644A.149X Altcode: 2020arXiv201108043X
Context. Protoplanetary disks contain structures such as gaps, rings,
and spirals, which are thought to be produced by the interaction
between the disk and embedded protoplanets. However, only a few
planet candidates are found orbiting within protoplanetary disks,
and most of them are being challenged as having been confused
with disk features. <BR /> Aims: The VLT/MUSE discovery of PDS 70 c
demonstrated a powerful way of searching for still-forming protoplanets
by targeting accretion signatures with medium-resolution integral field
spectroscopy. We aim to discover more proto-planetary candidates with
MUSE, with a secondary aim of improving the high-resolution spectral
differential imaging (HRSDI) technique by analyzing the instrumental
residuals of MUSE. <BR /> Methods: We analyzed MUSE observations of five
young stars with various apparent brightnesses and spectral types. We
applied the HRSDI technique to perform high-contrast imaging. The
detection limits were estimated using fake planet injections. <BR
/> Results: With a 30 min integration time, MUSE can reach 5σ
detection limits in apparent Hα line flux down to 10<SUP>-14</SUP>
and 10<SUP>-15</SUP> erg s<SUP>-1</SUP> cm<SUP>-2</SUP> at 0.075”
and 0.25”, respectively. In addition to PDS 70 b and c, we did not
detect any clear accretion signatures in PDS 70, J1850-3147, and V1094
Sco down to 0.1”. MUSE avoids the small sample statistics problem by
measuring the noise characteristics in the spatial direction at multiple
wavelengths. We detected two asymmetric atomic jets in HD 163296 with
a very high spatial resolution (down to 8 au) and medium spectral
resolution (R ~ 2500). <BR /> Conclusions: The HRSDI technique when
applied to MUSE data allows us to reach the photon noise limit at small
separations (i.e., <0.5”). With the combination of high-contrast
imaging and medium spectral resolution, MUSE can achieve fainter
detection limits in apparent line flux than SPHERE/ZIMPOL by a factor
of ~5. MUSE has some instrumental issues that limit the contrast that
appear in cases with strong point sources, which can be either a spatial
point source due to high Strehl observations or a spectral point source
due to a high line-to-continuum ratio. We modified the HRSDI technique
to better handle the instrumental artifacts and improve the detection
limits. To avoid the instrumental effects altogether, we suggest faint
young stars with relatively low Hα line-to-continuum ratio to be the
most suitable targets for MUSE to search for potential protoplanets.
---------------------------------------------------------
Title: Diffraction-limited integral-field spectroscopy for extreme
adaptive optics systems with the multicore fiber-fed integral-field
unit
Authors: Haffert, Sebastiaan Y.; Harris, Robert J.; Zanutta, Alessio;
Pike, Fraser A.; Bianco, Andrea; Redaelli, Eduardo; Benoît,
Aurélien; MacLachlan, David G.; Ross, Calum A.; Gris-Sánchez,
Itandehui; Trappen, Mareike D.; Xu, Yilin; Blaicher, Matthias; Maier,
Pascal; Riva, Giulio; Sinquin, Baptiste; Kulcsár, Caroline; Bharmal,
Nazim Ali; Gendron, Eric; Staykov, Lazar; Morris, Tim J.; Barboza,
Santiago; Muench, Norbert; Bardou, Lisa; Prengère, Léonard; Raynaud,
Henri-François; Hottinger, Phillip; Anagnos, Theodoros; Osborn,
James; Koos, Christian; Thomson, Robert R.; Birks, Tim A.; Snellen,
Ignas A. G.; Keller, Christoph U.
2020JATIS...6d5007H Altcode: 2020arXiv200903529H
Direct imaging instruments have the spatial resolution to resolve
exoplanets from their host star. This enables direct characterization
of the exoplanets atmosphere, but most direct imaging instruments do
not have spectrographs with high enough resolving power for detailed
atmospheric characterization. We investigate the use of a single-mode
diffraction-limited integral-field unit that is compact and easy to
integrate into current and future direct imaging instruments for
exoplanet characterization. This achieved by making use of recent
progress in photonic manufacturing to create a single-mode fiber-fed
image reformatter. The fiber link is created with three-dimensional
printed lenses on top of a single-mode multicore fiber that feeds an
ultrafast laser inscribed photonic chip that reformats the fiber into
a pseudoslit. We then couple it to a first-order spectrograph with a
triple stacked volume phase holographic grating for a high efficiency
over a large bandwidth. The prototype system has had a successful
first-light observing run at the 4.2-m William Herschel Telescope. The
measured on-sky resolving power is between 2500 and 3000, depending
on the wavelength. With our observations, we show that single-mode
integral-field spectroscopy is a viable option for current and future
exoplanet imaging instruments.
---------------------------------------------------------
Title: CS Cha B: A disc-obscured M-type star mimicking a polarised
planetary companion
Authors: Haffert, S. Y.; van Holstein, R. G.; Ginski, C.; Brinchmann,
J.; Snellen, I. A. G.; Milli, J.; Stolker, T.; Keller, C. U.;
Girard, J.
2020A&A...640L..12H Altcode: 2020arXiv200707831H
Context. Direct imaging provides a steady flow of newly discovered
giant planets and brown dwarf companions. These multi-object systems
can provide information about the formation of low-mass companions
in wide orbits and/or help us to speculate about possible migration
scenarios. Accurate classification of companions is crucial for testing
formation pathways. <BR /> Aims: In this work we further characterise
the recently discovered candidate for a planetary-mass companion CS
Cha b and determine if it is still accreting. <BR /> Methods: MUSE is
a four-laser-adaptive-optics-assisted medium-resolution integral-field
spectrograph in the optical part of the spectrum. We observed the CS
Cha system to obtain the first spectrum of CS Cha b. The companion
is characterised by modelling both the spectrum from 6300 Å to 9300
Å and the photometry using archival data from the visible to the
near-infrared (NIR). <BR /> Results: We find evidence of accretion
and outflow signatures in Hα and OI emission. The atmospheric models
with the highest likelihood indicate an effective temperature of
3450 ± 50 K with a log g of 3.6 ± 0.5 dex. Based on evolutionary
models, we find that the majority of the object is obscured. We
determine the mass of the faint companion with several methods to
be between 0.07 M<SUB>⊙</SUB> and 0.71 M<SUB>⊙</SUB> with an
accretion rate of Ṁ = 4 × 10<SUP>-11±0.4</SUP> M<SUB>⊙</SUB>
yr<SUP>-1</SUP>. <BR /> Conclusions: Our results show that CS Cha B is
most likely a mid-M-type star that is obscured by a highly inclined
disc, which has led to its previous classification using broadband
NIR photometry as a planetary-mass companion. This shows that it is
important and necessary to observe over a broad spectral range to
constrain the nature of faint companions. <P />The extracted spectrum
of CS Cha B is only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr/">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/L12">http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/640/L12</A>
---------------------------------------------------------
Title: Two Directly Imaged, Wide-orbit Giant Planets around the Young,
Solar Analog TYC 8998-760-1
Authors: Bohn, Alexander J.; Kenworthy, Matthew A.; Ginski, Christian;
Rieder, Steven; Mamajek, Eric E.; Meshkat, Tiffany; Pecaut, Mark J.;
Reggiani, Maddalena; de Boer, Jozua; Keller, Christoph U.; Snik,
Frans; Southworth, John
2020ApJ...898L..16B Altcode: 2020arXiv200710991B
Even though tens of directly imaged companions have been discovered in
the past decades, the number of directly confirmed multiplanet systems
is still small. Dynamical analysis of these systems imposes important
constraints on formation mechanisms of these wide-orbit companions. As
part of the Young Suns Exoplanet Survey we report the detection of a
second planetary-mass companion around the 17 Myr-old, solar-type star
TYC 8998-760-1 that is located in the Lower Centaurus Crux subgroup
of the Scorpius-Centaurus association. The companion has a projected
physical separation of 320 au and several individual photometric
measurements from 1.1 to 3.8 microns constrain a companion mass of 6
± 1 M<SUB>Jup</SUB>, which is equivalent to a mass ratio of q = 0.57
± 0.10% with respect to the primary. With the previously detected 14
± 3 M<SUB>Jup</SUB> companion that is orbiting the primary at 160 au,
TYC 8998-760-1 is the first directly imaged multiplanet system that is
detected around a young, solar analog. We show that circular orbits are
stable, but that mildly eccentric orbits for either/both components (e
> 0.1) are chaotic on gigayear timescales, implying in situ formation
or a very specific ejection by an unseen third companion. Due to the
wide separations of the companions TYC 8998-760-1 is an excellent
system for spectroscopic and photometric follow-up with space-based
observatories such as the James Webb Space Telescope. <SUP>*</SUP>
Based on observations collected at the European Organisation for
Astronomical Research in the Southern Hemisphere under ESO programs
099.C-0698(A), 0101.C-0341(A), 2103.C-5012(B), and 0104.C-0265(A).
---------------------------------------------------------
Title: On-sky verification of Fast and Furious focal-plane wavefront
sensing: Moving forward toward controlling the island effect at
Subaru/SCExAO
Authors: Bos, S. P.; Vievard, S.; Wilby, M. J.; Snik, F.; Lozi, J.;
Guyon, O.; Norris, B. R. M.; Jovanovic, N.; Martinache, F.; Sauvage,
J. -F.; Keller, C. U.
2020A&A...639A..52B Altcode: 2020arXiv200512097B
Context. High-contrast imaging (HCI) observations of exoplanets can
be limited by the island effect (IE). The IE occurs when the main
wavefront sensor (WFS) cannot measure sharp phase discontinuities
across the telescope's secondary mirror support structures (also
known as spiders). On the current generation of telescopes, the IE
becomes a severe problem when the ground wind speed is below a few
meters per second. During these conditions, the air that is in close
contact with the spiders cools down and is not blown away. This can
create a sharp optical path length difference between light passing on
opposite sides of the spiders. Such an IE aberration is not measured
by the WFS and is therefore left uncorrected. This is referred to
as the low-wind effect (LWE). The LWE severely distorts the point
spread function (PSF), significantly lowering the Strehl ratio and
degrading the contrast. <BR /> Aims: In this article, we aim to show
that the focal-plane wavefront sensing (FPWFS) algorithm, Fast and
Furious (F&F), can be used to measure and correct the IE/LWE. The
F&F algorithm is a sequential phase diversity algorithm and a
software-only solution to FPWFS that only requires access to images
of non-coronagraphic PSFs and control of the deformable mirror. <BR
/> Methods: We deployed the algorithm on the SCExAO HCI instrument
at the Subaru Telescope using the internal near-infrared camera in
H-band. We tested with the internal source to verify that F&F can
correct a wide variety of LWE phase screens. Subsequently, F&F
was deployed on-sky to test its performance with the full end-to-end
system and atmospheric turbulence. The performance of the algorithm was
evaluated by two metrics based on the PSF quality: (1) the Strehl ratio
approximation (SRA), and (2) variance of the normalized first Airy
ring (VAR). The VAR measures the distortion of the first Airy ring,
and is used to quantify PSF improvements that do not or barely affect
the PSF core (e.g., during challenging atmospheric conditions). <BR />
Results: The internal source results show that F&F can correct
a wide range of LWE phase screens. Random LWE phase screens with a
peak-to-valley wavefront error between 0.4 μm and 2 μm were all
corrected to a SRA > 90% and an VAR ⪅ 0.05. Furthermore, the
on-sky results show that F&F is able to improve the PSF quality
during very challenging atmospheric conditions (1.3-1.4″seeing at
500 nm). Closed-loop tests show that F&F is able to improve the
VAR from 0.27-0.03 and therefore significantly improve the symmetry
of the PSF. Simultaneous observations of the PSF in the optical (λ =
750 nm, Δλ = 50 nm) show that during these tests we were correcting
aberrations common to the optical and NIR paths within SCExAO. We
could not conclusively determine if we were correcting the LWE and/or
(quasi-)static aberrations upstream of SCExAO. <BR /> Conclusions:
The F&F algorithm is a promising focal-plane wavefront sensing
technique that has now been successfully tested on-sky. Going forward,
the algorithm is suitable for incorporation into observing modes,
which will enable PSFs of higher quality and stability during science
observations.
---------------------------------------------------------
Title: VizieR Online Data Catalog: CS Cha B spectrum (Haffert+, 2020)
Authors: Haffert, S. Y.; van Holstein, R. G.; Ginski, C.; Brinchmann,
J.; Snellen, I. A. G.; Milli, J.; Stolker, T.; Keller, C. U.;
Girard, J.
2020yCat..36409012H Altcode:
The extracted MUSE spectrum of CS Cha B. <P />(1 data file).
---------------------------------------------------------
Title: A universal smartphone add-on for portable spectroscopy and
polarimetry: iSPEX 2
Authors: Burggraaff, Olivier; Perduijn, Armand B.; van Hek, Robert F.;
Schmidt, Norbert; Keller, Christoph U.; Snik, Frans
2020SPIE11389E..2KB Altcode: 2020arXiv200601519B
Spectropolarimetry is a powerful technique for remote sensing
of the environment. It enables the retrieval of particle
shape and size distributions in air and water to an extent that
traditional spectroscopy cannot. SPEX is an instrument concept for
spectropolarimetry through spectral modulation, providing snapshot,
and hence accurate, hyperspectral intensity and degree and angle
of linear polarization. Successful SPEX instruments have included
groundSPEX and SPEX airborne, which both measure aerosol optical
thickness with high precision, and soon SPEXone, which will fly on
PACE. Here, we present a low-cost variant for consumer cameras, iSPEX 2,
with universal smartphone support. Smartphones enable citizen science
measurements which are significantly more scaleable, in space and time,
than professional instruments. Universal smartphone support is achieved
through a modular hardware design and SPECTACLE data processing. iSPEX
2 will be manufactured through injection molding and 3D printing. A
smartphone app for data acquisition and processing is in active
development. Production, calibration, and validation will commence
in the summer of 2020. Scientific applications will include citizen
science measurements of aerosol optical thickness and surface water
reflectance, as well as low-cost laboratory and portable spectroscopy.
---------------------------------------------------------
Title: IRDAP: SPHERE-IRDIS polarimetric data reduction pipeline
Authors: van Holstein, R. G.; Girard, J. H.; de Boer, J.; Snik, F.;
Milli, J.; Stam, D. M.; Ginski, C.; Mouillet, D.; Wahhaj, Z.; Schmid,
H. M.; Keller, C. U.; Langlois, M.; Dohlen, K.; Vigan, A.; Pohl, A.;
Carbillet, M.; Fantinel, D.; Maurel, D.; Origné, A.; Petit, C. Ramos,
J.; Rigal, F.; Sevin, A.; Boccaletti, A.; Le Coroller, H.; Dominik,
C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.; Udry, S.;
Chauvin, G.; Feldt, M.; Beuzit, J. -L.
2020ascl.soft04015V Altcode:
IRDAP (IRDIS Data reduction for Accurate Polarimetry) accurately
reduces SPHERE-IRDIS polarimetric data. It is a highly-automated
end-to-end pipeline; its core feature is model-based correction of
the instrumental polarization effects. IRDAP handles data taken both
in field- and pupil-tracking mode and using the broadband filters Y,
J, H and Ks. Data taken with the narrowband filters can be reduced
as well, although with a somewhat worse accuracy. For pupil-tracking
observations IRDAP can additionally apply angular differential imaging.
---------------------------------------------------------
Title: The Single-mode Complex Amplitude Refinement (SCAR)
coronagraph. II. Lab verification, and toward the characterization
of Proxima b
Authors: Haffert, S. Y.; Por, E. H.; Keller, C. U.; Kenworthy, M. A.;
Doelman, D. S.; Snik, F.; Escuti, M. J.
2020A&A...635A..56H Altcode:
We present the monochromatic lab verification of the newly developed
SCAR coronagraph that combines a phase plate (PP) in the pupil with
a microlens-fed single-mode fiber array in the focal plane. The
two SCAR designs that have been measured, create respectively a
360 degree and 180 degree dark region from 0.8-2.4λ/D around the
star. The 360 SCAR has been designed for a clear aperture and the
180 SCAR has been designed for a realistic aperture with central
obscuration and spiders. The 360 SCAR creates a measured stellar null
of 2-3 × 10<SUP>-4</SUP>, and the 180 SCAR reaches a null of 1 ×
10<SUP>-4</SUP>. Their monochromatic contrast is maintained within a
range of ±0.16λ/D peak-to-valley tip-tilt, which shows the robustness
against tip-tilt errors. The small inner working angle and tip-tilt
stability makes the SCAR coronagraph a very promising technique for
an upgrade of current high-contrast instruments to characterize and
detect exoplanets in the solar neighborhood.
---------------------------------------------------------
Title: Original use of MUSE's laser tomography adaptive optics to
directly image young accreting exoplanets
Authors: Girard, Julien H.; de Boer, Jozua; Haffert, Sebastiaan;
Zeidler, Peter; Bohn, Alexander; van Holstein, Rob G.; Snellen, Ignas;
Brinchmann, Jarle; Keller, Christoph; Bacon, Roland; Bae, Jaehan
2020arXiv200302145G Altcode:
We present recent results obtained with the VLT/MUSE Integral Field
Spectrograph fed by the 4LGSF and its laser tomography adaptive optics
module GALACSI. While this so-called narrow-field mode of MUSE was
not designed to perform directly imaging of exoplanets and outflows,
we show that it can be a game changer to detect and characterize
young exoplanets with a prominent emission lines (i.e H{\alpha},
tracer of accretion), at moderate contrasts. These performances are
achieved thanks to the combo of a near-diffraction limited PSF and
a medium resolution spectrograph and a cross-correlation approach in
post-processing . We discuss this in the context of ground and space,
infrared and visible wavelengths, preparing for missions like JWST
and WFIRST in great synergy and as pathfinder for future ELT/GSMT
(Extremely Large and/or Giant Segmented Mirror Telescopes) instruments.
---------------------------------------------------------
Title: SPHERE+: Imaging young Jupiters down to the snowline
Authors: Boccaletti, A.; Chauvin, G.; Mouillet, D.; Absil, O.;
Allard, F.; Antoniucci, S.; Augereau, J. -C.; Barge, P.; Baruffolo,
A.; Baudino, J. -L.; Baudoz, P.; Beaulieu, M.; Benisty, M.; Beuzit,
J. -L.; Bianco, A.; Biller, B.; Bonavita, B.; Bonnefoy, M.; Bos, S.;
Bouret, J. -C.; Brandner, W.; Buchschache, N.; Carry, B.; Cantalloube,
F.; Cascone, E.; Carlotti, A.; Charnay, B.; Chiavassa, A.; Choquet,
E.; Clenet, Y.; Crida, A.; De Boer, J.; De Caprio, V.; Desidera, S.;
Desert, J. -M.; Delisle, J. -B.; Delorme, P.; Dohlen, K.; Doelman,
D.; Dominik, C.; Orazi, V. D; Dougados, C.; Doute, S.; Fedele, D.;
Feldt, M.; Ferreira, F.; Fontanive, C.; Fusco, T.; Galicher, R.;
Garufi, A.; Gendron, E.; Ghedina, A.; Ginski, C.; Gonzalez, J. -F.;
Gratadour, D.; Gratton, R.; Guillot, T.; Haffert, S.; Hagelberg, J.;
Henning, T.; Huby, E.; Janson, M.; Kamp, I.; Keller, C.; Kenworthy,
M.; Kervella, P.; Kral, Q.; Kuhn, J.; Lagadec, E.; Laibe, G.; Langlois,
M.; Lagrange, A. -M.; Launhardt, R.; Leboulleux, L.; Le Coroller, H.;
Li Causi, G.; Loupias, M.; Maire, A. L.; Marleau, G.; Martinache,
F.; Martinez, P.; Mary, D.; Mattioli, M.; Mazoyer, J.; Meheut, H.;
Menard, F.; Mesa, D.; Meunier, N.; Miguel, Y.; Milli, J.; Min, M.;
Molliere, P.; Mordasini, C.; Moretto, G.; Mugnier, L.; Muro Arena,
G.; Nardetto, N.; Diaye, M. N; Nesvadba, N.; Pedichini, F.; Pinilla,
P.; Por, E.; Potier, A.; Quanz, S.; Rameau, J.; Roelfsema, R.; Rouan,
D.; Rigliaco, E.; Salasnich, B.; Samland, M.; Sauvage, J. -F.; Schmid,
H. -M.; Segransan, D.; Snellen, I.; Snik, F.; Soulez, F.; Stadler, E.;
Stam, D.; Tallon, M.; Thebault, P.; Thiebaut, E.; Tschudi, C.; Udry,
S.; van Holstein, R.; Vernazza, P.; Vidal, F.; Vigan, A.; Waters,
R.; Wildi, F.; Willson, M.; Zanutta, A.; Zavagno, A.; Zurlo, A.
2020arXiv200305714B Altcode:
SPHERE (Beuzit et al,. 2019) has now been in operation at the VLT for
more than 5 years, demonstrating a high level of performance. SPHERE
has produced outstanding results using a variety of operating modes,
primarily in the field of direct imaging of exoplanetary systems,
focusing on exoplanets as point sources and circumstellar disks as
extended objects. The achievements obtained thus far with SPHERE
(~200 refereed publications) in different areas (exoplanets, disks,
solar system, stellar physics...) have motivated a large consortium
to propose an even more ambitious set of science cases, and its
corresponding technical implementation in the form of an upgrade. The
SPHERE+ project capitalizes on the expertise and lessons learned
from SPHERE to push high contrast imaging performance to its limits
on the VLT 8m-telescope. The scientific program of SPHERE+ described
in this document will open a new and compelling scientific window for
the upcoming decade in strong synergy with ground-based facilities
(VLT/I, ELT, ALMA, and SKA) and space missions (Gaia, JWST, PLATO and
WFIRST). While SPHERE has sampled the outer parts of planetary systems
beyond a few tens of AU, SPHERE+ will dig into the inner regions
around stars to reveal and characterize by mean of spectroscopy the
giant planet population down to the snow line. Building on SPHERE's
scientific heritage and resounding success, SPHERE+ will be a dedicated
survey instrument which will strengthen the leadership of ESO and the
European community in the very competitive field of direct imaging
of exoplanetary systems. With enhanced capabilities, it will enable
an even broader diversity of science cases including the study of the
solar system, the birth and death of stars and the exploration of the
inner regions of active galactic nuclei.
---------------------------------------------------------
Title: The Young Suns Exoplanet Survey: Detection of a wide-orbit
planetary-mass companion to a solar-type Sco-Cen member
Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Manara, C. F.;
Pecaut, M. J.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat,
T.; Reggiani, M.; Todorov, K. O.; Snik, F.
2020MNRAS.492..431B Altcode: 2019MNRAS.tmp.3127B; 2019arXiv191204284B
The Young Suns Exoplanet Survey consists of a homogeneous sample of 70
young, solar-mass stars located in the Lower Centaurus-Crux subgroup
of the Scorpius-Centaurus association with an average age of 15 ±
3 Myr. We report the detection of a co-moving companion around the
K3IV star TYC 8998-760-1 (2MASSJ13251211-6456207) that is located at a
distance of 94.6 ± 0.3 pc using SPHERE/IRDIS on the VLT. Spectroscopic
observations with VLT/X-SHOOTER constrain the mass of the star to
1.00± 0.02 M_{⊙ } and an age of 16.7± 1.4 Myr. The companion
TYC 8998-760-1 b is detected at a projected separation of 1.71″,
which implies a projected physical separation of 162 au. Photometric
measurements ranging from Y to M band provide a mass estimate of
14± 3 M_jup by comparison to BT-Settl and AMES-dusty isochrones,
corresponding to a mass ratio of q = 0.013 ± 0.003 with respect
to the primary. We rule out additional companions to TYC 8998-760-1
that are more massive than 12 M_jup and farther than 12 au away from
the host. Future polarimetric and spectroscopic observations of this
system with ground and space based observatories will facilitate testing
of formation and evolution scenarios shaping the architecture of the
circumstellar environment around this `young Sun'.
---------------------------------------------------------
Title: RefPlanets: Search for reflected light from extrasolar planets
with SPHERE/ZIMPOL
Authors: Hunziker, S.; Schmid, H. M.; Mouillet, D.; Milli, J.; Zurlo,
A.; Delorme, P.; Abe, L.; Avenhaus, H.; Baruffolo, A.; Bazzon, A.;
Boccaletti, A.; Baudoz, P.; Beuzit, J. L.; Carbillet, M.; Chauvin, G.;
Claudi, R.; Costille, A.; Daban, J. -B.; Desidera, S.; Dohlen, K.;
Dominik, C.; Downing, M.; Engler, N.; Feldt, M.; Fusco, T.; Ginski,
C.; Gisler, D.; Girard, J. H.; Gratton, R.; Henning, Th.; Hubin,
N.; Kasper, M.; Keller, C. U.; Langlois, M.; Lagadec, E.; Martinez,
P.; Maire, A. L.; Menard, F.; Meyer, M. R.; Pavlov, A.; Pragt, J.;
Puget, P.; Quanz, S. P.; Rickman, E.; Roelfsema, R.; Salasnich, B.;
Sauvage, J. -F.; Siebenmorgen, R.; Sissa, E.; Snik, F.; Suarez, M.;
Szulágyi, J.; Thalmann, Ch.; Turatto, M.; Udry, S.; van Holstein,
R. G.; Vigan, A.; Wildi, F.
2020A&A...634A..69H Altcode: 2019arXiv191112759H
<BR /> Aims: RefPlanets is a guaranteed time observation
programme that uses the Zurich IMaging POLarimeter (ZIMPOL) of
Spectro-Polarimetric High-contrast Exoplanet REsearch instrument at
the Very Large Telescope to perform a blind search for exoplanets
in wavelengths from 600 to 900 nm. The goals of this study are the
characterisation of the unprecedented high polarimetic contrast and
polarimetric precision capabilities of ZIMPOL for bright targets,
the search for polarised reflected light around some of the closest
bright stars to the Sun, and potentially the direct detection of an
evolved cold exoplanet for the first time. <BR /> Methods: For our
observations of α Cen A and B, Sirius A, Altair, ɛ Eri and τ Ceti
we used the polarimetricdifferential imaging (PDI) mode of ZIMPOL
which removes the speckle noise down to the photon noise limit for
angular separations ≿0.6”. We describe some of the instrumental
effects that dominate the noise for smaller separations and explain
how to remove these additional noise effects in post-processing. We
then combine PDI with angular differential imaging as a final layer
of post-processing to further improve the contrast limits of our data
at these separations. <BR /> Results: For good observing conditions
we achieve polarimetric contrast limits of 15.0-16.3 mag at the
effective inner working angle of ~0.13”, 16.3-18.3 mag at 0.5”,
and 18.8-20.4 mag at 1.5”. The contrast limits closer in (≾0.6”)
display a significant dependence on observing conditions, while in the
photon-noise-dominated regime (≿0.6”) the limits mainly depend on
the brightness of the star and the total integration time. We compare
our results with contrast limits from other surveys and review
the exoplanet detection limits obtained with different detection
methods. For all our targets we achieve unprecedented contrast
limits. Despite the high polarimetric contrasts we are not able to
find any additional companions or extended polarised light sources
in the data obtained so far. <P />Based on observations made with
ESO Telescopes at the La Silla Paranal Observatory under programme
IDs: 095.C-0312(B), 096.C-0326(A), 097.C-0524(A), 097.C-0524(B),
098.C-0197(A), 099.C-0127(A), 099.C-0127(B), 0102.C-0435(A).
---------------------------------------------------------
Title: VizieR Online Data Catalog: A planetary-mass companion to a
solar-type star (Bohn+, 2020)
Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Manara, C. F.;
Pecaut, M. J.; de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat,
T.; Reggiani, M.; Todorov, K. O.; Snik, F.
2020yCat..74920431B Altcode:
Fits images corresponding to the imagery presented in the paper. The
data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed
with version 0.8.1 of PynPoint. We detect a co-moving companion around
The solar-type Sco-Cen member TYC 8998-760-1 with a mass of 14+/-3
Jupiter masses. The extracted astrometry and photometry is presented
in this catalogue. <P />(4 data files).
---------------------------------------------------------
Title: Polarimetric imaging mode of VLT/SPHERE/IRDIS. I. Description,
data reduction, and observing strategy
Authors: de Boer, J.; Langlois, M.; van Holstein, R. G.; Girard,
J. H.; Mouillet, D.; Vigan, A.; Dohlen, K.; Snik, F.; Keller, C. U.;
Ginski, C.; Stam, D. M.; Milli, J.; Wahhaj, Z.; Kasper, M.; Schmid,
H. M.; Rabou, P.; Gluck, L.; Hugot, E.; Perret, D.; Martinez, P.;
Weber, L.; Pragt, J.; Sauvage, J. -F.; Boccaletti, A.; Le Coroller,
H.; Dominik, C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.;
Udry, S.; Chauvin, G.; Feldt, M.; Beuzit, J. -L.
2020A&A...633A..63D Altcode: 2019arXiv190913107D
Context. Polarimetric imaging is one of the most effective techniques
for high-contrast imaging and for the characterization of protoplanetary
disks, and it has the potential of becoming instrumental in the
characterization of exoplanets. The Spectro-Polarimetric High-contrast
Exoplanet REsearch (SPHERE) instrument installed on the Very Large
Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph
(IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers
the capability of obtaining linear polarization images at high contrast
and resolution. <BR /> Aims: We aim to provide an overview of the
polarimetric imaging mode of VLT/SPHERE/IRDIS and study its optical
design to improve observing strategies and data reduction. <BR />
Methods: For H-band observations of TW Hydrae, we compared two data
reduction methods that correct for instrumental polarization effects in
different ways: a minimization of the "noise" image (U<SUB>ϕ</SUB>),
and a correction method based on a polarimetric model that we have
developed, as presented in Paper II of this study. <BR /> Results:
We use observations of TW Hydrae to illustrate the data reduction. In
the images of the protoplanetary disk around this star, we detect
variability in the polarized intensity and angle of linear polarization
that depend on the pointing-dependent instrument configuration. We
explain these variations as instrumental polarization effects and
correct for these effects using our model-based correction method. <BR
/> Conclusions: The polarimetric imaging mode of IRDIS has proven to
be a very successful and productive high-contrast polarimetric imaging
system. However, the instrument performance is strongly dependent
on the specific instrument configuration. We suggest adjustments
to future observing strategies to optimize polarimetric efficiency
in field-tracking mode by avoiding unfavorable derotator angles. We
recommend reducing on-sky data with the pipeline called IRDAP, which
includes the model-based correction method (described in Paper II)
to optimally account for the remaining telescope and instrumental
polarization effects and to retrieve the true polarization state of
the incident light. <P />Based on observations made with ESO Telescopes
at the La Silla Paranal Observatory under programme ID 095.C-0273(D).
---------------------------------------------------------
Title: Polarimetric imaging mode of
VLT/SPHERE/IRDIS. II. Characterization and correction of instrumental
polarization effects
Authors: van Holstein, R. G.; Girard, J. H.; de Boer, J.; Snik, F.;
Milli, J.; Stam, D. M.; Ginski, C.; Mouillet, D.; Wahhaj, Z.; Schmid,
H. M.; Keller, C. U.; Langlois, M.; Dohlen, K.; Vigan, A.; Pohl,
A.; Carbillet, M.; Fantinel, D.; Maurel, D.; Origné, A.; Petit,
C.; Ramos, J.; Rigal, F.; Sevin, A.; Boccaletti, A.; Le Coroller,
H.; Dominik, C.; Henning, T.; Lagadec, E.; Ménard, F.; Turatto, M.;
Udry, S.; Chauvin, G.; Feldt, M.; Beuzit, J. -L.
2020A&A...633A..64V Altcode: 2019arXiv190913108V
Context. Circumstellar disks and self-luminous giant exoplanets or
companion brown dwarfs can be characterized through direct-imaging
polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very
Large Telescope has the capabilities to perform such measurements,
but uncalibrated instrumental polarization effects limit the attainable
polarimetric accuracy. <BR /> Aims: We aim to characterize and correct
the instrumental polarization effects of the complete optical system,
that is, the telescope and SPHERE/IRDIS. <BR /> Methods: We created
a detailed Mueller matrix model in the broadband filters Y, J, H,
and K<SUB>s</SUB> and calibrated the model using measurements with
SPHERE's internal light source and observations of two unpolarized
stars. We developed a data-reduction method that uses the model to
correct for the instrumental polarization effects, and applied it
to observations of the circumstellar disk of T Cha. <BR /> Results:
The instrumental polarization is almost exclusively produced by the
telescope and SPHERE's first mirror and varies with telescope altitude
angle. The crosstalk primarily originates from the image derotator
(K-mirror). At some orientations, the derotator causes severe loss of
signal (> 90% loss in the H- and K<SUB>s</SUB>-band) and strongly
offsets the angle of linear polarization. With our correction method
we reach, in all filters, a total polarimetric accuracy of ≲0.1%
in the degree of linear polarization and an accuracy of a few degrees
in angle of linear polarization. <BR /> Conclusions: The correction
method enables us to accurately measure the polarized intensity and
angle of linear polarization of circumstellar disks, and is a vital
tool for detecting spatially unresolved (inner) disks and measuring
the polarization of substellar companions. We have incorporated the
correction method in a highly-automated end-to-end data-reduction
pipeline called IRDAP, which we made publicly available online. <P
/>Based on observations made with ESO telescopes at the La Silla
Paranal Observatory under program ID 60.A-9800(S), 60.A-9801(S) and
096.C-0248(C). <P />The data-reduction pipeline IRDAP is available at
<A href="https://irdap.readthedocs.io">https://irdap.readthedocs.io</A>
---------------------------------------------------------
Title: Overview of focal plane wavefront sensors to correct for the
Low Wind Effect on SUBARU/SCExAO
Authors: Vievard, Sebastien; Bos, Steven; Cassaing, Frederic; Ceau,
Alban; Guyon, Olivier; Jovanovic, Nemanja; Keller, Christoph U.; Lozi,
Julien; Martinache, Frantz; Montmerle-Bonnefois, Aurelie; Mugnier,
Laurent; NDiaye, Mamadou; Norris, Barnaby; Sahoo, Ananya; Sauvage,
Jean-Francois; Snik, Frans; Wilby, Michael J.; Wong, Alisson
2019arXiv191210179V Altcode:
The Low Wind Effect (LWE) refers to a phenomenon that occurs when
the wind speed inside a telescope dome drops below $3$m/s creating
a temperature gradient near the telescope spider. This produces
phase discontinuities in the pupil plane that are not detected
by traditional Adaptive Optics (AO) systems such as the pyramid
wavefront sensor or the Shack-Hartmann. Considering the pupil as
divided in 4 quadrants by regular spiders, the phase discontinuities
correspond to piston, tip and tilt aberrations in each quadrant of the
pupil. Uncorrected, it strongly decreases the ability of high contrast
imaging instruments utilizing coronagraphy to detect exoplanets at
small angular separations. Multiple focal plane wavefront sensors are
currently being developed and tested on the Subaru Coronagraphic Extreme
Adaptive Optics (SCExAO) instrument at Subaru Telescope: Among them,
the Zernike Asymmetric Pupil (ZAP) wavefront sensor already showed
on-sky that it could measure the LWE induced aberrations in focal
plane images. The Fast and Furious algorithm, using previous deformable
mirror commands as temporal phase diversity, showed in simulations its
efficiency to improve the wavefront quality in the presence of LWE. A
Neural Network algorithm trained with SCExAO telemetry showed promising
PSF prediction on-sky. The Linearized Analytic Phase Diversity (LAPD)
algorithm is a solution for multi-aperture cophasing and is studied to
correct for the LWE aberrations by considering the Subaru Telescope as
a 4 sub-aperture instrument. We present the different algorithms, show
the latest results and compare their implementation on SCExAO/SUBARU
as real-time wavefront sensors for the LWE compensation.
---------------------------------------------------------
Title: Focal-plane wavefront sensing with the vector-Apodizing
Phase Plate
Authors: Bos, S. P.; Doelman, D. S.; Lozi, J.; Guyon, O.; Keller,
C. U.; Miller, K. L.; Jovanovic, N.; Martinache, F.; Snik, F.
2019A&A...632A..48B Altcode: 2019arXiv190908317B
Context. One of the key limitations of the direct imaging of exoplanets
at small angular separations are quasi-static speckles that originate
from evolving non-common path aberrations (NCPA) in the optical train
downstream of the instrument's main wavefront sensor split-off. <BR />
Aims: In this article we show that the vector-Apodizing Phase Plate
(vAPP) coronagraph can be designed such that the coronagraphic point
spread functions (PSFs) can act as wavefront sensors to measure and
correct the (quasi-)static aberrations without dedicated wavefront
sensing holograms or modulation by the deformable mirror. The absolute
wavefront retrieval is performed with a non-linear algorithm. <BR />
Methods: The focal-plane wavefront sensing (FPWFS) performance of
the vAPP and the algorithm are evaluated via numerical simulations to
test various photon and read noise levels, the sensitivity to the 100
lowest Zernike modes, and the maximum wavefront error (WFE) that can be
accurately estimated in one iteration. We apply these methods to the
vAPP within SCExAO, first with the internal source and subsequently
on-sky. <BR /> Results: In idealized simulations we show that for
10<SUP>7</SUP> photons the root mean square (rms) WFE can be reduced to
∼λ/1000, which is 1 nm rms in the context of the SCExAO system. We
find that the maximum WFE that can be corrected in one iteration is
∼λ/8 rms or ∼200 nm rms (SCExAO). Furthermore, we demonstrate
the SCExAO vAPP capabilities by measuring and controlling the 30
lowest Zernike modes with the internal source and on-sky. On-sky,
we report a raw contrast improvement of a factor ∼2 between 2
and 4 λ/D after five iterations of closed-loop correction. When
artificially introducing 150 nm rms WFE, the algorithm corrects it
within five iterations of closed-loop operation. <BR /> Conclusions:
FPWFS with the vAPP coronagraphic PSFs is a powerful technique since
it integrates coronagraphy and wavefront sensing, eliminating the need
for additional probes and thus resulting in a 100% science duty cycle
and maximum throughput for the target.
---------------------------------------------------------
Title: Spatial linear dark field control and holographic modal
wavefront sensing with a vAPP coronagraph on MagAO-X
Authors: Miller, Kelsey; Males, Jared R.; Guyon, Olivier; Close,
Laird M.; Doelman, David; Snik, Frans; Por, Emiel; Wilby, Michael J.;
Keller, Christoph; Bohlman, Chris; Van Gorkom, Kyle; Rodack, Alexander;
Knight, Justin; Lumbres, Jennifer; Bos, Steven; Jovanovic, Nemanja
2019JATIS...5d9004M Altcode:
The Magellan Extreme Adaptive Optics (MagAO-X) Instrument is an extreme
AO system coming online at the end of 2019 that will be operating
within the visible and near-IR. With state-of-the-art wavefront sensing
and coronagraphy, MagAO-X will be optimized for high-contrast direct
exoplanet imaging at challenging visible wavelengths, particularly
Hα. To enable high-contrast imaging, the instrument hosts a vector
apodizing phase plate (vAPP) coronagraph. The vAPP creates a static
region of high contrast next to the star that is referred to as a dark
hole; on MagAO-X, the expected dark hole raw contrast is ∼4 × 10<SUP>
- 6</SUP>. The ability to maintain this contrast during observations,
however, is limited by the presence of non-common path aberrations
(NCPA) and the resulting quasi-static speckles that remain unsensed
and uncorrected by the primary AO system. These quasi-static speckles
within the dark hole degrade the high contrast achieved by the vAPP
and dominate the light from an exoplanet. The aim of our efforts here
is to demonstrate two focal plane wavefront sensing (FPWFS) techniques
for sensing NCPA and suppressing quasi-static speckles in the final
focal plane. To sense NCPA to which the primary AO system is blind, the
science image is used as a secondary wavefront sensor. With the vAPP,
a static high-contrast dark hole is created on one side of the PSF,
leaving the opposite side of the PSF unocculted. In this unobscured
region, referred to as the bright field, the relationship between
modulations in intensity and low-amplitude pupil plane phase aberrations
can be approximated as linear. The bright field can therefore be
used as a linear wavefront sensor to detect small NCPA and suppress
quasi-static speckles. This technique, known as spatial linear dark
field control (LDFC), can monitor the bright field for aberrations that
will degrade the high-contrast dark hole. A second form of FPWFS, known
as holographic modal wavefront sensing (hMWFS), is also employed with
the vAPP. This technique uses hologram-generated PSFs in the science
image to monitor the presence of low-order aberrations. With LDFC and
the hMWFS, high contrast across the dark hole can be maintained over
long observations, thereby allowing planet light to remain visible above
the stellar noise over the course of observations on MagAO-X. Here,
we present simulations and laboratory demonstrations of both spatial
LDFC and the hMWFS with a vAPP coronagraph at the University of Arizona
Extreme Wavefront Control Laboratory. We show both in simulation and in
the lab that the hMWFS can be used to sense low-order aberrations and
reduce the wavefront error (WFE) by a factor of 3 - 4 × . We also show
in simulation that, in the presence of a temporally evolving pupil plane
phase aberration with 27-nm root-mean-square (RMS) WFE, LDFC can reduce
the WFE to 18-nm RMS, resulting in factor of 6 to 10 gain in contrast
that is kept stable over time. This performance is also verified in
the lab, showing that LDFC is capable of returning the dark hole to
the average contrast expected under ideal lab conditions. These results
demonstrate the power of the hMWFS and spatial LDFC to improve MagAO-X's
high-contrast imaging capabilities for direct exoplanet imaging.
---------------------------------------------------------
Title: A snapshot full-Stokes spectropolarimeter for detecting life
on Earth
Authors: Snik, Frans; Keller, Christoph U.; Doelman, David S.;
Kühn, Jonas; Patty, C. H. Lucas; Hoeijmakers, H. Jens; Pallichadath,
Vidhya; Stam, Daphne M.; Pommerol, Antoine; Poch, Olivier; Demory,
Brice-Olivier
2019SPIE11132E..0AS Altcode: 2019arXiv190902283S
We present the design of a point-and-shoot non-imaging full-Stokes
spectropolarimeter dedicated to detecting life on Earth from an orbiting
platform like the ISS. We specifically aim to map circular polarization
in the spectral features of chorophyll and other biopigments for our
planet as a whole. These non-zero circular polarization signatures
are caused by homochirality of the molecular and supramolecular
configurations of organic matter, and are considered the most
unambiguous biomarker. To achieve a fully solid-state snapshot design,
we implement a novel spatial modulation that completely separates the
circular and linear polarization channels. The polarization modulator
consists of a patterned liquid-crystal quarter-wave plate inside the
spectrograph slit, which also constitutes the first optical element of
the instrument. This configuration eliminates cross-talk between linear
and circular polarization, which is crucial because linear polarization
signals are generally much stronger than the circular polarization
signals. This leads to a quite unorthodox optical concept for the
spectrograph, in which the object and the pupil are switched. We
discuss the general design requirements and trade-offs of LSDpol
(Life Signature Detection polarimeter), a prototype instrument that
is currently under development.
---------------------------------------------------------
Title: Insights into Terrestrial Planet Compositions and Geophysics
from Observations of Magma Worlds
Authors: Ridden-Harper, Andrew; Snellen, Ignas; Keller, Christoph;
Mollière, Paul; De Mooij, Ernst J. W.; Jayawardhana, Ray; de Kok,
Remco; Hoeijmakers, H. Jens; Brogi, Matteo; Fridlund, Carl Malcolm;
Vermeersen, Bert; Westrenen, Wim
2019ESS.....431305R Altcode:
There exists a remarkable population of short period transiting
rocky exoplanets with temperatures >2,000 K, and masses ranging
from about 8 Earth masses, such as the hot super-Earth 55 Cancri
e, to that of Mercury or smaller, such as K2-22b. These planets
are thought to have mineral atmospheres that are produced by the
vaporisation of their magma surfaces, or large exospheres that are
produced by sputtering of their atmospheres or exposed surfaces by
intense stellar winds. Additionally, the smaller, low surface gravity
hot rocky exoplanets have been found to be actively disintegrating
and forming 'comet-like' dust tails. <P />Since their atmospheres and
released gas and dust can be observationally constrained, these planets
present the tantalising prospect of directly probing the composition of
rocky planets. Sodium and calcium are promising species to detect given
their low sublimation temperatures, large absorption cross-sections,
likely presence in terrestrial planet compositions, and presence in
Mercury's exosphere. <P />This poster presents the insights we gained
from using high-resolution transmission spectroscopy to search for
Na and Ca around 55 Cnc e and K2-22 b using several ground based
telescopes. For 55 Cne e, we detected a tantilizing 5 σ signal of
Ca+ on one night of observation, but a similar signal has not been
detected since (despite our unprecedented limits). This may be related
to variability of the star-planet system and the planet's magnetic
field. <P />For K2-22 b, we did not detect absorption by Na or Ca+,
but found lower-limits that are smaller than the expected magnitude
of the signal based on the planet's estimated mass-loss rate and
assuming a terrestrial composition. We attribute this non-detection to
the probed gases being accelerated by the stellar wind and radiation
pressure to high velocities, resulting in very broad Doppler shifted
absorption signals that are hard to detect. <P />The implications of
these results on probing rocky exoplanet compositions, constraining
planetary magnetic fields, and understanding the environment around
short-period rocky exoplanets are also outlined.
---------------------------------------------------------
Title: ESA Voyage 2050 White Paper: Detecting life outside our solar
system with a large high-contrast-imaging mission
Authors: Snellen, Ignas; Albrecht, Simon; Anglada-Escude, Guillem;
Baraffe, Isabelle; Baudoz, Pierre; Benz, Willy; Beuzit, Jean-Luc;
Biller, Beth; Birkby, Jayne; Boccaletti, Anthony; van Boekel, Roy;
de Boer, Jos; Brogi, Matteo; Buchhave, Lars; Carone, Ludmila;
Claire, Mark; Claudi, Riccardo; Demory, Brice-Olivier; Desert,
Jean-Michel; Desidera, Silvano; Gaudi, Scott; Gratton, Raffaele;
Gillon, Michael; Grenfell, John Lee; Guyon, Olivier; Henning, Thomas;
Hinkley, Sasha; Huby, Elsa; Janson, Markus; Helling, Christiane;
Heng, Kevin; Kasper, Markus; Keller, Christoph; Kenworthy, Matthew;
Krause, Oliver; Kreidberg, Laura; Madhusudhan, Nikku; Lagrange,
Anne-Marie; Launhardt, Ralf; Lenton, Tim; Lopez-Puertas, Manuel;
Maire, Anne-Lise; Mayne, Nathan; Meadows, Victoria; Mennesson,
Bertrand; Micela, Giuseppina; Miguel, Yamila; Milli, Julien; Min,
Michiel; de Mooij, Ernst; Mouillet, David; N'Diaye, Mamadou; D'Orazi,
Valentina; Palle, Enric; Pagano, Isabella; Piotto, Giampaolo; Queloz,
Didier; Rauer, Heike; Ribas, Ignasi; Ruane, Garreth; Selsis, Franck;
Snik, Frans; Sozzetti, Alessandro; Stam, Daphne; Stark, Christopher;
Vigan, Arthur; de Visser, Pieter
2019arXiv190801803S Altcode:
In this white paper, we recommend the European Space Agency plays a
proactive role in developing a global collaborative effort to construct
a large high-contrast imaging space telescope, e.g. as currently
under study by NASA. Such a mission will be needed to characterize a
sizable sample of temperate Earth-like planets in the habitable zones
of nearby Sun-like stars and to search for extraterrestrial biological
activity. We provide an overview of relevant European expertise,
and advocate ESA to start a technology development program towards
detecting life outside the Solar system.
---------------------------------------------------------
Title: Search for gas from the disintegrating rocky exoplanet K2-22b
Authors: Ridden-Harper, A. R.; Snellen, I. A. G.; Keller, C. U.;
Mollière, P.
2019A&A...628A..70R Altcode: 2019arXiv190608795R
Context. The red dwarf star K2-22 is transited every 9.14 h by
an object which is best explained by being a disintegrating rocky
exoplanet featuring a variable comet-like dust tail. While the dust is
thought to dominate the transit light curve, gas is also expected to
be present, either from being directly evaporated off the planet or by
being produced by the sublimation of dust particles in the tail. <BR
/> Aims: Both ionized calcium and sodium have large cross-sections,
and although present at low abundance, exhibit the strongest atomic
absorption features in comets. We therefore also identify these species
as the most promising tracers of circumplanetary gas in evaporating
rocky exoplanets and search for them in the tail of K2-22 b to constrain
the gas-loss and sublimation processes in this enigmatic object. <BR
/> Methods: We observed four transits of K2-22 b with X-shooter on
the Very Large Telescope operated by ESO to obtain time series of
intermediate-resolution (R 11 400) spectra. Our analysis focussed on
the two sodium D lines (588.995 and 589.592 nm) and the Ca<SUP>+</SUP>
triplet (849.802, 854.209, and 866.214 nm). The stellar calcium and
sodium absorption was removed using the out-of-transit spectra. We
searched for planet-related absorption in the velocity rest frame of the
planet, which changes from approximately -66 to +66 km s<SUP>-1</SUP>
during the transit. <BR /> Results: Since K2-22 b exhibits highly
variable transit depths, we analysed the individual nights and their
average. By injecting signals we reached 5σ upper limits on the
individual nights that range from 11-13% and 1.7-2.0% for the sodium
and ionized calcium absorption of the tail, respectively. Night 1 was
contaminated by its companion star so we considered weighted averages
with and without Night 1 and quote conservative 5σ limits without
Night 1 of 9 and 1.4%, respectively. Assuming their mass fractions to
be similar to those in the Earth's crust, these limits correspond to
scenarios in which 0.04 and 35% of the transiting dust is sublimated and
observed as absorbing gas. However, this assumes the gas to be co-moving
with the planet. We show that for the high irradiation environment of
K2-22 b, sodium and ionized calcium could be quickly accelerated to 100s
of km s<SUP>-1</SUP> owing to radiation pressure and entrainment by the
stellar wind, making these species much more difficult to detect. No
evidence for such possibly broad and blue-shifted signals are seen in
our data. <BR /> Conclusions: Future observations aimed at observing
circumplanetary gas should take into account the possible broad and
blue-shifted velocity field of atomic and ionized species.
---------------------------------------------------------
Title: Polarimetric imaging of circumstellar disks. I. Artifacts
due to limited angular resolution
Authors: Heikamp, S.; Keller, C. U.
2019A&A...627A.156H Altcode:
Context. Polarimetric images of circumstellar environments,
even when corrected with adaptive optics, have a limited angular
resolution. Finite resolution greatly affects polarimetric images
because of the canceling of adjacent polarization signals with opposite
signs. In radio astronomy this effect is called beam depolarization and
is well known. However, radio techniques to mitigate beam depolarization
are not directly applicable to optical images as a consequence of
the inherent lack of phase information at optical wavelengths. <BR />
Aims: We explore the effects of a finite point-spread function (PSF) on
polarimetric images and the application of Richardson-Lucy deconvolution
to polarimetric images. <BR /> Methods: We simulated polarimetric
images of highly simplified, circumstellar disk models and convolved
these with simulated and actual SPHERE/ZIMPOL PSFs. We attempted
to deconvolve simulated images in orthogonal linear polarizations
and polarized intensity images. <BR /> Results: The most significant
effect of finite angular resolution is the loss of polarimetric signal
close to the central star where large polarization signals of opposite
signs average out. The finite angular resolution can also introduce
polarized light in areas beyond the original, polarized signal such
as outside of disks. These effects are particularly severe for disks
that are not rotationally symmetric. The deconvolution of polarimetric
images is far from trivial. Richardson-Lucy deconvolution applied to
images in opposite linear polarization states, which are subsequently
subtracted from each other, cannot recover the signal close to the
star. Sources that lack rotational symmetry cannot be recovered with
this deconvolution approach.
---------------------------------------------------------
Title: Two accreting protoplanets around the young star PDS 70
Authors: Haffert, S. Y.; Bohn, A. J.; de Boer, J.; Snellen, I. A. G.;
Brinchmann, J.; Girard, J. H.; Keller, C. U.; Bacon, R.
2019NatAs...3..749H Altcode: 2019NatAs.tmp..329H; 2019arXiv190601486H
Newly forming protoplanets are expected to create cavities and
substructures in young, gas-rich protoplanetary disks<SUP>1-3</SUP>,
but they are difficult to detect as they could be confused
with disk features affected by advanced image analysis
techniques<SUP>4,5</SUP>. Recently, a planet was discovered
inside the gap of the transitional disk of the T Tauri star PDS
70<SUP>6,7</SUP>. Here, we report on the detection of strong Hα
emission from two distinct locations in the PDS 70 system, one
corresponding to the previously discovered planet PDS 70 b, which
confirms the earlier Hα detection<SUP>8</SUP>, and another located
close to the outer edge of the gap, coinciding with a previously
identified bright dust spot in the disk and with a small opening
in a ring of molecular emission<SUP>6,7,9</SUP>. We identify this
second Hα peak as a second protoplanet in the PDS 70 system. The Hα
emission spectra of both protoplanets indicate ongoing accretion onto
the protoplanets<SUP>10,11</SUP>, which appear to be near a 2:1 mean
motion resonance. Our observations show that adaptive-optics-assisted,
medium-resolution integral field spectroscopy with MUSE<SUP>12</SUP>
targeting accretion signatures will be a powerful way to trace ongoing
planet formation in transitional disks at different stages of their
evolution. Finding more young planetary systems in mean motion resonance
would give credibility to the Grand Tack hypothesis in which Jupiter
and Saturn migrated in a resonance orbit during the early formation
period of our Solar System<SUP>13</SUP>.
---------------------------------------------------------
Title: Discovery of a directly imaged disk in scattered light around
the Sco-Cen member Wray 15-788
Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Benisty, M.;
de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Muro-Arena,
G. A.; Pecaut, M. J.; Snik, F.; Wolff, S. G.; Reggiani, M.
2019A&A...624A..87B Altcode: 2019arXiv190207723B
Context. Protoplanetary disks are the birth environments of planetary
systems. Therefore, the study of young, circumstellar environments
is essential in understanding the processes taking place in planet
formation and the evolution of planetary systems. <BR /> Aims:
We detect and characterize circumstellar disks and potential
companions around solar-type, pre-main sequence stars in the
Scorpius-Centaurus association (Sco-Cen). <BR /> Methods: As part
of our ongoing survey we carried out high-contrast imaging with
VLT/SPHERE/IRDIS to obtain polarized and total intensity images of
the young (11<SUB>-7</SUB><SUP>+16</SUP> Myr old) K3IV star Wray
15-788 within the Lower Centaurus Crux subgroup of Sco-Cen. For the
total intensity images, we remove the stellar halo via an approach
based on reference star differential imaging in combination with
principal component analysis. <BR /> Results: Both total intensity
and polarimetric data resolve a disk around the young, solar-like
Sco-Cen member Wray 15-788. Modeling of the stellar spectral energy
distribution suggests that this is a protoplanetary disk at a transition
stage. We detect a bright outer ring at a projected separation of 370
mas (≈56 au), hints of inner substructures at 170 mas (≈28 au),
and a gap in between. Within a position angle range of only 60° <
ϕ < 240°, we are confident at the 5σ level that we detect actual
scattered light flux from the outer ring of the disk; the remaining
part is indistinguishable from background noise. For the detected part
of the outer ring we determine a disk inclination of i = 21° ± 6°
and a position angle of φ = 76° ± 16°. Furthermore, we find that
Wray 15-788 is part of a binary system with the A2V star HD 98363 at a
separation of 50” (≈6900 au). <BR /> Conclusions: The detection of
only half of the outer ring might be due to shadowing by a misaligned
inner disk. A potential substellar companion can cause the misalignment
of the inner structures and can be responsible for clearing the
detected gap from scattering material. However, we cannot rule out the
possibility of a non-detection due to our limited signal-to-noise ratio
(S/N), combined with brightness azimuthal asymmetry. From our data we
can exclude companions more massive than 10 M<SUB>jup</SUB> within the
gap at a separation of 230 mas (≈35 au). Additional data are required
to characterize the disk's peculiar morphology and to set tighter
constraints on the potential perturber's orbital parameters and mass. <P
/>Reduced images are only available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A87">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A87</A>Based
on observations collected at the European Organisation for Astronomical
Research in the Southern Hemisphere under ESO programs 099.C-0698(A),
0101.C-0153(A), and 0101.C-0464(A).
---------------------------------------------------------
Title: VizieR Online Data Catalog: Discovery of a resolved disk
around Wray 15-788 (Bohn+, 2019)
Authors: Bohn, A. J.; Kenworthy, M. A.; Ginski, C.; Benisty, M.;
de Boer, J.; Keller, C. U.; Mamajek, E. E.; Meshkat, T.; Muro-Arena,
G. A.; Pecaut, M. J.; Snik, F.; Wolff, S. G.; Reggiani, M.
2019yCat..36240087B Altcode:
Fits images corresponding to the imagery presented in the paper. The
data are obtained with VLT/SPHERE/IRDIS. The data reduction is performed
with the latest version of PynPoint. In both total and polarized
intensity we resolve a disk around the K-type pre-main sequence star
Wray 15-788. SED analysis suggests that this is a protoplanetary disk
at a transition stage. <P />(4 data files).
---------------------------------------------------------
Title: SPHERE/ZIMPOL high resolution polarimetric imager. I. System
overview, PSF parameters, coronagraphy, and polarimetry
Authors: Schmid, H. M.; Bazzon, A.; Roelfsema, R.; Mouillet, D.;
Milli, J.; Menard, F.; Gisler, D.; Hunziker, S.; Pragt, J.; Dominik,
C.; Boccaletti, A.; Ginski, C.; Abe, L.; Antoniucci, S.; Avenhaus,
H.; Baruffolo, A.; Baudoz, P.; Beuzit, J. L.; Carbillet, M.; Chauvin,
G.; Claudi, R.; Costille, A.; Daban, J. -B.; de Haan, M.; Desidera,
S.; Dohlen, K.; Downing, M.; Elswijk, E.; Engler, N.; Feldt, M.;
Fusco, T.; Girard, J. H.; Gratton, R.; Hanenburg, H.; Henning,
Th.; Hubin, N.; Joos, F.; Kasper, M.; Keller, C. U.; Langlois, M.;
Lagadec, E.; Martinez, P.; Mulder, E.; Pavlov, A.; Podio, L.; Puget,
P.; Quanz, S. P.; Rigal, F.; Salasnich, B.; Sauvage, J. -F.; Schuil,
M.; Siebenmorgen, R.; Sissa, E.; Snik, F.; Suarez, M.; Thalmann, Ch.;
Turatto, M.; Udry, S.; van Duin, A.; van Holstein, R. G.; Vigan, A.;
Wildi, F.
2018A&A...619A...9S Altcode: 2018arXiv180805008S
Context. The SPHERE "planet finder" is an extreme adaptive optics (AO)
instrument for high resolution and high contrast observations at the
Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter
(ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the
limits of current AO systems to shorter wavelengths, higher spatial
resolution, and much improved polarimetric performance. <BR /> Aims:
We present a detailed characterization of SPHERE/ZIMPOL which should
be useful for an optimal planning of observations and for improving
the data reduction and calibration. We aim to provide new benchmarks
for the performance of high contrast instruments, in particular for
polarimetric differential imaging. <BR /> Methods: We have analyzed
SPHERE/ZIMPOL point spread functions (PSFs) and measure the normalized
peak surface brightness, the encircled energy, and the full width half
maximum (FWHM) for different wavelengths, atmospheric conditions, star
brightness, and instrument modes. Coronagraphic images are described
and the peak flux attenuation and the off-axis flux transmission are
determined. Simultaneous images of the coronagraphic focal plane and the
pupil plane are analyzed and the suppression of the diffraction rings
by the pupil stop is investigated. We compared the performance at small
separation for different coronagraphs with tests for the binary α Hyi
with a separation of 92 mas and a contrast of Δm ≈ 6<SUP>m</SUP>. For
the polarimetric mode we made the instrument calibrations using zero
polarization and high polarization standard stars and here we give a
recipe for the absolute calibration of polarimetric data. The data
show small (< 1 mas) but disturbing differential polarimetric
beam shifts, which can be explained as Goos-Hähnchen shifts from
the inclined mirrors, and we discuss how to correct this effect. The
polarimetric sensitivity is investigated with non-coronagraphic and
deep, coronagraphic observations of the dust scattering around the
symbiotic Mira variable R Aqr. <BR /> Results: SPHERE/ZIMPOL reaches
routinely an angular resolution (FWHM) of 22-28 mas, and a normalized
peak surface brightness of SB<SUB>0</SUB> - m<SUB>star</SUB> ≈
-6.5<SUP>m</SUP> arcsec<SUP>-2</SUP> for the V-, R- and I-band. The
AO performance is worse for mediocre ≳1.0″ seeing conditions,
faint stars m<SUB>R</SUB> ≳ 9<SUP>m</SUP>, or in the presence of the
"low wind" effect (telescope seeing). The coronagraphs are effective in
attenuating the PSF peak by factors of > 100, and the suppression
of the diffracted light improves the contrast performance by a factor
of approximately two in the separation range 0.06″-0.20″. The
polarimetric sensitivity is Δp < 0.01% and the polarization zero
point can be calibrated to better than Δp ≈ 0.1%. The contrast limits
for differential polarimetric imaging for the 400 s I-band data of R
Aqr at a separation of ρ = 0.86″ are for the surface brightness
contrast SB<SUB>pol</SUB>( ρ)-m<SUB>star</SUB> ≈ 8<SUP>m</SUP>
arcsec<SUP>-2</SUP> and for the point source contrast m<SUB>pol</SUB>(
ρ)-m<SUB>star</SUB> ≈ 15<SUP>m</SUP> and much lower limits are
achievable with deeper observations. <BR /> Conclusions: SPHERE/ZIMPOL
achieves imaging performances in the visual range with unprecedented
characteristics, in particular very high spatial resolution and very
high polarimetric contrast. This instrument opens up many new research
opportunities for the detailed investigation of circumstellar dust,
in scattered and therefore polarized light, for the investigation of
faint companions, and for the mapping of circumstellar Hα emission. <P
/>Based on observations collected at La Silla and Paranal Observatory,
ESO (Chile), Program ID: 60.A-9249 and 60.A-9255.
---------------------------------------------------------
Title: Chromatic transit light curves of disintegrating rocky planets
Authors: Ridden-Harper, A. R.; Keller, C. U.; Min, M.; van Lieshout,
R.; Snellen, I. A. G.
2018A&A...618A..97R Altcode: 2018arXiv180707973R
Context. Kepler observations have revealed a class of short-period
exoplanets, of which Kepler-1520 b is the prototype, which have
comet-like dust tails thought to be the result of small, rocky planets
losing mass. The shape and chromaticity of the transits constrain the
properties of the dust particles originating from the planet's surface,
offering a unique opportunity to probe the composition and geophysics
of rocky exoplanets. <BR /> Aims: We aim to approximate the average
Kepler long-cadence light curve of Kepler-1520 b and investigate how
the optical thickness and transit cross section of a general dust tail
can affect the observed wavelength dependence and depth of transit
light curves. <BR /> Methods: We developed a new 3D model that ejects
sublimating particles from the planet surface to build up a dust tail,
assuming it to be optically thin, and used 3D radiative transfer
computations that fully treat scattering using the distribution of
hollow spheres (DHS) method, to generate transit light curves between
0.45 and 2.5 μm. <BR /> Results: We show that the transit depth is
wavelength independent of optically thick tails, potentially explaining
why only some observations indicate a wavelength dependence. From the 3D
nature of our simulated tails, we show that their transit cross sections
are related to the component of particle ejection velocity perpendicular
to the planets orbital plane and use this to derive a minimum ejection
velocity of 1.2 km s<SUP>-1</SUP>. To fit the average transit depth
of Kepler-1520 b of 0.87%, we require a high dust mass-loss rate of
7-80 M<SUB>⊕</SUB> Gyr<SUP>-1</SUP> which implies planet lifetimes
that may be inconsistent with the observed sample. Therefore, these
mass loss rates should be considered to be upper limits.
---------------------------------------------------------
Title: Review of high-contrast imaging systems for current and
future ground- and space-based telescopes I: coronagraph design
methods and optical performance metrics
Authors: Ruane, G.; Riggs, A.; Mazoyer, J.; Por, E. H.; N'Diaye,
M.; Huby, E.; Baudoz, P.; Galicher, R.; Douglas, E.; Knight, J.;
Carlomagno, B.; Fogarty, K.; Pueyo, L.; Zimmerman, N.; Absil, O.;
Beaulieu, M.; Cady, E.; Carlotti, A.; Doelman, D.; Guyon, O.; Haffert,
S.; Jewell, J.; Jovanovic, N.; Keller, C.; Kenworthy, M. A.; Kuhn, J.;
Miller, K.; Sirbu, D.; Snik, F.; Wallace, J. Kent; Wilby, M.; Ygouf, M.
2018SPIE10698E..2SR Altcode: 2018arXiv180707042R
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center
in September 2017 in Leiden, the Netherlands gathered a diverse group
of 25 researchers working on exoplanet instrumentation to stimulate
the emergence and sharing of new ideas. In this first installment of a
series of three papers summarizing the outcomes of the OOC workshop, we
present an overview of design methods and optical performance metrics
developed for coronagraph instruments. The design and optimization
of coronagraphs for future telescopes has progressed rapidly over the
past several years in the context of space mission studies for Exo-C,
WFIRST, HabEx, and LUVOIR as well as ground-based telescopes. Design
tools have been developed at several institutions to optimize a variety
of coronagraph mask types. We aim to give a broad overview of the
approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function
of coronagraphs is to suppress starlight while maintaining light
from off-axis sources, our community lacks a general set of standard
performance metrics that apply to both detecting and characterizing
exoplanets. The attendees of the OOC workshop agreed that it would
benefit our community to clearly define quantities for comparing the
performance of coronagraph designs and systems. Therefore, we also
present a set of metrics that may be applied to theoretical designs,
testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument
to the detection significance of the given point source in the presence
of realistic noise.
---------------------------------------------------------
Title: First direct detection of a polarized companion outside a
resolved circumbinary disk around CS Chamaeleonis
Authors: Ginski, C.; Benisty, M.; van Holstein, R. G.; Juhász,
A.; Schmidt, T. O. B.; Chauvin, G.; de Boer, J.; Wilby, M.; Manara,
C. F.; Delorme, P.; Ménard, F.; Pinilla, P.; Birnstiel, T.; Flock,
M.; Keller, C.; Kenworthy, M.; Milli, J.; Olofsson, J.; Pérez, L.;
Snik, F.; Vogt, N.
2018A&A...616A..79G Altcode: 2018arXiv180502261G
<BR /> Aims: To understand planet formation it is necessary to study
the birth environment of planetary systems. Resolved imaging of young
planet forming disks allows us to study this environment in great detail
and find signs of planet-disk interaction and disk evolution. In the
present study we aim to investigate the circumstellar environment of
the spectroscopic binary T Tauri star CS Cha. From unresolved mid-to
far-infrared photometry it is predicted that CS Cha hosts a disk with
a large cavity. In addition, spectral energy distribution modeling
suggests significant dust settling, pointing toward an evolved disk
that may show signs of ongoing or completed planet formation. <BR />
Methods: We observed CS Cha with the high contrast imager SPHERE at the
ESO Very Large Telescope (VLT) in polarimetric differential imaging mode
to resolve the circumbinary disk in near-infrared scattered light. These
observations were followed up by VLT/NACO L-band observations and
complemented by archival VLT/NACO K-band and Hubble Space Telescope
WFPC2 I-band data. <BR /> Results: We resolve the compact circumbinary
disk around CS Cha for the first time in scattered light. We find a
smooth, low inclination disk with an outer radius of 55 au (at 165
pc). We do not detect the inner cavity but find an upper limit for the
cavity size of 15 au. Furthermore, we find a faint comoving companion
with a projected separation of 210 au from the central binary outside
of the circumbinary disk. The companion is detected in polarized light
and shows an extreme degree of polarization (13.7 ± 0.4% in the J
band). The J- and H-band magnitudes of the companion are compatible
with masses of a few M<SUB>Jup</SUB>. However, K-, L-, and I-band
data draw this conclusion into question. We explore with radiative
transfer modeling whether an unresolved circum-companion disk can be
responsible for the high polarization and complex photometry. We find
that the set of observations is best explained by a heavily extincted
low-mass ( 20 M<SUB>Jup</SUB>) brown dwarf or high-mass planet with an
unresolved disk and dust envelope. <P />Based on observations performed
with VLT/SPHERE under program ID 098.C-0760(B) and 099.C-0891(B)
and VLT/NACO under program ID 298.C-5054(B) and 076.C-0292(A)The
reduced images are only available at the CDS via anonymous ftp to
<A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/616/A79">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/616/A79</A>
---------------------------------------------------------
Title: High contrast imaging for the enhanced resolution imager and
spectrometer (ERIS)
Authors: Kenworthy, Matthew A.; Snik, Frans; Keller, Christoph U.;
Doelman, David; Por, Emiel H.; Absil, Olivier; Carlomagno, Brunella;
Karlsson, Mikael; Huby, Elsa; Glauser, Adrian M.; Quanz, Sascha P.;
Taylor, William D.
2018SPIE10702E..46K Altcode: 2020arXiv201201963K
ERIS is a diffraction limited thermal infrared imager and spectrograph
for the Very Large Telescope UT4. One of the science cases for ERIS
is the detection and characterization of circumstellar structures and
exoplanets around bright stars that are typically much fainter than the
stellar diffraction halo. Enhanced sensitivity is provided through the
combination of (i) suppression of the diffraction halo of the target
star using coronagraphs, and (ii) removal of any residual diffraction
structure through focal plane wavefront sensing and subsequent active
correction. In this paper we present the two coronagraphs used for
diffraction suppression and enabling high contrast imaging in ERIS.
---------------------------------------------------------
Title: On-sky results of the Leiden EXoplanet Instrument (LEXI)
Authors: Haffert, S. Y.; Wilby, M. J.; Keller, C. U.; Snellen,
I. A. G.; Doelman, D. S.; Por, E. H.; van Kooten, M.; Bos, S. P.;
Wardenier, J.
2018SPIE10703E..23H Altcode:
The Leiden EXoplanet Instrument (LEXI) is a bench-mounted
high-contrast spectrograph (HCS) and high- contrast imager(HCI). Both
science instruments are mounted behind a common adaptive optics (AO)
system. The AO can be controlled by several new wavefront sensors for
which we will show the first on-sky results. There is a new pupil-plane
wavefront sensors; the Generalized Optical Differentiation Wavefront
sensor(g-ODWFS). LEXI can switch between two observing modes, the
HCI mode or the HD-IFS mode. The spectrograph is very compact because
it is fed by single-mode fiber. The HD-IFS is an IFS that covers the
spectral range of 600 800 nm with a constant spectral resolving power
of 96000. The 2kx3k detector makes it possible to deliver diffraction
limited spectra of up to 20 input fibers. The high-dispersion spectra
of the HD-IFS allow for robust post-processing technique to remove
residual stellar speckles and allows for direct characterization of
the faint stellar environment. We will show the first sucesful on-sky
results of the injection into a single-mode fiber with LEXI. In HCI
mode an Apodizing Phase Plate (APP) is used to create a dark region
around the star with an average design contrast of 1E-4. The APP is
multiplexed with holographic modes to create the Coronagraphic Modal
Wavefront sensor (cMWS) for non-common path error (NCPE) correction. The
cMWS creates holographic copies in the focal plane that react linearly
to aberrations. The holographic copies are measured simultaneously with
the science target. There is no downtime for NCPE correction. We will
show the first on-sky closed-loop correction of (NCPEs) with the cMWS.
---------------------------------------------------------
Title: Optimization of contrast in adaptive optics for exoplanet
imaging
Authors: Radhakrishnan, Vikram Mark; Keller, Christoph U.; Doelman,
Niek
2018SPIE10703E..4NR Altcode:
Direct imaging of exoplanets requires overcoming the enormous contrast
between the exoplanet and its host star, to distinguish the reflected
light from the exoplanet, from the diffracted light of the star. Direct
optimization of contrast, using nonlinear optimization techniques
of the Adaptive Optics (AO) system coupled with coronagraphy, shows
significant promise in achieving high contrast, beyond the limits of
what can be achieved with traditional AO systems. Using a coronagraph
optic as a "static" phase modifying element, and a deformable mirror
as a "dynamic" element, we create an adaptive coronagraph, capable of
engineering the point spread function (PSF) of the imaging system,
to create a deep, dark hole in the focal plane, within which the
exoplanet can be imaged. We present the results of simulations of a
system, consisting of a vector Apodizing Phase Plate (vAPP) coronagraph,
a deformable mirror (DM), and an imaging camera. The vAPP coronagraph
reroutes starlight within the pupil plane, to create a designated dark
hole region, which in the ideal case would be devoid of starlight
in the focal plane. Off-axis exoplanet light is transmitted through
to the dark hole and hence can be imaged. Atmospheric turbulence is
simulated to generate a distorted wavefront, and a nonlinear, gradient
climbing based optimization algorithm is implemented to drive the DM
to optimize a merit function. This merit function is chosen with a
dual objective to maximize average raw contrast in the dark hole,
while maintaining a sufficiently high Strehl ratio. Preliminary
results show that in a setup with a coronagraph designed to create a
6×6 (λ/D)<SUP>2</SUP> rectangular dark hole with a raw contrast of
10<SUP>-5</SUP>, the optimization procedure results in a raw contrast
of 10<SUP>-7</SUP> at the dark hole while maintaining a Strehl ratio
above 40%. It is observed that by tweaking the merit function, this
non-linear optimization procedure can be adjusted to result in either
higher Strehl or higher contrast. We discuss potential strategies to
extend the non-linear optimization techniques to real-time, non-linear
control for the AO system, thereby achieving a real-time, dynamic,
adaptive coronagraph. Toward this end, we investigate the results
of using the fast wavefront sensor data to reconstruct the wavefront
phase, virtually propagate this through the science optical path, and
optimize contrast on this virtual science image, as opposed to using
the slower science camera to optimize contrast on the true science
image. One potential approach to implement true real-time control would
be to use deep neural networks, trained using deep deterministic policy
gradients, to identify and remove speckles of diffracted starlight in
the dark hole region in real-time.
---------------------------------------------------------
Title: Review of high-contrast imaging systems for current and
future ground-based and space-based telescopes: Part II. Common path
wavefront sensing/control and coherent differential imaging
Authors: Jovanovic, Nemanja; Absil, Olivier; Baudoz, Pierre; Beaulieu,
Mathilde; Bottom, Michael; Cady, Eric; Carlomagno, Brunella; Carlotti,
Alexis; Doelman, David; Fogarty, Kevin; Galicher, Raphaël.; Guyon,
Olivier; Haffert, Sebastiaan; Huby, Elsa; Jewell, Jeffrey; Keller,
Christoph; Kenworthy, Matthew A.; Knight, Justin; Kühn, Jonas; Miller,
Kelsey; Mazoyer, Johan; N'Diaye, Mamadou; Por, Emiel; Pueyo, Laurent;
Riggs, A. J. E.; Ruane, Garreth; Sirbu, Dan; Snik, Frans; Wallace,
J. K.; Wilby, Michael; Ygouf, Marie
2018SPIE10703E..1UJ Altcode: 2018arXiv180707043J
The Optimal Optical Coronagraph (OOC) Workshop held at the Lorentz
Center in September 2017 in Leiden, the Netherlands, gathered a
diverse group of 25 researchers working on exoplanet instrumentation
to stimulate the emergence and sharing of new ideas. In this second
installment of a series of three papers summarizing the outcomes of
the OOC workshop, we present an overview of common path wavefront
sensing/control and Coherent Differential Imaging techniques,
highlight the latest results, and expose their relative strengths
and weaknesses. We layout critical milestones for the field with the
aim of enhancing future ground/space based high contrast imaging
platforms. Techniques like these will help to bridge the daunting
contrast gap required to image a terrestrial planet in the zone where
it can retain liquid water, in reflected light around a G type star
from space.
---------------------------------------------------------
Title: Laboratory verification of Fast & Furious phase diversity:
Towards controlling the low wind effect in the SPHERE instrument
Authors: Wilby, M. J.; Keller, C. U.; Sauvage, J. -F.; Dohlen, K.;
Fusco, T.; Mouillet, D.; Beuzit, J. -L.
2018A&A...615A..34W Altcode: 2018arXiv180303258W
Context. The low wind effect (LWE) refers to a characteristic set
of quasi-static wavefront aberrations seen consistently by the SPHERE
instrument when dome-level wind speeds drop below 3 ms<SUP>-1</SUP>. The
LWE produces bright low-order speckles in the stellar point-spread
function (PSF), which severely limit the contrast performance of SPHERE
under otherwise optimal observing conditions. <BR /> Aims: In this paper
we propose the Fast & Furious (F&F) phase diversity algorithm
as a viable software-only solution for real-time LWE compensation,
which would utilise image sequences from the SPHERE differential
tip-tilt sensor (DTTS) and apply corrections via reference slope
offsets on the AO system's Shack-Hartmann wavefront sensor. <BR />
Methods: We evaluated the closed-loop performance of F&F on the
MITHIC high-contrast test-bench, under conditions emulating LWE-affected
DTTS images. These results were contrasted with predictive simulations
for a variety of convergence tests, in order to assess the expected
performance of an on-sky implementation of F&F in SPHERE. <BR />
Results: The algorithm was found to be capable of returning LWE-affected
images to Strehl ratios of greater than 90% within five iterations,
for all appropriate laboratory test cases. These results are highly
representative of predictive simulations, and demonstrate stability
of the algorithm against a wide range of factors including low image
signal-to-noise ratio (S/N), small image field of view, and amplitude
errors. It was also found in simulation that closed-loop stability can
be preserved down to image S/N as low as five while still improving
overall wavefront quality, allowing for reliable operation even on
faint targets. <BR /> Conclusions: The Fast & Furious algorithm is
an extremely promising solution for real-time compensation of the LWE,
which can operate simultaneously with science observations and may
be implemented in SPHERE without requiring additional hardware. The
robustness and relatively large effective dynamic range of F&F
also make it suitable for general wavefront optimisation applications,
including the co-phasing of segmented ELT-class telescopes.
---------------------------------------------------------
Title: MagAO-X: project status and first laboratory results
Authors: Males, Jared R.; Close, Laird M.; Miller, Kelsey; Schatz,
Lauren; Doelman, David; Lumbres, Jennifer; Snik, Frans; Rodack, Alex;
Knight, Justin; Van Gorkom, Kyle; Long, Joseph D.; Hedglen, Alex;
Kautz, Maggie; Jovanovic, Nemanja; Morzinski, Katie; Guyon, Olivier;
Douglas, Ewan; Follette, Katherine B.; Lozi, Julien; Bohlman, Chris;
Durney, Olivier; Gasho, Victor; Hinz, Phil; Ireland, Michael; Jean,
Madison; Keller, Christoph; Kenworthy, Matt; Mazin, Ben; Noenickx,
Jamison; Alfred, Dan; Perez, Kevin; Sanchez, Anna; Sauve, Corwynn;
Weinberger, Alycia; Conrad, Al
2018SPIE10703E..09M Altcode: 2018arXiv180704315M
MagAO-X is an entirely new extreme adaptive optics system for the
Magellan Clay 6.5 m telescope, funded by the NSF MRI program starting
in Sep 2016. The key science goal of MagAO-X is high-contrast imaging
of accreting protoplanets at Hα. With 2040 actuators operating at
up to 3630 Hz, MagAO-X will deliver high Strehls (> 70%), high
resolution (19 mas), and high contrast (< 1 × 10<SUP>-4 </SUP>)
at Hα (656 nm). We present an overview of the MagAO-X system, review
the system design, and discuss the current project status.
---------------------------------------------------------
Title: ERIS: revitalising an adaptive optics instrument for the VLT
Authors: Davies, R.; Esposito, S.; Schmid, H. -M.; Taylor, W.; Agapito,
G.; Agudo Berbel, A.; Baruffolo, A.; Biliotti, V.; Biller, B.; Black,
M.; Boehle, A.; Briguglio, B.; Buron, A.; Carbonaro, L.; Cortes,
A.; Cresci, G.; Deysenroth, M.; Di Cianno, A.; Di Rico, G.; Doelman,
D.; Dolci, M.; Dorn, R.; Eisenhauer, F.; Fantinel, D.; Ferruzzi, D.;
Feuchtgruber, H.; Förster Schreiber, N.; Gao, X.; Gemperlein, H.;
Genzel, R.; George, E.; Gillessen, S.; Giordano, C.; Glauser, A.;
Glindemann, A.; Grani, P.; Hartl, M.; Heijmans, J.; Henry, D.; Huber,
H.; Kasper, M.; Keller, C.; Kenworthy, M.; Kühn, J.; Kuntschner,
H.; Lightfoot, J.; Lunney, D.; MacIntosh, M.; Mannucci, F.; March,
S.; Neeser, M.; Patapis, P.; Pearson, D.; Plattner, M.; Puglisi, A.;
Quanz, S.; Rau, C.; Riccardi, A.; Salasnich, B.; Schubert, J.; Snik,
F.; Sturm, E.; Valentini, A.; Waring, C.; Wiezorrek, E.; Xompero, M.
2018SPIE10702E..09D Altcode: 2018arXiv180705089D
ERIS is an instrument that will both extend and enhance the fundamental
diffraction limited imaging and spectroscopy capability for the VLT. It
will replace two instruments that are now being maintained beyond their
operational lifetimes, combine their functionality on a single focus,
provide a new wavefront sensing module that makes use of the facility
Adaptive Optics System, and considerably improve their performance. The
instrument will be competitive with respect to JWST in several regimes,
and has outstanding potential for studies of the Galactic Center,
exoplanets, and high redshift galaxies. ERIS had its final design
review in 2017, and is expected to be on sky in 2020. This contribution
describes the instrument concept, outlines its expected performance,
and highlights where it will most excel.
---------------------------------------------------------
Title: Review of high-contrast imaging systems for current and future
ground-based and space-based telescopes III: technology opportunities
and pathways
Authors: Snik, Frans; Absil, Olivier; Baudoz, Pierre; Beaulieu,
Mathilde; Bendek, Eduardo; Cady, Eric; Carlomagno, Brunella; Carlotti,
Alexis; Cvetojevic, Nick; Doelman, David; Fogarty, Kevin; Galicher,
Raphaël.; Guyon, Olivier; Haffert, Sebastiaan; Huby, Elsa; Jewell,
Jeffrey; Jovanovic, Nemanja; Keller, Christoph; Kenworthy, Matthew
A.; Knight, Justin; Kuhn, Jonas; Mazoyer, Johan; Miller, Kelsey;
N'Diaye, Mamadou; Norris, Barnaby; Por, Emiel; Pueyo, Laurent; Riggs,
A. J. Eldorado; Ruane, Garreth; Sirbu, Dan; Wallace, J. Kent; Wilby,
Michael; Ygouf, Marie
2018SPIE10706E..2LS Altcode: 2018arXiv180707100S
The Optimal Optical CoronagraphWorkshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of
30 researchers working on exoplanet instrumentation to stimulate the
emergence and sharing of new ideas. This contribution is the final part
of a series of three papers summarizing the outcomes of the workshop,
and presents an overview of novel optical technologies and systems that
are implemented or considered for high-contrast imaging instruments
on both ground-based and space telescopes. The overall objective
of high contrast instruments is to provide direct observations
and characterizations of exoplanets at contrast levels as extreme
as 10<SUP>-10</SUP>. We list shortcomings of current technologies,
and identify opportunities and development paths for new technologies
that enable quantum leaps in performance. Specifically, we discuss the
design and manufacturing of key components like advanced deformable
mirrors and coronagraphic optics, and their amalgamation in "adaptive
coronagraph" systems. Moreover, we discuss highly integrated system
designs that combine contrast-enhancing techniques and characterization
techniques (like high-resolution spectroscopy) while minimizing the
overall complexity. Finally, we explore extreme implementations using
all-photonics solutions for ground-based telescopes and dedicated huge
apertures for space telescopes.
---------------------------------------------------------
Title: Multiplexed holographic aperture masking with liquid-crystal
geometric phase masks
Authors: Doelman, D. S.; Tuthill, P.; Norris, B.; Wilby, M. J.; Por,
E. H.; Keller, C. U.; Escuti, M. J.; Snik, F.
2018SPIE10701E..0TD Altcode: 2018arXiv180800751D
Sparse Aperture Masking (SAM) allows for high-contrast imaging at small
inner working angles, however the performance is limited by the small
throughput and the number of baselines. We present the concept and
first lab results of Holographic Aperture Masking (HAM) with extreme
liquid-crystal geometric phase patterns. We multiplex subapertures
using holographic techniques to combine the same subaperture in multiple
non-redundant PSFs in combination with a non-interferometric reference
spot. This way arbitrary subaperture combinations and PSF configurations
can be realized, giving HAM more uv-coverage, better throughput and
improved calibration as compared to SAM, at the cost of detector space.
---------------------------------------------------------
Title: A Planet with a Disc? A Surprising Detection in Polarised
Light with VLT/SPHERE
Authors: Ginski, C.; van Holstein, R.; Juhász, A.; Benisty, M.;
Schmidt, T.; Chauvin, G.; de Boer, J.; Wilby, M.; Manara, C. F.;
Delorme, P.; Ménard, F.; Muro-Arena, G.; Pinilla, P.; Birnstiel,
T.; Flock, M.; Keller, C.; Kenworthy, M.; Milli, J.; Olofsson, J.;
Pérez, L.; Snik, F.; Vogt, N.
2018Msngr.172...27G Altcode:
With the Spectro-Polarimetric High- contrast Exoplanet REsearch (SPHERE)
instrument at ESO's Very Large Telescope (VLT) we can study the linear
polarisation of directly detected planets and brown dwarfs, to learn
about their atmospheres and immediate environments. We summarise
here the recent discovery of a low-mass companion in polarised
light by Ginski et al. (2018). The object shows an extreme degree of
polarisation, indicating the presence of a circumplanetary disc.
---------------------------------------------------------
Title: The Single-mode Complex Amplitude Refinement (SCAR)
coronagraph: II. Lab verification, and toward the characterization
of Proxima b
Authors: Haffert, S. Y.; Por, E. H.; Keller, C. U.; Kenworthy, M. A.;
Doelman, D. S.; Snik, F.; Escuti, M. J.
2018arXiv180310693H Altcode:
We present the monochromatic lab verification of the newly developed
SCAR coronagraph that combines a phase plate (PP) in the pupil with
a microlens-fed single-mode fiber array in the focal plane. The two
SCAR designs that have been measured, create respectively a 360 degree
and 180 degree dark region from 0.8-2.4 \lambda/D around the star. The
360 SCAR has been designed for a clear aperture and the 180 SCAR has
been designed for a realistic aperture with central obscuration and
spiders. The 360 SCAR creates a measured stellar null of $2-3 \times
10^{-4}$ , and the 180 SCAR reaches a null of $1 \times 10^{-4}$
. Their monochromatic contrast is maintained within a range of $\pm$
0.16 \lambda/D peak-to-valley tip-tilt, which shows the robustness
against tip-tilt errors. The small inner working angle and tip-tilt
stability makes the SCAR coronagraph a very promising technique for
an upgrade of current high-contrast instruments to characterize and
detect exoplanets in the solar neighborhood.
---------------------------------------------------------
Title: SPEX: a highly accurate spectropolarimeter for atmospheric
aerosol characterization
Authors: Rietjens, J. H. H.; Smit, J. M.; di Noia, A.; Hasekamp,
O. P.; van Harten, G.; Snik, F.; Keller, C. U.
2017SPIE10563E..44R Altcode:
Global characterization of atmospheric aerosol in terms of the
microphysical properties of the particles is essential for understanding
the role aerosols in Earth climate [1]. For more accurate predictions
of future climate the uncertainties of the net radiative forcing of
aerosols in the Earth's atmosphere must be reduced [2]. Essential
parameters that are needed as input in climate models are not only
the aerosol optical thickness (AOT), but also particle specific
properties such as the aerosol mean size, the single scattering albedo
(SSA) and the complex refractive index. The latter can be used to
discriminate between absorbing and non-absorbing aerosol types, and
between natural and anthropogenic aerosol. Classification of aerosol
types is also very important for air-quality and health-related
issues [3]. Remote sensing from an orbiting satellite platform
is the only way to globally characterize atmospheric aerosol at a
relevant timescale of 1 day [4]. One of the few methods that can be
employed for measuring the microphysical properties of aerosols is to
observe both radiance and degree of linear polarization of sunlight
scattered in the Earth atmosphere under different viewing directions
[5][6][7]. The requirement on the absolute accuracy of the degree
of linear polarization P<SUB>L</SUB> is very stringent: the absolute
error in P<SUB>L</SUB> must be smaller then 0.001+0.005.P<SUB>L</SUB>
in order to retrieve aerosol parameters with sufficient accuracy to
advance climate modelling and to enable discrimination of aerosol types
based on their refractive index for air-quality studies [6][7]. In
this paper we present the SPEX instrument, which is a multi-angle
spectropolarimeter that can comply with the polarimetric accuracy needed
for characterizing aerosols in the Earth's atmosphere. We describe
the implementation of spectral polarization modulation in a prototype
instrument of SPEX and show results of ground based measurements from
which aerosol microphysical properties are retrieved.
---------------------------------------------------------
Title: The Spectropolarimeter for Planetary Exploration: SPEX
Authors: Laan, Erik; Stam, Daphne; Snik, Frans; Karalidi, Theodora;
Keller, Christoph; ter Horst, Rik; Navarro, Ramon; Oomen, Gijsbert;
de Vries, Johan; Hoogeveen, Ruud
2017SPIE10566E..2GL Altcode:
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative,
compact remotesensing instrument for measuring and characterizing
aerosols in the atmosphere. The shoebox size instrument is capable of
accurate full linear spectropolarimetry without moving parts or liquid
crystals. High precision polarimetry is performed through encoding
the degree and angle of linear polarization of the incoming light
in a sinusoidal modulation of the spectrum. Measuring this intensity
spectrum thus provides the spectral dependence of the degree and angle
of linear polarization. Polarimetry has proven to be an excellent
tool to study microphysical properties of atmospheric particles. Such
information is essential to better understand the weather and climate of
a planet. Although SPEX can be used to study any planetary atmosphere,
including the Earth's, the current design of SPEX is tailored to study
Martian dust and clouds from an orbiting platform. SPEX' 9 entrance
pupils can simultaneously measure intensity spectra from 0.4 to 0.8
microns, in different directions along the flight direction (including
two limb viewing directions). This way, the scattering phase functions
of dust and cloud particles within a ground pixel are sampled while
flying over it. SPEX can provide synergy with instruments on rovers and
landers, as it provides an overview of spatial and temporal variations
of the Martian atmosphere.
---------------------------------------------------------
Title: SPEX: the Spectropolarimeter for Planetary Exploration
Authors: Rietjens, J. H. H.; Snik, F.; Stam, D. M.; Smit, J. M.; van
Harten, G.; Keller, C. U.; Verlaan, A. L.; Laan, E. C.; ter Horst,
R.; Navarro, R.; Wielinga, K.; Moon, S. G.; Voors, R.
2017SPIE10565E..1CR Altcode:
We present SPEX, the Spectropolarimeter for Planetary Exploration,
which is a compact, robust and low-mass spectropolarimeter designed
to operate from an orbiting or in situ platform. Its purpose is to
simultaneously measure the radiance and the state (degree and angle)
of linear polarization of sunlight that has been scattered in a
planetary atmosphere and/or reflected by a planetary surface with high
accuracy. The degree of linear polarization is extremely sensitive to
the microphysical properties of atmospheric or surface particles (such
as size, shape, and composition), and to the vertical distribution
of atmospheric particles, such as cloud top altitudes. Measurements
as those performed by SPEX are therefore crucial and often the only
tool for disentangling the many parameters that describe planetary
atmospheres and surfaces. SPEX uses a novel, passive method for its
radiance and polarization observations that is based on a carefully
selected combination of polarization optics. This method, called
spectral modulation, is the modulation of the radiance spectrum in both
amplitude and phase by the degree and angle of linear polarization,
respectively. The polarization optics consists of an achromatic
quarter-wave retarder, an athermal multiple-order retarder, and a
polarizing beam splitter. We will show first results obtained with
the recently developed prototype of the SPEX instrument, and present
a performance analysis based on a dedicated vector radiative transport
model together with a recently developed SPEX instrument simulator.
---------------------------------------------------------
Title: Polarization modeling and predictions for DKIST part 2:
application of the Berreman calculus to spectral polarization fringes
of beamsplitters and crystal retarders
Authors: Harrington, David M.; Snik, Frans; Keller, Christoph U.;
Sueoka, Stacey R.; van Harten, Gerard
2017JATIS...3d8001H Altcode: 2017arXiv171006067H
We outline polarization fringe predictions derived from an application
of the Berreman calculus for the Daniel K. Inouye Solar Telescope
(DKIST) retarder optics. The DKIST retarder baseline design used six
crystals, single-layer antireflection coatings, thick cover windows,
and oil between all optical interfaces. This tool estimates polarization
fringes and optic Mueller matrices as functions of all optical design
choices. The amplitude and period of polarized fringes under design
changes, manufacturing errors, tolerances, and several physical factors
can now be estimated. This tool compares well with observations of
fringes for data collected with the spectropolarimeter for infrared
and optical regions at the Dunn Solar Telescope using bicrystalline
achromatic retarders as well as laboratory tests. With this tool, we
show impacts of design decisions on polarization fringes as impacted by
antireflection coatings, oil refractive indices, cover window presence,
and part thicknesses. This tool helped DKIST decide to remove retarder
cover windows and also recommends reconsideration of coating strategies
for DKIST. We anticipate this tool to be essential in designing future
retarders for mitigation of polarization and intensity fringe errors
in other high spectral resolution astronomical systems.
---------------------------------------------------------
Title: Combining angular differential imaging and accurate polarimetry
with SPHERE/IRDIS to characterize young giant exoplanets
Authors: van Holstein, Rob G.; Snik, Frans; Girard, Julien H.;
de Boer, Jozua; Ginski, C.; Keller, Christoph U.; Stam, Daphne M.;
Beuzit, Jean-Luc; Mouillet, David; Kasper, Markus; Langlois, Maud;
Zurlo, Alice; de Kok, Remco J.; Vigan, Arthur
2017SPIE10400E..15V Altcode: 2017arXiv170907519V
Young giant exoplanets emit infrared radiation that can be linearly
polarized up to several percent. This linear polarization can trace:
1) the presence of atmospheric cloud and haze layers, 2) spatial
structure, e.g. cloud bands and rotational flattening, 3) the spin axis
orientation and 4) particle sizes and cloud top pressure. We introduce
a novel high-contrast imaging scheme that combines angular differential
imaging (ADI) and accurate near-infrared polarimetry to characterize
self-luminous giant exoplanets. We implemented this technique at
VLT/SPHEREIRDIS and developed the corresponding observing strategies,
the polarization calibration and the data-reduction approaches. The
combination of ADI and polarimetry is challenging, because the
field rotation required for ADI negatively affects the polarimetric
performance. By combining ADI and polarimetry we can characterize
planets that can be directly imaged with a very high signal-to-noise
ratio. We use the IRDIS pupil-tracking mode and combine ADI and
principal component analysis to reduce speckle noise. We take advantage
of IRDIS' dual-beam polarimetric mode to eliminate differential effects
that severely limit the polarimetric sensitivity (flat-fielding errors,
differential aberrations and seeing), and thus further suppress
speckle noise. To correct for instrumental polarization effects, we
apply a detailed Mueller matrix model that describes the telescope
and instrument and that has an absolute polarimetric accuracy <=
0.1%. Using this technique we have observed the planets of HR 8799 and
the (sub-stellar) companion PZ Tel B. Unfortunately, we do not detect
a polarization signal in a first analysis. We estimate preliminary 1σ
upper limits on the degree of linear polarization of ∼ 1% and ∼ 0.1%
for the planets of HR 8799 and PZ Tel B, respectively. The achieved
sub-percent sensitivity and accuracy show that our technique has great
promise for characterizing exoplanets through direct-imaging polarimetry
---------------------------------------------------------
Title: Three Years of SPHERE: The Latest View of the Morphology and
Evolution of Protoplanetary Discs
Authors: Garufi, A.; Benisty, M.; Stolker, T.; Avenhaus, H.; de Boer,
J. .; Pohl, A.; Quanz, S. P.; Dominik, C.; Ginski, C.; Thalmann, C.;
van Boekel, R.; Boccaletti, A.; Henning, T.; Janson, M.; Salter, G.;
Schmid, H. M.; Sissa, E.; Langlois, M.; Beuzit, J. -L.; Chauvin, G.;
Mouillet, D.; Augereau, J. -C.; Bazzon, A.; Biller, B.; Bonnefoy,
M.; Buenzli, E.; Cheetham, A.; Daemgen, S.; Desidera, S.; Engler,
N.; Feldt, M.; Girard, J.; Gratton, R.; Hagelberg, J.; Keller, C.;
Keppler, M.; Kenworthy, M.; Kral, Q.; Lopez, B.; Maire, A. -L.; Menard,
F.; Mesa, D.; Messina, S.; Meyer, M. R.; Milli, J.; Min, M.; Muller,
A.; Olofsson, J.; Pawellek, N.; Pinte, C.; Szulagyi, J.; Vigan, A.;
Wahhaj, Z.; Waters, R.; Zurlo, A.
2017Msngr.169...32G Altcode: 2017arXiv171002795G
Spatially resolving the immediate surroundings of young stars is a
key challenge for the planet formation community. SPHERE on the VLT
represents an important step forward by increasing the opportunities
offered by optical or near-infrared imaging instruments to image
protoplanetary discs. The Guaranteed Time Observation Disc team has
concentrated much of its efforts on polarimetric differential imaging,
a technique that enables the efficient removal of stellar light and
thus facilitates the detection of light scattered by the disc within a
few au from the central star. These images reveal intriguing complex
disc structures and diverse morphological features that are possibly
caused by ongoing planet formation in the disc. An overview of the
recent advances enabled by SPHERE is presented.
---------------------------------------------------------
Title: Investigating circular patterns in linear polarization
observations of Venus
Authors: Mahapatra, G.; Stam, D. M.; Rossi, L.; Rodenhuis, M.; Snik,
F.; Keller, C. U.
2017EPSC...11..885M Altcode:
In this work, we report our observations of Venus using the polarized
flux. Our observations show certain curious looking concentric rings
around the sub-solar point of Venus. We use our radiative transfer
model to explain these fluctuations and discuss what the possible
explanations might be.
---------------------------------------------------------
Title: Rigorous vector wave propagation for arbitrary flat media
Authors: Bos, Steven P.; Haffert, Sebastiaan Y.; Keller, Christoph U.
2017SPIE10407E..09B Altcode: 2018arXiv181109777B
Precise modelling of the (off-axis) point spread function (PSF)
to identify geometrical and polarization aberrations is important for
many optical systems. In order to characterise the PSF of the system in
all Stokes parameters, an end-to-end simulation of the system has to
be performed in which Maxwell's equations are rigorously solved. We
present the first results of a python code that we are developing
to perform multiscale end-to-end wave propagation simulations that
include all relevant physics. Currently we can handle plane-parallel
near- and far-field vector diffraction effects of propagating waves
in homogeneous isotropic and anisotropic materials, refraction and
reflection of flat parallel surfaces, interference effects in thin
films and unpolarized light. We show that the code has a numerical
precision on the order of 10<SUP>-16 </SUP>for non-absorbing isotropic
and anisotropic materials. For absorbing materials the precision is
on the order of 10<SUP>-8</SUP>. The capabilities of the code are
demonstrated by simulating a converging beam reflecting from a flat
aluminium mirror at normal incidence.
---------------------------------------------------------
Title: BP Piscium: its flaring disc imaged with
SPHERE/ZIMPOL<SUP>★</SUP>
Authors: de Boer, J.; Girard, J. H.; Canovas, H.; Min, M.; Sitko,
M.; Ginski, C.; Jeffers, S. V.; Mawet, D.; Milli, J.; Rodenhuis, M.;
Snik, F.; Keller, C. U.
2017MNRAS.466L...7D Altcode: 2016arXiv161006609D
Whether BP Piscium (BP Psc) is either a pre-main sequence T Tauri
star at d ≈ 80 pc, or a post-main sequence G giant at d ≈ 300
pc is still not clear. As a first-ascent giant, it is the first to
be observed with a molecular and dust disc. Alternatively, BP Psc
would be among the nearest T Tauri stars with a protoplanetary disc
(PPD). We investigate whether the disc geometry resembles typical PPDs,
by comparing polarimetric images with radiative transfer models. Our
Very Large Telescope/Spectro-Polarimetric High-contrast Exoplanet
REsearch (SPHERE)/Zurich IMaging Polarimeter (ZIMPOL) observations
allow us to perform polarimetric differential imaging, reference star
differential imaging, and Richardson-Lucy deconvolution. We present the
first visible light polarization and intensity images of the disc of
BP Psc. Our deconvolution confirms the disc shape as detected before,
mainly showing the southern side of the disc. In polarized intensity
the disc is imaged at larger detail and also shows the northern side,
giving it the typical shape of high-inclination flared discs. We explain
the observed disc features by retrieving the large-scale geometry with
MCMAX radiative transfer modelling, which yields a strongly flared
model, atypical for discs of T Tauri stars.
---------------------------------------------------------
Title: On-sky Performance Analysis of the Vector Apodizing Phase
Plate Coronagraph on MagAO/Clio2
Authors: Otten, Gilles P. P. L.; Snik, Frans; Kenworthy, Matthew A.;
Keller, Christoph U.; Males, Jared R.; Morzinski, Katie M.; Close,
Laird M.; Codona, Johanan L.; Hinz, Philip M.; Hornburg, Kathryn J.;
Brickson, Leandra L.; Escuti, Michael J.
2017ApJ...834..175O Altcode: 2017arXiv170204193O
We report on the performance of a vector apodizing phase plate
coronagraph that operates over a wavelength range of 2-5 μm and
is installed in MagAO/Clio2 at the 6.5 m Magellan Clay telescope
at Las Campanas Observatory, Chile. The coronagraph manipulates the
phase in the pupil to produce three beams yielding two coronagraphic
point-spread functions (PSFs) and one faint leakage PSF. The phase
pattern is imposed through the inherently achromatic geometric phase,
enabled by liquid crystal technology and polarization techniques. The
coronagraphic optic is manufactured using a direct-write technique
for precise control of the liquid crystal pattern and multitwist
retarders for achromatization. By integrating a linear phase ramp to
the coronagraphic phase pattern, two separated coronagraphic PSFs are
created with a single pupil-plane optic, which makes it robust and easy
to install in existing telescopes. The two coronagraphic PSFs contain
a 180° dark hole on each side of a star, and these complementary
copies of the star are used to correct the seeing halo close to the
star. To characterize the coronagraph, we collected a data set of a
bright (m<SUB>L</SUB> = 0-1) nearby star with ∼1.5 hr of observing
time. By rotating and optimally scaling one PSF and subtracting it
from the other PSF, we see a contrast improvement by 1.46 magnitudes
at 3.5 λ /D. With regular angular differential imaging at 3.9 μm,
the MagAO vector apodizing phase plate coronagraph delivers a 5σ {{Δ
}}{mag} contrast of 8.3 (={10}<SUP>-3.3</SUP>) at 2 λ /D and 12.2
(={10}<SUP>-4.8</SUP>) at 3.5 λ /D.
---------------------------------------------------------
Title: The coronagraphic Modal Wavefront Sensor: a hybrid focal-plane
sensor for the high-contrast imaging of circumstellar environments
Authors: Wilby, M. J.; Keller, C. U.; Snik, F.; Korkiakoski, V.;
Pietrow, A. G. M.
2017A&A...597A.112W Altcode: 2016arXiv161004235W
The raw coronagraphic performance of current high-contrast imaging
instruments is limited by the presence of a quasi-static speckle
(QSS) background, resulting from instrumental Non-Common Path Errors
(NCPEs). Rapid development of efficient speckle subtraction techniques
in data reduction has enabled final contrasts of up to 10<SUP>-6</SUP>
to be obtained, however it remains preferable to eliminate the
underlying NCPEs at the source. In this work we introduce the
coronagraphic Modal Wavefront Sensor (cMWS), a new wavefront sensor
suitable for real-time NCPE correction. This combines the Apodizing
Phase Plate (APP) coronagraph with a holographic modal wavefront sensor
to provide simultaneous coronagraphic imaging and focal-plane wavefront
sensing with the science point-spread function. We first characterise
the baseline performance of the cMWS via idealised closed-loop
simulations, showing that the sensor is able to successfully recover
diffraction-limited coronagraph performance over an effective dynamic
range of ±2.5 radians root-mean-square (rms) wavefront error within
2-10 iterations, with performance independent of the specific choice
of mode basis. We then present the results of initial on-sky testing
at the William Herschel Telescope, which demonstrate that the sensor
is capable of NCPE sensing under realistic seeing conditions via the
recovery of known static aberrations to an accuracy of 10 nm (0.1
radians) rms error in the presence of a dominant atmospheric speckle
foreground. We also find that the sensor is capable of real-time
measurement of broadband atmospheric wavefront variance (50% bandwidth,
158 nm rms wavefront error) at a cadence of 50 Hz over an uncorrected
telescope sub-aperture. When combined with a suitable closed-loop
adaptive optics system, the cMWS holds the potential to deliver an
improvement of up to two orders of magnitude over the uncorrected
QSS floor. Such a sensor would be eminently suitable for the direct
imaging and spectroscopy of exoplanets with both existing and future
instruments, including EPICS and METIS for the E-ELT.
---------------------------------------------------------
Title: Dusty tails of evaporating exoplanets. II. Physical modelling
of the KIC 12557548b light curve
Authors: van Lieshout, R.; Min, M.; Dominik, C.; Brogi, M.; de
Graaff, T.; Hekker, S.; Kama, M.; Keller, C. U.; Ridden-Harper, A.;
van Werkhoven, T. I. M.
2016A&A...596A..32V Altcode: 2016arXiv160900275V
Context. Evaporating rocky exoplanets, such as KIC 12557548b, eject
large amounts of dust, which can trail the planet in a comet-like
tail. When such objects occult their host star, the resulting transit
signal contains information about the dust in the tail. <BR /> Aims:
We aim to use the detailed shape of the Kepler light curve of KIC
12557548b to constrain the size and composition of the dust grains that
make up the tail, as well as the mass loss rate of the planet. <BR />
Methods: Using a self-consistent numerical model of the dust dynamics
and sublimation, we calculated the shape of the tail by following dust
grains from their ejection from the planet to their destruction due to
sublimation. From this dust cloud shape, we generated synthetic light
curves (incorporating the effects of extinction and angle-dependent
scattering), which were then compared with the phase-folded Kepler light
curve. We explored the free-parameter space thoroughly using a Markov
chain Monte Carlo method. <BR /> Results: Our physics-based model is
capable of reproducing the observed light curve in detail. Good fits
are found for initial grain sizes between 0.2 and 5.6 μm and dust
mass loss rates of 0.6 to 15.6 M<SUB>⊕</SUB> Gyr<SUP>-1</SUP> (2σ
ranges). We find that only certain combinations of material parameters
yield the correct tail length. These constraints are consistent with
dust made of corundum (Al<SUB>2</SUB>O<SUB>3</SUB>), but do not agree
with a range of carbonaceous, silicate, or iron compositions. <BR />
Conclusions: Using a detailed, physically motivated model, it is
possible to constrain the composition of the dust in the tails of
evaporating rocky exoplanets. This provides a unique opportunity to
probe to interior composition of the smallest known exoplanets.
---------------------------------------------------------
Title: Direct detection of scattered light gaps in the transitional
disk around HD 97048 with VLT/SPHERE
Authors: Ginski, C.; Stolker, T.; Pinilla, P.; Dominik, C.; Boccaletti,
A.; de Boer, J.; Benisty, M.; Biller, B.; Feldt, M.; Garufi, A.;
Keller, C. U.; Kenworthy, M.; Maire, A. L.; Ménard, F.; Mesa, D.;
Milli, J.; Min, M.; Pinte, C.; Quanz, S. P.; van Boekel, R.; Bonnefoy,
M.; Chauvin, G.; Desidera, S.; Gratton, R.; Girard, J. H. V.; Keppler,
M.; Kopytova, T.; Lagrange, A. -M.; Langlois, M.; Rouan, D.; Vigan, A.
2016A&A...595A.112G Altcode: 2016arXiv160904027G
<BR /> Aims: We studied the well-known circumstellar disk around the
Herbig Ae/Be star HD 97048 with high angular resolution to reveal
undetected structures in the disk which may be indicative of disk
evolutionary processes such as planet formation. <BR /> Methods: We used
the IRDIS near-IR subsystem of the extreme adaptive optics imager SPHERE
at the ESO/VLT to study the scattered light from the circumstellar disk
via high resolution polarimetry and angular differential imaging. <BR
/> Results: We imaged the disk in unprecedented detail and revealed
four ring-like brightness enhancements and corresponding gaps in the
scattered light from the disk surface with radii between 39 au and 341
au. We derived the inclination and position angle as well as the height
of the scattering surface of the disk from our observational data. We
found that the surface height profile can be described by a single
power law up to a separation 270 au. Using the surface height profile
we measured the scattering phase function of the disk and found that it
is consistent with theoretical models of compact dust aggregates. We
discuss the origin of the detected features and find that low mass
(≤1 M<SUB>Jup</SUB>) nascent planets are a possible explanation. <P
/>Based on data collected at the European Southern Observatory, Chile
(ESO Programs 096.C-0248, 096.C-0241, 077.C-0106).
---------------------------------------------------------
Title: Multiple rings in the transition disk and companion candidates
around RX J1615.3-3255. High contrast imaging with VLT/SPHERE
Authors: de Boer, J.; Salter, G.; Benisty, M.; Vigan, A.; Boccaletti,
A.; Pinilla, P.; Ginski, C.; Juhasz, A.; Maire, A. -L.; Messina,
S.; Desidera, S.; Cheetham, A.; Girard, J. H.; Wahhaj, Z.; Langlois,
M.; Bonnefoy, M.; Beuzit, J. -L.; Buenzli, E.; Chauvin, G.; Dominik,
C.; Feldt, M.; Gratton, R.; Hagelberg, J.; Isella, A.; Janson, M.;
Keller, C. U.; Lagrange, A. -M.; Lannier, J.; Menard, F.; Mesa, D.;
Mouillet, D.; Mugrauer, M.; Peretti, S.; Perrot, C.; Sissa, E.; Snik,
F.; Vogt, N.; Zurlo, A.; SPHERE Consortium
2016A&A...595A.114D Altcode: 2016arXiv161004038D
Context. The effects of a planet sculpting the disk from which it formed
are most likely to be found in disks that are in transition between
being classical protoplanetary and debris disks. Recent direct imaging
of transition disks has revealed structures such as dust rings, gaps,
and spiral arms, but an unambiguous link between these structures and
sculpting planets is yet to be found. <BR /> Aims: We aim to find signs
of ongoing planet-disk interaction and study the distribution of small
grains at the surface of the transition disk around RX J1615.3-3255 (RX
J1615). <BR /> Methods: We observed RX J1615 with VLT/SPHERE. From these
observations, we obtained polarimetric imaging with ZIMPOL (R'-band)
and IRDIS (J), and IRDIS (H2H3) dual-band imaging with simultaneous
spatially resolved spectra with the IFS (YJ). <BR /> Results: We image
the disk for the first time in scattered light and detect two arcs,
two rings, a gap and an inner disk with marginal evidence for an inner
cavity. The shapes of the arcs suggest that they are probably segments
of full rings. Ellipse fitting for the two rings and inner disk yield
a disk inclination I = 47 ± 2° and find semi-major axes of 1.50 ±
0.01” (278 au), 1.06 ± 0.01” (196 au) and 0.30 ± 0.01” (56 au),
respectively. We determine the scattering surface height above the
midplane, based on the projected ring center offsets. Nine point sources
are detected between 2.1” and 8.0” separation and considered as
companion candidates. With NACO data we recover four of the nine point
sources, which we determine to be not co-moving, and therefore unbound
to the system. <BR /> Conclusions: We present the first detection
of the transition disk of RX J1615 in scattered light. The height of
the rings indicate limited flaring of the disk surface, which enables
partial self-shadowing in the disk. The outermost arc either traces the
bottom of the disk or it is another ring with semi-major axis ≳ 2.35”
(435 au). We explore both scenarios, extrapolating the complete shape
of the feature, which will allow us to distinguish between the two
in future observations. The most attractive scenario, where the arc
traces the bottom of the outer ring, requires the disk to be truncated
at r ≈ 360 au. If the closest companion candidate is indeed orbiting
the disk at 540 au, then it would be the most likely cause for such
truncation. This companion candidate, as well as the remaining four,
all require follow up observations to determine if they are bound to
the system. <P />Based on observations made with ESO Telescopes at
the La Silla Paranal Observatory under programme IDs 095.C-0298(A),
095.C-0298(B), and 095.C-0693(A) during guaranteed and open time
observations of the SPHERE consortium, and on NACO observations: program
IDs: 085.C-0012(A), 087.C-0111(A), and 089.C-0133(A). The reduced
images as FITS files are only available at the CDS via anonymous ftp
to <A href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(<A href="http://130.79.128.5">http://130.79.128.5</A>) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/595/A114</A>
---------------------------------------------------------
Title: The Hera Saturn entry probe mission
Authors: Mousis, O.; Atkinson, D. H.; Spilker, T.; Venkatapathy, E.;
Poncy, J.; Frampton, R.; Coustenis, A.; Reh, K.; Lebreton, J. -P.;
Fletcher, L. N.; Hueso, R.; Amato, M. J.; Colaprete, A.; Ferri, F.;
Stam, D.; Wurz, P.; Atreya, S.; Aslam, S.; Banfield, D. J.; Calcutt,
S.; Fischer, G.; Holland, A.; Keller, C.; Kessler, E.; Leese, M.;
Levacher, P.; Morse, A.; Muñoz, O.; Renard, J. -B.; Sheridan, S.;
Schmider, F. -X.; Snik, F.; Waite, J. H.; Bird, M.; Cavalié, T.;
Deleuil, M.; Fortney, J.; Gautier, D.; Guillot, T.; Lunine, J. I.;
Marty, B.; Nixon, C.; Orton, G. S.; Sánchez-Lavega, A.
2016P&SS..130...80M Altcode: 2015arXiv151007685M
The Hera Saturn entry probe mission is proposed as an M-class mission
led by ESA with a contribution from NASA. It consists of one atmospheric
probe to be sent into the atmosphere of Saturn, and a Carrier-Relay
spacecraft. In this concept, the Hera probe is composed of ESA
and NASA elements, and the Carrier-Relay Spacecraft is delivered
by ESA. The probe is powered by batteries, and the Carrier-Relay
Spacecraft is powered by solar panels and batteries. We anticipate
two major subsystems to be supplied by the United States, either by
direct procurement by ESA or by contribution from NASA: the solar
electric power system (including solar arrays and the power management
and distribution system), and the probe entry system (including the
thermal protection shield and aeroshell). Hera is designed to perform
in situ measurements of the chemical and isotopic compositions as
well as the dynamics of Saturn's atmosphere using a single probe, with
the goal of improving our understanding of the origin, formation, and
evolution of Saturn, the giant planets and their satellite systems,
with extrapolation to extrasolar planets. Hera's aim is to probe
well into the cloud-forming region of the troposphere, below the
region accessible to remote sensing, to the locations where certain
cosmogenically abundant species are expected to be well mixed. By
leading to an improved understanding of the processes by which giant
planets formed, including the composition and properties of the local
solar nebula at the time and location of giant planet formation,
Hera will extend the legacy of the Galileo and Cassini missions by
further addressing the creation, formation, and chemical, dynamical,
and thermal evolution of the giant planets, the entire solar system
including Earth and the other terrestrial planets, and formation of
other planetary systems.
---------------------------------------------------------
Title: Search for an exosphere in sodium and calcium in the
transmission spectrum of exoplanet 55 Cancri e
Authors: Ridden-Harper, A. R.; Snellen, I. A. G.; Keller, C. U.; de
Kok, R. J.; Di Gloria, E.; Hoeijmakers, H. J.; Brogi, M.; Fridlund,
M.; Vermeersen, B. L. A.; van Westrenen, W.
2016A&A...593A.129R Altcode: 2016arXiv160608447R
Context. The atmospheric and surface characterization of rocky
planets is a key goal of exoplanet science. Unfortunately, the
measurements required for this are generally out of reach of present-day
instrumentation. However, the planet Mercury in our own solar system
exhibits a large exosphere composed of atomic species that have been
ejected from the planetary surface by the process of sputtering. Since
the hottest rocky exoplanets known so far are more than an order of
magnitude closer to their parent star than Mercury is to the Sun,
the sputtering process and the resulting exospheres could be orders
of magnitude larger and potentially detectable using transmission
spectroscopy, indirectly probing their surface compositions. <BR />
Aims: The aim of this work is to search for an absorption signal from
exospheric sodium (Na) and singly ionized calcium (Ca<SUP>+</SUP>)
in the optical transmission spectrum of the hot rocky super-Earth 55
Cancri e. Although the current best-fitting models to the planet mass
and radius require a possible atmospheric component, uncertainties in
the radius exist, making it possible that 55 Cancri e could be a hot
rocky planet without an atmosphere. <BR /> Methods: High resolution
(R ~ 110 000) time-series spectra of five transits of 55 Cancri e,
obtained with three different telescopes (UVES/VLT, HARPS/ESO 3.6 m and
HARPS-N/TNG) were analysed. Targeting the sodium D lines and the calcium
H and K lines, the potential planet exospheric signal was filtered out
from the much stronger stellar and telluric signals, making use of the
change of the radial component of the orbital velocity of the planet
over the transit from -57 to +57 km s<SUP>-1</SUP>. <BR /> Results:
Combining all five transit data sets, we detect a signal potentially
associated with sodium in the planet exosphere at a statistical
significance level of 3σ. Combining the four HARPS transits that
cover the calcium H and K lines, we also find a potential signal from
ionized calcium (4.1σ). Interestingly, this latter signal originates
from just one of the transit measurements - with a 4.9σ detection at
this epoch. Unfortunately, due to the low significance of the measured
sodium signal and the potentially variable Ca<SUP>+</SUP> signal, we
estimate the p-values of these signals to be too high (corresponding to
<4σ) to claim unambiguous exospheric detections. By comparing the
observed signals with artificial signals injected early in the analysis,
the absorption by Na and Ca<SUP>+</SUP> are estimated to be at a level
of ~2.3 × 10<SUP>-3</SUP> and ~7.0 × 10<SUP>-2</SUP> respectively,
relative to the stellar spectrum. <BR /> Conclusions: If confirmed,
the 3σ signal would correspond to an optically thick sodium exosphere
with a radius of 5 R<SUB>⊕</SUB>, which is comparable to the Roche
lobe radius of the planet. The 4.9σ detection of Ca<SUP>+</SUP>
in a single HARPS data set would correspond to an optically thick
Ca<SUP>+</SUP> exosphere approximately five times larger than the
Roche lobe radius. If this were a real detection, it would imply
that the exosphere exhibits extreme variability. Although no formal
detection has been made, we advocate that probing the exospheres of
hot super-Earths in this way has great potential, also knowing that
Mercury's exosphere varies significantly over time. It may be a fast
route towards the first characterization of the surface properties of
this enigmatic class of planets. <P />Based on observations collected
at the European Organisation for Astronomical Research in the Southern
Hemisphere under ESO programmes 092.C-0178 and 288.C-5010 and the
Telescopio Nazionale Galileo under programme CAT13B_33.
---------------------------------------------------------
Title: HERA: an atmospheric probe to unveil the depths of Saturn
Authors: Mousis, Olivier; Atkinson, David H.; Amato, Michael; Aslam,
Shahid; Atreya, Sushil K.; Blanc, Michel; Bolton, Scott J.; Brugger,
Bastien; Calcutt, Simon; Cavalié, Thibault; Charnoz, Sébastien;
Coustenis, Athena; DELEUIL, Magali; Ferri, Francesca; Fletcher,
Leigh N.; Guillot, Tristan; Hartogh, Paul; Holland, Andrew; Hueso,
Ricardo; Keller, Christoph; Kessler, Ernst; Lebreton, Jean-Pierre;
leese, Mark; Lellouch, Emmanuel; Levacher, Patrick; Marty, Bernard;
Morse, Andrew; Nixon, Conor; Reh, Kim R.; Renard, Jean-Baptiste;
Sanchez-Lavega, Agustin; Schmider, François-Xavier; Sheridan, Simon;
Simon, Amy A.; Snik, Frans; Spilker, Thomas R.; Stam, Daphne M.;
Venkatapathy, Ethiraj; Vernazza, Pierre; Waite, J. Hunter; Wurz, Peter
2016DPS....4812328M Altcode:
The Hera Saturn entry probe mission is proposed as an M-class mission
led by ESA with a significant collaboration with NASA. It consists
of a Saturn atmospheric probe and a Carrier-Relay spacecraft. Hera
will perform in situ measurements of the chemical and isotopic
compositions as well as the dynamics of Saturn's atmosphere, with
the goal of improving our understanding of the origin, formation, and
evolution of Saturn, the giant planets and their satellite systems,
with extrapolation to extrasolar planets.The primary science objectives
will be addressed by an atmospheric entry probe that would descend
under parachute and carry out in situ measurements beginning in the
stratosphere to help characterize the location and properties of the
tropopause, and continue into the troposphere to pressures of at least
10 bars. All of the science objectives, except for the abundance of
oxygen, which may be only addressed indirectly via observations of
species whose abundances are tied to the abundance of water, can
be achieved by reaching 10 bars. As in previous highly successful
collaborative efforts between ESA and NASA, the proposed mission has a
baseline concept based on a NASA-provided carrier/data relay spacecraft
that would deliver the ESA-provided atmospheric probe to the desired
atmospheric entry point at Saturn. ESA's proposed contribution should
fit well into the M5 Cosmic Vision ESA call cost envelope.A nominal
mission configuration would consist of a probe that detaches from
the carrier one to several months prior to probe entry. Subsequent to
probe release, the carrier trajectory would be deflected to optimize
the over-flight phasing of the probe descent location for both probe
data relay as well as performing carrier approach and flyby science,
and would allow multiple retransmissions of the probe data for
redundancy. The Saturn atmospheric entry probe would in many respects
resemble the Jupiter Galileo probe. It is anticipated that the probe
architecture for this mission would be battery-powered and accommodate
a data relay to the carrier for data collection, storage on board the
carrier/data relay, for later retransmission to Earth.
---------------------------------------------------------
Title: The Leiden EXoplanet Instrument (LEXI): a high-contrast
high-dispersion spectrograph
Authors: Haffert, S. Y.; Wilby, M. J.; Keller, C. U.; Snellen, I. A. G.
2016SPIE.9908E..67H Altcode:
The Leiden EXoplanet Instrument (LEXI) will be the first instrument
designed for high-contrast, high-dispersion integral field spectroscopy
at optical wavelengths. High-contrast imaging (HCI) and high-dispersion
spectroscopy (HDS) techniques are used to reach contrasts of
10<SUP>-7</SUP>. LEXI will be a bench-mounted, high dispersion integral
field spectrograph that will record spectra in a small area around the
star with high spatial resolution and high dynamic range. A prototype
is being setup to The Leiden EXoplanet Instrument (LEXI) will be the
first instrument designed for high-contrast, high-dispersion integral
field spectroscopy at optical wavelengths. High-contrast imaging
(HCI) and high-dispersion spectroscopy (HDS) techniques are used to
reach contrasts of 10<SUP>-7</SUP>. LEXI will be a bench-mounted,
high dispersion integral field spectrograph that will record spectra
in a small area around the star with high spatial resolution and high
dynamic range. A prototype is being setup to test the combination of
HCI+HDS and its first light is expected in 2016.
---------------------------------------------------------
Title: Novel instrument concepts for characterizing directly imaged
exoplanets
Authors: Keller, Christoph U.
2016SPIE.9908E..9VK Altcode:
Current high-contrast exoplanet imagers are optimized to find new
exoplanets; they minimize diffracted starlight in a large area around a
star. I present four novel instrumental approaches that are optimized
to characterize these discoveries by minimizing starlight in a small
area around the known location of an exoplanet: 1) coronagraphs that
remove virtually all starlight over an octave in wavelength while
transmitting more than 90% of the exoplanet signal; 2) holographic
wavefront sensors that measure aberrations in the science focal plane;
3) ultra-fast adaptive optics systems that minimize these aberrations;
and 4) direct minimization of the remaining starlight. By integrating
these technologies with a high spectral- resolution, integral-field
spectrograph that can resolve the Doppler shift and the polarization
difference between the starlight and the reflected light from
the exoplanet, it will be possible to determine the atmospheric
composition, temperature and velocity structures of exoplanets and
their spin rotation rate and orbital velocity. This will ultimately
allow the upcoming extremely large telescopes to characterize rocky
exoplanets in the habitable zone to look for signatures of life.
---------------------------------------------------------
Title: End-to-end simulations of the E-ELT/METIS coronagraphs
Authors: Carlomagno, Brunella; Absil, Olivier; Kenworthy, Matthew;
Ruane, Garreth; Keller, Christoph U.; Otten, Gilles; Feldt, Markus;
Hippler, Stefan; Huby, Elsa; Mawet, Dimitri; Delacroix, Christian;
Surdej, Jean; Habraken, Serge; Forsberg, Pontus; Karlsson, Mikael;
Vargas Catalan, Ernesto; Brandl, Bernhard R.
2016SPIE.9909E..73C Altcode:
The direct detection of low-mass planets in the habitable zone of
nearby stars is an important science case for future E-ELT instruments
such as the mid-infrared imager and spectrograph METIS, which features
vortex phase masks and apodizing phase plates (APP) in its baseline
design. In this work, we present end-to-end performance simulations,
using Fourier propagation, of several METIS coronagraphic modes,
including focal-plane vortex phase masks and pupil-plane apodizing
phase plates, for the centrally obscured, segmented E-ELT pupil. The
atmosphere and the AO contributions are taken into account. Hybrid
coronagraphs combining the advantages of vortex phase masks and APPs
are considered to improve the METIS coronagraphic performance.
---------------------------------------------------------
Title: A "Fast and Furious'" solution to the low-wind effect for
SPHERE at the VLT
Authors: Wilby, M. J.; Keller, C. U.; Sauvage, J. -F.; Fusco, T.;
Mouillet, D.; Beuzit, J. -L.; Dohlen, K.
2016SPIE.9909E..6CW Altcode:
We present a potential non-invasive solution to sensing the so-called
low-wind effect (LWE) seen in the SPHERE instrument at the VLT, based on
the "Fast and Furious (F&F) sequential phase diversity wavefront
reconstruction algorithm. This uses non-coronagraphic focal-plane
images available from the near-infra-red Differential Tip-Tilt Sensor
(DTTS), with the closed-loop correction cycle itself providing the
necessary phase diversity between frames required to reconstruct
the full wavefront phase. Crucially, this means F&F does not
need to apply large artificial phase probes as required by standard
phase diversity algorithms, allowing it to operate in a real-time (
10 Hz) correction mode without impacting science observations. In this
paper we present the results of realistic closed-loop AO simulations
designed to emulate SPHERE/DTTS observations of the LWE. With this
we demonstrate that the F&F algorithm is capable of effective
removal of the characteristic point-spread function (PSF) aberrations
of strongly LWE-affected images within a few closed-loop iterations,
with the final wavefront quality limited only by the corrective order of
the deformable mirror. The ultimate goal of this project is to provide
an independent, real-time and focal-plane wavefront sensor for SPHERE
which is capable of detecting and directly compensating the LWE as it
arises, thus improving coronagraph performance under the best 15-20 %
of observing conditions where the effect is most pronounced.
---------------------------------------------------------
Title: Focal-plane electric field sensing with pupil-plane holograms
Authors: Por, Emiel H.; Keller, Christoph U.
2016SPIE.9909E..59P Altcode:
The direct detection and spectral characterization of exoplanets
requires a coronagraph to suppress the diffracted star light. Amplitude
and phase aberrations in the optical train fill the dark zone of
the coronagraph with quasi-static speckles that limit the achievable
contrast. Focal-plane electric field sensing, such as phase diversity
introduced by a deformable mirror (DM), is a powerful tool to minimize
this residual star light. The residual electric field can be estimated
by sequentially applying phase probes on the DM to inject star light
with a well-known amplitude and phase into the dark zone and analyzing
the resulting intensity images. The DM can then be used to add light
with the same amplitude but opposite phase to destructively interfere
with this residual star light. Using a static phase-only pupil-plane
element we create holographic copies of the point spread function (PSF),
each superimposed with a certain pupil-plane phase probe. We therefore
obtain all intensity images simultaneously while still retaining a
central, unaltered science PSF. The electric field sensing method
only makes use of the holographic copies, allowing for correction
of the residual electric field while retaining the central PSF for
uninterrupted science data collection. In this paper we demonstrate
the feasibility of this method with numerical simulations.
---------------------------------------------------------
Title: Designing and testing the coronagraphic Modal Wavefront Sensor:
a fast non-common path error sensor for high-contrast imaging
Authors: Wilby, M. J.; Keller, C. U.; Haffert, S.; Korkiakoski, V.;
Snik, F.; Pietrow, A. G. M.
2016SPIE.9909E..21W Altcode:
Non-Common Path Errors (NCPEs) are the dominant factor limiting
the performance of current astronomical high-contrast imaging
instruments. If uncorrected, the resulting quasi-static speckle noise
floor limits coronagraph performance to a raw contrast of typically
10<SUP>-4</SUP>, a value which does not improve with increasing
integration time. The coronagraphic Modal Wavefront Sensor (cMWS)
is a hybrid phase optic which uses holographic PSF copies to supply
focal-plane wavefront sensing information directly from the science
camera, whilst maintaining a bias-free coronagraphic PSF. This
concept has already been successfully implemented on-sky at the
William Herschel Telescope (WHT), La Palma, demonstrating both
real-time wavefront sensing capability and successful extraction of
slowly varying wavefront errors under a dominant and rapidly changing
atmospheric speckle foreground. In this work we present an overview of
the development of the cMWS and recent first light results obtained
using the Leiden EXoplanet Instrument (LEXI), a high-contrast imager
and high-dispersion spectrograph pathfinder instrument for the WHT.
---------------------------------------------------------
Title: The ZIMPOL high contrast imaging polarimeter for SPHERE:
polarimetric high contrast commissioning results
Authors: Roelfsema, Ronald; Bazzon, Andreas; Schmid, Hans Martin;
Pragt, Johan; Govaert, Alain; Gisler, Daniel; Dominik, Carsten;
Baruffolo, Andrea; Beuzit, Jean-Luc; Costille, Anne; Dohlen, Kjetil;
Downing, Mark; Elswijk, Eddy; de Haan, Menno; Hubin, Norbert; Kasper,
Markus; Keller, Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov,
Alexey; Puget, Pascal; Salasnich, Bernardo; Sauvage, Jean-Francois;
Wildi, Francois
2016SPIE.9909E..27R Altcode:
SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is
a second generation VLT instrument aimed at the direct detection
of exo-planets. It has received its first light in May 2014. ZIMPOL
(Zurich Imaging Polarimeter) is the imaging polarimeter subsystem of
the SPHERE instrument. It's capable of both high accuracy and high
sensitivity polarimetry but can also be used as a classical imager. It
is located behind an extreme AO system and a stellar coronagraph. ZIMPOL
operates at visible wavelengths which is best suited to detect the
very faint reflected and hence polarized visible light from extra solar
planets. During the SPHERE fourth commissioning period (October 2014)
we have made deep coronagraphic observations of the bright star alpha
Gru (mR = 1.75) to assess the high contrast polarimetric performance
of SPHERE-ZIMPOL. We have integrated on the target for a total time of
about 45 minutes during the meridian transit in the Very Broad Band
filter (600 - 900 nm) with a classical Lyot coronagraph with 3 λ/D
radius focal mask. We reduce the data by a combination of Polarized
Background subtraction, Polarimetric Differential Imaging (PDI) and
Angular Differential Imaging (ADI). We reach contrasts of 10-6 and
10-7 at a radial distances of respectively 7 and 14 lambda/D from
the PSF core. At these radial distances we are respectively a factor
of 10 and 2 above the photon noise limit. We discuss our results by
considering the temporal and spatial speckle behavior close to the
PSF core in combination with low order polarimetric aberrations.
---------------------------------------------------------
Title: Instrumemtation
Authors: Keller, Christoph U.; Snik, Frans; Harrington, David M.;
Packham, Chris
2015psps.book...35K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Combining high-dispersion spectroscopy with high contrast
imaging: Probing rocky planets around our nearest neighbors
Authors: Snellen, I.; de Kok, R.; Birkby, J. L.; Brandl, B.; Brogi,
M.; Keller, C.; Kenworthy, M.; Schwarz, H.; Stuik, R.
2015A&A...576A..59S Altcode: 2015arXiv150301136S
Context. Ground-based high-dispersion (R ~ 100 000) spectroscopy (HDS)
is proving to be a powerful technique with which to characterize
extrasolar planets. The planet signal is distilled from the bright
starlight, combining ral and time-differential filtering techniques. In
parallel, high-contrast imaging (HCI) is developing rapidly, aimed at
spatially separating the planet from the star. While HDS is limited by
the overwhelming noise from the host star, HCI is limited by residual
quasi-static speckles. Both techniques currently reach planet-star
contrast limits down to ~10<SUP>-5</SUP>, albeit for very different
types of planetary systems. <BR /> Aims: In this work, we discuss
a way to combine HDS and HCI (HDS+HCI). For a planet located at a
resolvable angular distance from its host star, the starlight can be
reduced up to several orders of magnitude using adaptive optics and/or
coronography. In addition, the remaining starlight can be filtered
out using high-dispersion spectroscopy, utilizing the significantly
different (or Doppler shifted) high-dispersion spectra of the planet
and star. In this way, HDS+HCI can in principle reach contrast limits
of ~10<SUP>-5</SUP> × 10<SUP>-5</SUP>, although in practice this
will be limited by photon noise and/or sky-background. In contrast to
current direct imaging techniques, such as Angular Differential Imaging
and Spectral Differential Imaging, it will work well at small working
angles and is much less sensitive to speckle noise. For the discovery
of previously unknown planets HDS+HCI requires a high-contrast adaptive
optics system combined with a high-dispersion R ~ 100 000 integral field
spectrograph (IFS). This combination currently does not exist, but is
planned for the European Extremely Large Telescope. <BR /> Methods:
We present simulations of HDS+HCI observations with the E-ELT, both
probing thermal emission from a planet at infrared wavelengths, and
starlight reflected off a planet atmosphere at optical wavelengths. For
the infrared simulations we use the baseline parameters of the E-ELT
and METIS instrument, with the latter combining extreme adaptive
optics with an R = 100 000 IFS. We include realistic models of
the adaptive optics performance and atmospheric transmission and
emission. For the optical simulation we also assume R = 100 000 IFS
with adaptive optics capabilities at the E-ELT. <BR /> Results: One
night of HDS+HCI observations with the E-ELT at 4.8 μm (Δλ = 0.07
μm) can detect a planet orbiting α Cen A with a radius of R = 1.5
R<SUB>earth</SUB> and a twin-Earth thermal spectrum of T<SUB>eq</SUB>
= 300 K at a signal-to-noise (S/N) of 5. In the optical, with a Strehl
ratio performance of 0.3, reflected light from an Earth-size planet
in the habitable zone of Proxima Centauri can be detected at a S/N of
10 in the same time frame. Recently, first HDS+HCI observations have
shown the potential of this technique by determining the spin-rotation
of the young massive exoplanet β Pictoris b. <BR /> Conclusions:
The exploration of the planetary systems of our neighbor stars is
of great scientific and philosophical value. The HDS+HCI technique
has the potential to detect and characterize temperate rocky planets
in their habitable zones. Exoplanet scientists should not shy away
from claiming a significant fraction of the future ELTs to make such
observations possible.
---------------------------------------------------------
Title: Magnetic field topology and chemical spot distributions in
the extreme Ap star HD 75049
Authors: Kochukhov, O.; Rusomarov, N.; Valenti, J. A.; Stempels, H. C.;
Snik, F.; Rodenhuis, M.; Piskunov, N.; Makaganiuk, V.; Keller, C. U.;
Johns-Krull, C. M.
2015A&A...574A..79K Altcode: 2014arXiv1411.7518K
Context. Intermediate-mass, magnetic chemically peculiar (Ap) stars
provide a unique opportunity to study the topology of stellar magnetic
fields in detail and to investigate magnetically driven processes
of spot formation. <BR /> Aims: Here we aim to derive the surface
magnetic field geometry and chemical abundance distributions for the
extraordinary Ap star HD 75049. This object hosts a surface field of
~30 kG, one of the strongest known for any non-degenerate star. <BR />
Methods: We used time-series of high-resolution HARPS intensity and
circular polarisation observations. These data were interpreted with the
help of magnetic Doppler imaging and model atmospheres incorporating
effects of a non-solar chemical composition and a strong magnetic
field. <BR /> Results: Based on high-precision measurements of the
mean magnetic field modulus, we refined the rotational period of HD
75049 to P<SUB>rot</SUB> = 4.048267 ± 0.000036 d. We also derived
basic stellar parameters, T<SUB>eff</SUB> = 10 250 ± 250 K and log g =
4.3 ± 0.1. Magnetic Doppler imaging revealed that the field topology
of HD 75049 is poloidal and dominated by a dipolar contribution with
a peak surface field strength of 39 kG. At the same time, deviations
from the classical axisymmetric oblique dipolar configuration are
significant. Chemical surface maps of Si, Cr, Fe, and Nd show abundance
contrasts of 0.5-1.4 dex, which is low compared with many other Ap
stars. Of the chemical elements, Nd is found to be enhanced close to
the magnetic pole, whereas Si and Cr are concentrated predominantly
at the magnetic equator. The iron distribution shows low-contrast
features both at the magnetic equator and the pole. <BR /> Conclusions:
The morphology of the magnetic field and the properties of chemical
spots in HD 75049 are qualitatively similar to those of Ap stars with
weaker fields. Consequently, whatever mechanism forms and sustains
global magnetic fields in intermediate-mass main-sequence stars, it
operates in the same way over the entire observed range of magnetic
field strengths. <P />Based on observations collected at the European
Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296,
086.D-0240, 088.D-0066, 090.D-0256, 078.D-0192, 080.D-0170).
---------------------------------------------------------
Title: Surprising detection of an equatorial dust lane on the AGB
star IRC+10216
Authors: Jeffers, S. V.; Min, M.; Waters, L. B. F. M.; Canovas,
H.; Pols, O. R.; Rodenhuis, M.; de Juan Ovelar, M.; Keller, C. U.;
Decin, L.
2014A&A...572A...3J Altcode: 2014arXiv1412.5063J
<BR /> Aims: Understanding the formation of planetary nebulae remains
elusive because in the preceding asymptotic giant branch (AGB)
phase these stars are heavily enshrouded in an optically thick dusty
envelope. <BR /> Methods: To further understand the morphology of
the circumstellar environments of AGB stars we observe the closest
carbon-rich AGB star IRC+10216 in scattered light. <BR /> Results:
When imaged in scattered light at optical wavelengths, IRC+10216
surprisingly shows a narrow equatorial density enhancement, in contrast
to the large-scale spherical rings that have been imaged much further
out. We use radiative transfer models to interpret this structure
in terms of two models: firstly, an equatorial density enhancement,
commonly observed in the more evolved post-AGB stars, and secondly,
in terms of a dust rings model, where a local enhancement of mass-loss
creates a spiral ring as the star rotates. <BR /> Conclusions: We
conclude that both models can be used to reproduce the dark lane in
the scattered light images, which is caused by an equatorially density
enhancement formed by dense dust rather than a bipolar outflow as
previously thought. We are unable to place constraints on the formation
of the equatorial density enhancement by a binary system. <P />Final
reduced images (FITS) are available at the CDS via anonymous ftp to <A
href="http://cdsarc.u-strasbg.fr">http://cdsarc.u-strasbg.fr</A>
(ftp://130.79.128.5) or via <A
href="http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A3">http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/572/A3</A>Based
on observations made with the William Herschel Telescope operated on the
island of La Palma by the Isaac Newton Group in the Spanish Observatorio
del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.
---------------------------------------------------------
Title: Mapping atmospheric aerosols with a citizen science network
of smartphone spectropolarimeters
Authors: Snik, Frans; Rietjens, Jeroen H. H.; Apituley, Arnoud;
Volten, Hester; Mijling, Bas; Di Noia, Antonio; Heikamp, Stephanie;
Heinsbroek, Ritse C.; Hasekamp, Otto P.; Smit, J. Martijn; Vonk, Jan;
Stam, Daphne M.; Harten, Gerard; Boer, Jozua; Keller, Christoph U.
2014GeoRL..41.7351S Altcode:
To assess the impact of atmospheric aerosols on health, climate, and
air traffic, aerosol properties must be measured with fine spatial
and temporal sampling. This can be achieved by actively involving
citizens and the technology they own to form an atmospheric measurement
network. We establish this new measurement strategy by developing
and deploying iSPEX, a low-cost, mass-producible optical add-on for
smartphones with a corresponding app. The aerosol optical thickness
(AOT) maps derived from iSPEX spectropolarimetric measurements of the
daytime cloud-free sky by thousands of citizen scientists throughout
the Netherlands are in good agreement with the spatial AOT structure
derived from satellite imagery and temporal AOT variations derived
from ground-based precision photometry. These maps show structures
at scales of kilometers that are typical for urban air pollution,
indicating the potential of iSPEX to provide information about aerosol
properties at locations and at times that are not covered by current
monitoring efforts.
---------------------------------------------------------
Title: Drift scanning technique for mid-infrared background
subtraction
Authors: Heikamp, Stephanie; Brandl, Bernhard R.; Keller, Christoph
U.; Venema, Lars; Pantin, Eric; Siebenmorgen, Ralf; Ives, Derek;
Kerber, Florian
2014SPIE.9147E..9TH Altcode:
Accurate calibration of ground-based, mid-infrared observations is
challenging due to the strong and rapidly varying thermal background
emission. The classical solution is the chopping/nodding technique where
the secondary mirror and the telescope are being moved by several tens
of arcseconds on the sky. However, chopping is generally inefficient and
limited in accuracy and frequency by the mass and size of the secondary
mirror. A more elegant solution is a drift scan where the telescope
slowly drifts across or around the region of interest; the source
moves on the detector by at least one FWHM of the PSF within the time
over which the detector performance and the background emission can be
considered stable. The final image of a drift scan is mathematically
reconstructed from a series of adjacent short exposures. The drift
scan approach has recently received a lot of interest, mainly for two
reasons: first, some of the new, large-format mid-IR Si:As detectors
(AQUARIUS) suffer from excess low frequency noise (ELFN). To reach the
nominal performance limit of the detectors, chopping would have to be
performed at a high frequency, faster than what most telescopes can
handle; second, the next generation of extremely large telescopes will
not offer chopping/nodding, and alternative methods need to be developed
and tested. In this paper we present the results from simulated drift
scan data. We use drift scanning to simultaneously obtain an accurate
detector flat field and the sky background. The results are relevant
for the future operation and calibration of VISIR at the VLT as well
as for METIS, the thermal infrared instrument for the E-ELT.
---------------------------------------------------------
Title: Focal-plane wavefront sensing with high-order adaptive
optics systems
Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek;
Kenworthy, Matthew; Otten, Gilles; Verhaegen, Michel
2014SPIE.9148E..5DK Altcode: 2014arXiv1407.5846K
We investigate methods to calibrate the non-common path aberrations at
an adaptive optics system having a wavefront-correcting device working
with an extremely high resolution (larger than 150x150 correcting
elements). We use focal-plane images collected successively, the
corresponding phase-diversity information and numerically efficient
algorithms to calculate the required wavefront updates. Different
approaches are considered in numerical simulations, and laboratory
experiments are shown to confirm the results. We compare the
performances of the standard Gerchberg-Saxton algorithm, Fast
and Furious (use of small-phase assumption to take advantage of
linearisation) and recently proposed phase-retrieval methods based on
convex optimisation. The results indicate that the calibration task
is easiest with algorithms similar to Fast and Furious, at least in
the framework we considered.
---------------------------------------------------------
Title: Fast & Furious focal-plane wavefront sensing
Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek;
Kenworthy, Matthew; Otten, Gilles; Verhaegen, Michel
2014ApOpt..53.4565K Altcode: 2014arXiv1406.1006K
We present two complementary algorithms suitable for using
focal-plane measurements to control a wavefront corrector with
an extremely high spatial resolution. The algorithms use linear
approximations to iteratively minimize the aberrations seen by the
focal-plane camera. The first algorithm, Fast & Furious (FF),
uses a weak-aberration assumption and pupil symmetries to achieve fast
wavefront reconstruction. The second algorithm, an extension to FF,
can deal with an arbitrary pupil shape; it uses a Gerchberg-Saxton
style error reduction to determine the pupil amplitudes. Simulations and
experimental results are shown for a spatial light modulator controlling
the wavefront with a resolution of 170 x 170 pixels. The algorithms
increase the Strehl ratio from ~0.75 to 0.98-0.99, and the intensity
of the scattered light is reduced throughout the whole recorded image
of 320 x 320 pixels. The remaining wavefront rms error is estimated
to be ~0.15 rad with FF and ~0.10 rad with FF-GS.
---------------------------------------------------------
Title: The ZIMPOL high contrast imaging polarimeter for SPHERE:
system test results
Authors: Roelfsema, Ronald; Bazzon, Andreas; Schmid, Hans Martin;
Pragt, Johan; Gisler, Daniel; Dominik, Carsten; Baruffolo, Andrea;
Beuzit, Jean-Luc; Costille, Anne; Dohlen, Kjetil; Downing, Mark;
Elswijk, Eddy; de Haan, Menno; Hubin, Norbert; Kasper, Markus; Keller,
Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov, Alexey; Puget,
Pascal; Salasnich, Bernardo; Sauvage, Jean-Francois; Wildi, Francois
2014SPIE.9147E..3WR Altcode:
SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is a new
instrument for the VLT aimed at the direct detection of exo-planets. It
has received its first light in May 2014. ZIMPOL (Zurich Imaging
Polarimeter) is the imaging polarimeter subsystem of the SPHERE
instrument. It's capable of both high accuracy and high sensitivity
polarimetry but can also be used as a classical imager. It is located
behind an extreme AO system and a stellar coronagraph. ZIMPOL operates
at visible wavelengths (600-900 nm) which is best suited to detect
the very faint reflected and hence polarized visible light from extra
solar planets. It has an instantaneous Field of View of 3 x 3 arcsec2
(extendable to 8 arcsec diameter) with an angular resolution of 14
mili-arcsec. We discuss the results that are obtained from the full
SPHERE-ZIMPOL system testing. In particular the optical, polarimetric
and high contrast performance.
---------------------------------------------------------
Title: Characterizing instrumental effects on polarization at a
Nasmyth focus using NaCo
Authors: de Boer, Jozua; Girard, Julien H.; Mawet, Dimitri; Snik,
Frans; Keller, Christoph U.; Milli, Julien
2014SPIE.9147E..87D Altcode:
We propose a new calibration scheme to determine the instrumental
polarization (IP) and crosstalk induced by either the telescope or
an instrument at Nasmyth focus. We measure the polarized blue sky at
zenith with VLT/UT4/NaCo for different NaCo derotator and telescope
azimuth angles. Taking multiple measurements after rotating both the
instrument and the telescope with angles of 90° allows use to determine
the IP and most crosstalk components separately for the telescope and
the instrument. This separation of the Mueller matrices of UT4 and the
NaCo is especially important for measurements taken in the conventional
polarimetric mode (field stabilized), because the rotation of the
instrument with respect to M3 causes a variation in the IP and crosstalk
throughout the measurement. The technique allows us to determine the
IP with an accuracy of 0.4%, and constrain or determine lower or upper
limits for most crosstalk components. Most notably, the UT4 U -->
V crosstalk is substantially larger than theory predicts. An angular
offset in NaCo's half wave plate orientation is a possible source of
systematic errors. We measure this offset to be 1.8° +/- 0.5°.
---------------------------------------------------------
Title: LOUPE: Spectropolarimetry of the Earth from the surface of
the Moon
Authors: Hoeijmakers, H. J.; Snik, F.; Stam, D. M.; Keller, C. U.
2014EPSC....9..574H Altcode:
We present our prototype for the LOUPE instrument: A small and robust
imaging spectropolarimeter that can observe the Earth from the surface
of the moon, with as primary objective to characterize the Earth's
linear polarization spectrum throughout the Earth's daily rotation
and monthly phase angle changes. The purpose of LOUPE is to provide
benchmark data for future polarization observations of possibly
habitable exoplanets. Our instrument concept has been proven to work
in a laboratory setting, and efforts are being made to design and
produce a flight model.
---------------------------------------------------------
Title: Towards Polarimetric Exoplanet Imaging with ELTs
Authors: Keller, C. U.; Korkiakoski, V.; Rodenhuis, M.; Snik, F.
2014ebi..conf..4.6K Altcode:
A prime science goal of Extremely Large Telescopes (ELTs) is the
detection and characterization of exoplanets to answer the question: are
we alone? ELTs will obtain the first direct images of rocky exoplanets
in the habitable zone and search for atmospheric biomarkers. However,
the required instrumental technologies are not yet at a level where an
instrument could be built that would achieve this goal. Polarimetry will
be an important ingredient in future high-contrast instruments as it
will provide a major contrast improvement for planets located within
the first two Airy rings and offers unique diagnostic capabilities
for liquid water (ocean glint, water clouds and their rainbows),
hazes and dust in exoplanetary atmospheres. <P />We will describe
novel instrumental approaches to improving subsystems, in particular
polarimetry, wavefront sensing and adaptive optics control. To reach
contrasts of 10-9 and beyond to image rocky exoplanets from the ground,
a series of individually optimized subsystems cannot succeed; rather,
entire combinations of subsystems must be optimized together. We will
describe our efforts at measuring and controlling wavefronts with 40'000
degrees of freedom, reaching the photon-noise limit in high-contrast
polarimetric imaging at telescopes and our plans to reach a contrast of
at least 10-9 in broadband light under realistic, simulated ground-based
conditions in the laboratory and to test new approaches at telescopes,
in particular achromatic aperture and focal-plane coronagraphs,
focal-plane wavefront-sensing and speckle suppression, integral-field
polarimetry and high-contrast data reduction algorithms.
---------------------------------------------------------
Title: Instrumental polarisation at the Nasmyth focus of the E-ELT
Authors: de Juan Ovelar, M.; Snik, F.; Keller, C. U.; Venema, L.
2014A&A...562A...8D Altcode: 2013arXiv1312.6148D; 2013arXiv1312.6148O
The ~39-m European Extremely Large Telescope (E-ELT) will be the
largest telescope ever built. This makes it particularly suitable for
sensitive polarimetric observations, as polarimetry is a photon-starved
technique. However, the telescope mirrors may severely limit the
polarimetric accuracy of instruments on the Nasmyth platforms by
creating instrumental polarisation and/or modifying the polarisation
signal of the object. In this paper we characterise the polarisation
effects of the two currently considered designs for the E-ELT Nasmyth
ports as well as the effect of ageing of the mirrors. By means of
the Mueller matrix formalism, we compute the response matrices of each
mirror arrangement for a range of zenith angles and wavelengths. We then
present two techniques to correct for these effects that require the
addition of a modulating device at the "polarisation-free" intermediate
focus that acts either as a switch or as a part of a two-stage
modulator. We find that the values of instrumental polarisation,
Stokes transmission reduction and cross-talk vary significantly
with wavelength, and with pointing, for the lateral Nasmyth case,
often exceeding the accuracy requirements for proposed polarimetric
instruments. Realistic ageing effects of the mirrors after perfect
calibration of these effects may cause polarimetric errors beyond the
requirements. We show that the modulation approach with a polarimetric
element located in the intermediate focus reduces the instrumental
polarisation effects down to tolerable values, or even removes them
altogether. The E-ELT will be suitable for sensitive and accurate
polarimetry, provided frequent calibrations are carried out, or a
dedicated polarimetric element is installed at the intermediate focus.
---------------------------------------------------------
Title: Analysis and interpretation of 15 quarters of Kepler data of
the disintegrating planet KIC 12557548 b
Authors: van Werkhoven, T. I. M.; Brogi, M.; Snellen, I. A. G.;
Keller, C. U.
2014A&A...561A...3V Altcode: 2013arXiv1311.5688V
Context. The Kepler object KIC 12557548 shows irregular eclipsing
behaviour with a constant 15.685 h period, but strongly varying
transit depth. The object responsible for this is believed to be
a disintegrating planet forming a trailing dust cloud transiting
the star. A 1D model of an exponentially decaying dust tail was
found to reproduce the average eclipse in intricate detail. Based
on radiative hydrodynamic modelling, the upper limit for the planet
mass was found to be twice the mass of the Moon. <BR /> Aims: In this
paper we fit individual eclipses, in addition to fitting binned light
curves, to learn more about the process underlying the eclipse depth
variation. Additionally, we put forward observational constraints that
any model of this planet-star system will have to match. <BR /> Methods:
We manually de-correlated and de-trended 15 quarters of Kepler data,
three of which were observed in short cadence mode. We determined the
transit depth, egress depth, and stellar intensity for each orbit and
search for dependencies between these parameters. We investigated the
full orbit by comparing the flux distribution of a moving phase window
of interest versus the out-of-eclipse flux distribution. We fit short
cadence data on a per-orbit basis using a two-parameter tail model,
allowing us to investigate potential dust tail property variations. <BR
/> Results: We find two quiescent spells of ~30 orbital periods each
where the transit depth is <0.1%, followed by relatively deep
transits. Additionally, we find periods of on-off behaviour where
>0.5% deep transits are followed by apparently no transit at
all. Apart from these isolated events we find neither significant
correlation between consecutive transit depths nor a correlation
between transit depth and stellar intensity. We find a three-sigma
upper limit for the secondary eclipse of 4.9 × 10<SUP>-5</SUP>,
consistent with a planet candidate with a radius of less than 4600
km. Using the short cadence data we find that a 1D exponential dust tail
model is insufficient to explain the data. We improved our model to a
2D, two-component dust model with an opaque core and an exponential
tail. Using this model we fit individual eclipses observed in short
cadence mode. We find an improved fit of the data, quantifying earlier
suggestions by Budaj (2013, A&A, 557, A72) of the necessity of at
least two components. We find that deep transits have most absorption
in the tail, and not in a disk-shaped, opaque coma, but the transit
depth and the total absorption show no correlation with the tail length.
---------------------------------------------------------
Title: Imaging the circumstellar environment of the young T Tauri
star SU Aurigae
Authors: Jeffers, S. V.; Min, M.; Canovas, H.; Rodenhuis, M.; Keller,
C. U.
2014A&A...561A..23J Altcode: 2013arXiv1311.4832J
The circumstellar environments of classical T Tauri stars are
challenging to directly image because of their high star-to-disk
contrast ratio. One method to overcome this is by using imaging
polarimetry where scattered and consequently polarised starlight from
the star's circumstellar disk can be separated from the unpolarised
light of the central star. We present images of the circumstellar
environment of SU Aur, a classical T Tauri star at the transition of T
Tauri to Herbig stars. The images directly show that the disk extends
out to 500 AU with an inclination angle of ~50°. Using interpretive
models, we derived very small grains in the surface layers of its disk,
with a very steep size- and surface-density distribution. Additionally,
we resolved a large and extended nebulosity in our images that is most
likely a remnant of the prenatal molecular cloud. The position angle
of the disk, determined directly from our images, rules out a polar
outflow or jet as the cause of this large-scale nebulosity.
---------------------------------------------------------
Title: Calibrating a high-resolution wavefront corrector with a
static focal-plane camera
Authors: Korkiakoski, Visa; Doelman, Niek; Codona, Johanan; Kenworthy,
Matthew; Otten, Gilles; Keller, Christoph U.
2013ApOpt..52.7554K Altcode: 2013arXiv1310.1241K
We present a method to calibrate a high-resolution wavefront-correcting
device with a single, static camera, located in the focal plane; no
moving of any component is needed. The method is based on a localized
diversity and differential optical transfer functions (dOTF) to compute
both the phase and amplitude in the pupil plane located upstream of the
last imaging optics. An experiment with a spatial light modulator shows
that the calibration is sufficient to robustly operate a focal-plane
wavefront sensing algorithm controlling a wavefront corrector with ~40
000 degrees of freedom. We estimate that the locations of identical
wavefront corrector elements are determined with a spatial resolution
of 0.3% compared to the pupil diameter.
---------------------------------------------------------
Title: Three-dimensional magnetic and abundance mapping of the cool
Ap star HD 24712 . I. Spectropolarimetric observations in all four
Stokes parameters
Authors: Rusomarov, N.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Makaganiuk, V.; Rodenhuis, M.;
Snik, F.; Stempels, H. C.; Valenti, J. A.
2013A&A...558A...8R Altcode: 2013arXiv1306.0997R
Context. High-resolution spectropolarimetric observations provide
simultaneous information about stellar magnetic field topologies and
three-dimensional distributions of chemical elements. High-quality
spectra in the Stokes IQUV parameters are currently available for very
few early-type magnetic chemically peculiar stars. Here we present
analysis of a unique full Stokes vector spectropolarimetric data
set, acquired for the cool magnetic Ap star HD 24712 with a recently
commissioned spectropolarimeter. <BR /> Aims: The goal of our work
is to examine the circular and linear polarization signatures inside
spectral lines and to study variation of the stellar spectrum and
magnetic observables as a function of rotational phase. <BR /> Methods:
HD 24712 was observed with the HARPSpol instrument at the 3.6-m ESO
telescope over a period of 2010-2011. We achieved full rotational phase
coverage with 43 individual Stokes parameter observations. The resulting
spectra have a signal-to-noise ratio of 300-600 and resolving power
exceeding 10<SUP>5</SUP>. The multiline technique of least-squares
deconvolution (LSD) was applied to combine information from the
spectral lines of Fe-peak and rare earth elements. <BR /> Results: We
used the HARPSPol spectra of HD 24712 to study the morphology of the
Stokes profile shapes in individual spectral lines and in LSD Stokes
profiles corresponding to different line masks. From the LSD Stokes V
profiles we measured the longitudinal component of the magnetic field,
⟨B<SUB>z</SUB>⟩, with an accuracy of 5-10 G. We also determined the
net linear polarization from the LSD Stokes Q and U profiles. Combining
previous ⟨B<SUB>z</SUB>⟩ measurements with our data allowed us to
determine an improved rotational period of the star, P<SUB>rot</SUB>
= 12.45812 ± 0.00019 d. We also measured the longitudinal
magnetic field from the cores of Hα and Hβ lines. The analysis of
⟨B<SUB>z</SUB>⟩ measurements showed no evidence for a significant
radial magnetic field gradient in the atmosphere of HD 24712. We used
our ⟨B<SUB>z</SUB>⟩ and net linear polarization measurements to
determine parameters of the dipolar magnetic field topology. We found
that magnetic observables can be reasonably well reproduced by the
dipolar model, although significant discrepancies remain at certain
rotational phases. We discovered rotational modulation of the Hα
core and related it to a non-uniform surface distribution of rare
earth elements. <P />Based on observations collected at the European
Southern Observatory, Chile (ESO programs 084.D-0338, 085.D-0296,
086.D-0240).Figure 3 and Appendix A are available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: The color dependent morphology of the post-AGB star HD 161796
Authors: Min, M.; Jeffers, S. V.; Canovas, H.; Rodenhuis, M.; Keller,
C. U.; Waters, L. B. F. M.
2013A&A...554A..15M Altcode: 2013arXiv1303.1704M
Context. Many protoplanetary nebulae show strong asymmetries in their
surrounding shells, pointing to asymmetries during the mass loss
phase. Questions concerning the origin and the onset of deviations
from spherical symmetry are important for our understanding of the
evolution of these objects. Here we focus on the circumstellar shell of
the post-AGB star HD 161796. <BR /> Aims: We aim to detect signatures of
an aspherical outflow, and to derive its properties. <BR /> Methods: We
used the imaging polarimeter the Extreme Polarimeter (ExPo), a visitor
instrument at the William Herschel Telescope, to accurately image the
dust shell surrounding HD 161796 in various wavelength filters. Imaging
polarimetry allows us to separate the faint, polarized, light that comes
from circumstellar material from the bright, unpolarized, light from
the central star. <BR /> Results: The shell around HD 161796 is highly
aspherical. A clear signature of an equatorial density enhancement can
be seen. This structure is optically thick at short wavelengths and
changes its appearance to optically thin at longer wavelengths. In the
classification of the two different appearances of planetary nebulae
from HST images it changes from being classified as DUst-Prominent
Longitudinally-EXtended (DUPLEX) at short wavelengths to star-obvious
low-level-elongated (SOLE) at longer wavelengths. This strengthens
the interpretation that these two appearances are manifestations of
the same physical structure. Furthermore, we find that the central
star is hotter than often assumed and the relatively high observed
reddening is a consequence of circumstellar rather than interstellar
extinction. <P />Based on observations made with the William Herschel
Telescope operated on the island of La Palma by the Isaac Newton Group
in the Spanish Observatorio del Roque de los Muchachos of the Instituto
de Astrofsicaŋsica de Canarias.
---------------------------------------------------------
Title: Are there tangled magnetic fields on HgMn stars?
Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels,
H. C.; Valenti, J. A.
2013A&A...554A..61K Altcode: 2013arXiv1304.6717K
Context. Several recent spectrophotometric studies failed to detect
significant global magnetic fields in late-B HgMn chemically peculiar
stars, but some investigations have suggested the presence of strong
unstructured or tangled fields in these objects. <BR /> Aims: We used
detailed spectrum synthesis analysis to search for evidence of tangled
magnetic fields in high-quality observed spectra of eight slowly
rotating HgMn stars and one normal late-B star. We also evaluated
recent sporadic detections of weak longitudinal magnetic fields in
HgMn stars based on the moment technique. <BR /> Methods: Our spectrum
synthesis code calculated the Zeeman broadening of metal lines in HARPS
spectra, assuming an unstructured, turbulent magnetic field. A simple
line formation model with a homogeneous radial field distribution was
applied to assess compatibility between previous longitudinal field
measurements and the observed mean circular polarization signatures. <BR
/> Results: Our analysis of the Zeeman broadening of magnetically
sensitive spectral lines reveals no evidence of tangled magnetic fields
in any of the studied HgMn or normal stars. We infer upper limits
of 200-700 G for the mean magnetic field modulus - much smaller than
the field strengths implied by studies based on differential magnetic
line intensification and quadratic field diagnostics. The new HARPSpol
longitudinal field measurements for the extreme HgMn star HD 65949 and
the normal late-B star 21 Peg are consistent with zero at a precision of
3-6 G. Re-analysis of our Stokes V spectra of the spotted HgMn star HD
11753 shows that the recent moment technique measurements retrieved from
the same data are incompatible with the lack of circular polarization
signatures in the spectrum of this star. <BR /> Conclusions: We conclude
that there is no evidence for substantial tangled magnetic fields on
the surfaces of studied HgMn stars. We cannot independently confirm
the presence of very strong quadratic or marginal longitudinal fields
for these stars, so results from the moment technique are likely to be
spurious. <P />Based on observations collected at the European Southern
Observatory, Chile (ESO programmes 084.D-0338, 085.D-0296, 086.D-0240).
---------------------------------------------------------
Title: Magnetically Controlled Accretion on the Classical T Tauri
Stars GQ Lupi and TW Hydrae
Authors: Johns-Krull, Christopher M.; Chen, Wei; Valenti, Jeff A.;
Jeffers, Sandra V.; Piskunov, Nikolai E.; Kochukhov, Oleg; Makaganiuk,
V.; Stempels, H. C.; Snik, Frans; Keller, Christoph; Rodenhuis, M.
2013ApJ...765...11J Altcode: 2013arXiv1301.3182J
We present high spectral resolution (R ≈ 108, 000) Stokes V
polarimetry of the classical T Tauri stars (CTTSs) GQ Lup and TW Hya
obtained with the polarimetric upgrade to the HARPS spectrometer on
the ESO 3.6 m telescope. We present data on both photospheric lines
and emission lines, concentrating our discussion on the polarization
properties of the He I emission lines at 5876 Å and 6678 Å. The He
I lines in these CTTSs contain both narrow emission cores, believed
to come from near the accretion shock region on these stars, and broad
emission components which may come from either a wind or the large-scale
magnetospheric accretion flow. We detect strong polarization in the
narrow component of the two He I emission lines in both stars. We
observe a maximum implied field strength of 6.05 ± 0.24 kG in the
5876 Å line of GQ Lup, making it the star with the highest field
strength measured in this line for a CTTS. We find field strengths in
the two He I lines that are consistent with each other, in contrast to
what has been reported in the literature on at least one star. We do
not detect any polarization in the broad component of the He I lines
on these stars, strengthening the conclusion that they form over a
substantially different volume relative to the formation region of
the narrow component of the He I lines.
---------------------------------------------------------
Title: Sterrekundig Instituut Utrecht: The Last Years
Authors: Keller, C. U.
2013ASPC..470....3K Altcode: 2012arXiv1208.4095K
I describe the last years of the 370-year long life of the Sterrekundig
Instituut Utrecht, which was the second-oldest university observatory in
the world and was closed in early 2012 after the Faculty of Science and
the Board of Utrecht University decided, without providing qualitative
or quantitative arguments, to remove astrophysics from its research
and education portfolio.
---------------------------------------------------------
Title: Observing Circumstellar Neighbourhoods with the Extreme
Polarimeter
Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Min, M.; Keller,
C. U.
2013ASPC..470..407R Altcode:
The study of circumstellar environments at ever higher contrasts has
generated considerable interest in recent years. One method to increase
the contrast is to observe the linearly polarized light scattered by
the circumstellar material while suppressing the unpolarized stellar
flux. This paper presents some sample imaging polarimetry results
obtained with the Extreme Polarimeter (ExPo). ExPo operates in the
visible part of the spectrum, and currently achieves a polarimetric
sensitivity of 10<SUP>-4</SUP>. Despite the demise of the Utrecht
Astronomical Institute, where this instrument was developed, the
instrument is still being used and upgraded. It has now moved to the
Leiden Observatory.
---------------------------------------------------------
Title: HARPS Spectropolarimetry of the Classical T Tauri Stars GQ
Lup and TW Hya
Authors: Johns-Krull, Christopher M.; Chen, W.; Valenti, J. A.;
Jeffers, S. V.; Piskunov, N. E.; Kochukhov, O.; Makaganiuk, V.;
Stempels, H. C.; Snik, F.; Keller, C.; Rodenhuis, M.
2013AAS...22125614J Altcode:
We present high spectral resolution Stokes V polarimetery of the
Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with
the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m
telescope. We present data on both photospheric lines and emission
lines, concentrating our discussion on the polarization properties
of the He I emission lines at 5876 A and 6678 A. The He I lines
in both these CTTS contain both narrow emission cores, believed to
come from near the accretion shock region on these stars, and broad
emission components which may come from either a wind or the large
scale magnetospheric accretion flow. We detect strong polarization in
the narrow component of both the He I emission lines in both stars. We
observe a maximum implied field strength of 5.8 +/- 0.3 kG in the 5876
A line of GQ Lup, the highest field strength measured to date in this
line for a CTTS. We find field strengths in the two He I lines that
are consistent with each other, unlike what has been reported in the
literature on at least one star. We do not detect any polarization in
the broad component of the He I lines on these stars, strengthening
the conclusion that they form over a substantially different volume
relative the formation region of the narrow component of the He I lines.
---------------------------------------------------------
Title: Astronomical Polarimetry: Polarized Views of Stars and Planets
Authors: Snik, Frans; Keller, Christoph U.
2013pss2.book..175S Altcode:
Polarization is a fundamental property of light from astronomical
objects, and measuring that polarization often yields crucial
information, which is unobtainable otherwise.This chapter reviews the
useful formalisms for describing polarization in the optical regime,
the mechanisms for the creation of such polarization, and methods
for measuring it. Particular emphasis is given on how to implement a
polarimeter within an astronomical facility, and on how to deal with
systematic effects that often limit the polarimetric performance.
---------------------------------------------------------
Title: Observing the Earth as an exoplanet with LOUPE, the lunar
observatory for unresolved polarimetry of Earth
Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Bagnulo, S.; Sparks,
W. B.; Keller, C. U.
2012P&SS...74..202K Altcode: 2012arXiv1203.0209K
The detections of small, rocky exoplanets have surged in recent years
and will likely continue to do so. To know whether a rocky exoplanet
is habitable, we have to characterize its atmosphere and surface. A
promising characterization method for rocky exoplanets is direct
detection using spectropolarimetry. This method will be based on single
pixel signals, because spatially resolving exoplanets is impossible with
current and near-future instruments. Well-tested retrieval algorithms
are essential to interpret these single pixel signals in terms of
atmospheric composition, cloud and surface coverage. Observations
of Earth itself provide the obvious benchmark data for testing such
algorithms. The observations should provide signals that are integrated
over the Earth's disk, that capture day and night variations, and
all phase angles. The Moon is a unique platform from where the Earth
can be observed as an exoplanet, undisturbed, all of the time. Here,
we present LOUPE, the Lunar Observatory for Unresolved Polarimetry of
Earth, a small and robust spectropolarimeter to observe our Earth as
an exoplanet.
---------------------------------------------------------
Title: Electric Properties of Water Ice doped with Hydrogen Peroxide
(H2O2): Implications for Icy Moons such as Europa
Authors: Keller, C.; Freund, F. T.; Cruikshank, D. P.
2012AGUFM.P51A2026K Altcode:
Large floats of ice on Jupiter's moon Europa drift and collide. The
float boundaries are marked by brownish-reddish colors. The origin
of these colors is poorly understood. Maybe upwelling of water along
the active float boundaries brings finely divided suspended matter
or organic compounds from the ocean below to the surface, where
the intense, high energy environment in Jupiter's radiation belt
would lead to photochemical oxidation. At the same time it has been
suggested that Europa's ice contains traces of H2O2, presumably due to
micro-meteorite impacts and other processes. We measured the electric
currents generated in pure and H2O2-doped water ice when we subjected
one end of ice blocks to uniaxial stress. Ice samples with 0%, 0.3%
and 0.03% H2O2 were formed in polyethylene troughs, 4.1 x 13.5 x 3.8 cm,
with Cu contacts at both ends, at 263K (-10°C), 190K (-78°C, dry ice)
and 77K (-196°C,liquid N2). At 77K the ice samples detached themselves
from at least one of the Cu contacts, due to thermal contraction. At
190K, when stressing one end, essentially no currents were produced
in the pure water ice. By contrast, H2O2-doped ices produced several
hundred picoamperes (pA) of positive currents, indicating defect
electrons (holes) flowing down the stress gradient. At 263K the
results are ambiguous. These (as yet preliminary) results indicate
that stresses might break the peroxy bonds of imbedded H2O2 molecules,
releasing the same type of positive hole charge carriers as observed
during stress experiments with silicate rocks. Since positive holes
are defect electrons associated with O 2sp levels at the upper edge of
the valence band, they seem to have the capability to spread through
the ices. Chemically positive holes are equivalent to highly oxidizing
oxygen radicals. They may be responsible for oxidation reactions along
the boundaries of active ice floats on Europa.
---------------------------------------------------------
Title: Unusual Stokes V profiles during flaring activity of a
delta sunspot
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.; Fletcher, L.;
Socas-Navarro, H.
2012A&A...547A..34F Altcode: 2012arXiv1209.0983F
<BR /> Aims: We analyze a set of full Stokes profile observations of
the flaring active region NOAA 10808. The region was recorded with
the Vector-Spectromagnetograph of the Synoptic Optical Long-term
Investigations of the Sun facility. The active region produced
several successive X-class flares between 19:00 UT and 24:00 UT on
September 13, 2005 and we aim to quantify transient and permanent
changes in the magnetic field and velocity field during one of the
flares, which has been fully captured. <BR /> Methods: The Stokes
profiles were inverted using the height-dependent inversion code
LILIA to analyze magnetic field vector changes at the flaring site. We
report multilobed asymmetric Stokes V profiles found in the δ-sunspot
umbra. We fit the asymmetric Stokes V profiles assuming an atmosphere
consisting of two components (SIR inversions) to interpret the profile
shape. The results are put in context with Michelson Doppler Imager
(MDI) magnetograms and reconstructed X-ray images from the Reuven
Ramaty High Energy Solar Spectroscopic Imager. <BR /> Results: We
obtain the magnetic field vector and find signs of restructuring
of the photospheric magnetic field during the flare close to the
polarity inversion line at the flaring site. At two locations in the
umbra we encounter strong fields (~3 kG), as inferred from the Stokes
I profiles, which, however, exhibit a low polarization signal. During
the flare we observe in addition asymmetric Stokes V profiles at one
of these sites. The asymmetric Stokes V profiles appear co-spatial
and co-temporal with a strong apparent polarity reversal observed
in MDI-magnetograms and a chromospheric hard X-ray source. The
two-component atmosphere fits of the asymmetric Stokes profiles
result in line-of-sight velocity differences in the range of ~12 km
s<SUP>-1</SUP> to 14 km s<SUP>-1</SUP> between the two components in
the photosphere. Another possibility is that local atmospheric heating
is causing the observed asymmetric Stokes V profile shape. In either
case our analysis shows that a very localized patch of ~5″ in the
photospheric umbra, co-spatial with a flare footpoint, exhibits a
subresolution fine structure.
---------------------------------------------------------
Title: SPICES: spectro-polarimetric imaging and characterization of
exoplanetary systems. From planetary disks to nearby Super Earths
Authors: Boccaletti, Anthony; Schneider, Jean; Traub, Wes; Lagage,
Pierre-Olivier; Stam, Daphne; Gratton, Raffaele; Trauger, John;
Cahoy, Kerri; Snik, Frans; Baudoz, Pierre; Galicher, Raphael; Reess,
Jean-Michel; Mawet, Dimitri; Augereau, Jean-Charles; Patience,
Jenny; Kuchner, Marc; Wyatt, Mark; Pantin, Eric; Maire, Anne-Lise;
Vérinaud, Christophe; Ronayette, Samuel; Dubreuil, Didier; Min,
Michiel; Rodenhuis, Michiel; Mesa, Dino; Belikov, Russ; Guyon, Olivier;
Tamura, Motohide; Murakami, Naoshi; Beerer, Ingrid Mary; SPICES Team;
Mas, M.; Rouan, D.; Perrin, G.; Lacour, S.; Thébault, P.; Nguyen,
N.; Ibgui, L.; Arenou, F.; Lestrade, J. F.; N'Diaye, M.; Dohlen, K.;
Ferrari, M.; Hugot, E.; Beuzit, J. -L.; Lagrange, A. -M.; Martinez,
P.; Barthelemey, M.; Mugnier, L.; Keller, C.; Marley, M.; Kalas, P.;
Stapelfeldt, K.; Brown, R.; Kane, S.; Desidera, S.; Sozzetti, A.;
Mura, A.; Martin, E. L.; Bouy, H.; Allan, A.; King, R.; Vigan, A.;
Churcher, L.; Udry, S.; Matsuo, T.; Nishikawa, J.; Hanot, C.; Wolf,
S.; Kaltenegger, L.; Klahr, H.; Pilat-Lohinger, E.
2012ExA....34..355B Altcode: 2012arXiv1203.0507B; 2012ExA...tmp....8B
SPICES (Spectro-Polarimetric Imaging and Characterization of
Exoplanetary Systems) is a five-year M-class mission proposed to ESA
Cosmic Vision. Its purpose is to image and characterize long-period
extrasolar planets and circumstellar disks in the visible (450-900
nm) at a spectral resolution of about 40 using both spectroscopy and
polarimetry. By 2020/2022, present and near-term instruments will have
found several tens of planets that SPICES will be able to observe
and study in detail. Equipped with a 1.5 m telescope, SPICES can
preferentially access exoplanets located at several AUs (0.5-10 AU)
from nearby stars (<25 pc) with masses ranging from a few Jupiter
masses to Super Earths (∼2 Earth radii, ∼10 M<SUB>⊕</SUB>)
as well as circumstellar disks as faint as a few times the zodiacal
light in the Solar System.
---------------------------------------------------------
Title: A spectro-polarimetric integral field spectrograph for
EPICS-EPOL
Authors: Rodenhuis, M.; Sprenger, B.; Keller, C. U.
2012SPIE.8446E..9JR Altcode:
Imaging polarimetry offers a way to increase the contrast of light
scattered from circumstellar material, enabling direct observation of
exoplanets -possibly rocky- with the E-ELT. To actually characterize
these planets, some spectral resolution is essential. With sufficient
resolution -both spectral and spatial- the spectral differential
imaging technique can be used in addition to the polarimetry to
detect circumstellar point sources. We present the concept for a
spectro-polarimetric integral field spectrograph for the EPICS-EPOL
instrument and our current efforts to demonstrate this concept with
our existing imaging polarimeter ExPo.
---------------------------------------------------------
Title: Evidence for the disintegration of KIC 12557548 b
Authors: Brogi, M.; Keller, C. U.; de Juan Ovelar, M.; Kenworthy,
M. A.; de Kok, R. J.; Min, M.; Snellen, I. A. G.
2012A&A...545L...5B Altcode: 2012arXiv1208.2988B
Context. The Kepler object KIC 12557548 b is peculiar. It exhibits
transit-like features every 15.7 h that vary in depth between 0.2% and
1.2%. Rappaport et al. (2012, ApJ, 752, 1) explain the observations
in terms of a disintegrating, rocky planet that has a trailing
cloud of dust created and constantly replenished by thermal surface
erosion. The variability of the transit depth is then a consequence
of changes in the cloud optical depth. <BR /> Aims: We aim to
validate the disintegrating-planet scenario by modeling the detailed
shape of the observed light curve, and thereby constrain the cloud
particle properties to better understand the nature of this intriguing
object. <BR /> Methods: We analyzed the six publicly-available quarters
of raw Kepler data, phase-folded the light curve and fitted it to
a model for the trailing dust cloud. Constraints on the particle
properties were investigated with a light-scattering code. <BR />
Results: The light curve exhibits clear signatures of light scattering
and absorption by dust, including a brightening in flux just before
ingress correlated with the transit depth and explained by forward
scattering, and an asymmetry in the transit light curve shape, which
is easily reproduced by an exponentially decaying distribution of
optically thin dust, with a typical grain size of 0.1 μm. <BR />
Conclusions: Our quantitative analysis supports the hypothesis that the
transit signal of KIC 12557548 b is due to a variable cloud of dust,
most likely originating from a disintegrating object.
---------------------------------------------------------
Title: Searching for signs of habitability with LOUPE, the Lunar
Observatory of Unresolved Polarimetry of Earth
Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Bagnulo, S.; Sparks,
W. B.; Keller, C. U.
2012epsc.conf..537K Altcode: 2012espc.conf..537K
We present LOUPE, a novel type of spectropolarimeter to measure the
flux and state of polarization of sunlight that is reflected by the
Earth from 0.4 to 0.8 μm. LOUPE has been designed as payload of a
lunar lander. From the moon, the Earth can be observed as a whole,
during its daily rotation and at all phase angles, just as if it were
an exoplanet. LOUPE will provide benchmark data for the development
of instruments for Earth-like exoplanet characterization, and for the
testing of numerical retrieval algorithms.
---------------------------------------------------------
Title: The extreme polarimeter: design, performance, first results
and upgrades
Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; de Juan Ovelar,
Maria; Min, M.; Homs, L.; Keller, C. U.
2012SPIE.8446E..9IR Altcode: 2012arXiv1211.6300R
Well over 700 exoplanets have been detected to date. Only a handful of
these have been observed directly. Direct observation is extremely
challenging due to the small separation and very large contrast
involved. Imaging polarimetry offers a way to decrease the contrast
between the unpolarized starlight and the light that has become
linearly polarized after scattering by circumstellar material. This
material can be the dust and debris found in circumstellar disks, but
also the atmosphere or surface of an exoplanet. We present the design,
calibration approach, polarimetric performance and sample observation
results of the Extreme Polarimeter, an imaging polarimeter for the
study of circumstellar environments in scattered light at visible
wavelengths. The polarimeter uses the beam-exchange technique, in
which the two orthogonal polarization states are imaged simultaneously
and a polarization modulator is swaps the polarization states of the
two beams before the next image is taken. The instrument currently
operates without the aid of Adaptive Optics. To reduce the effects
of atmospheric seeing on the polarimetry, the images are taken at
a frame rate of 35 fps, and large numbers of frames are combined to
obtain the polarization images. Four successful observing runs have
been performed using this instrument at the 4.2 m William Herschel
Telescope on La Palma, targeting young stars with protoplanetary disks
as well as evolved stars surrounded by dusty envelopes. In terms of
fractional polarization, the instrument sensitivity is better than
10<SUP>-4</SUP>. The contrast achieved between the central star and
the circumstellar source is of the order 10<SUP>-6</SUP>. We show that
our calibration approach yields absolute polarization errors below 1%.
---------------------------------------------------------
Title: Modeling the instrumental polarization of the VLT and E-ELT
telescopes with the M&m's code
Authors: de Juan Ovelar, M.; Diamantopoulou, S.; Roelfsema, R.;
van Werkhoven, T.; Snik, F.; Pragt, Johan; Keller, C.
2012SPIE.8449E..12D Altcode:
Polarimetry is a particularly powerful technique when imaging
circumstellar environments. Currently most telescopes include more
or less advanced polarimetric facilities and large telescopes count
on it for their planet-finder instruments like SPHERE-ZIMPOL on the
VLT or EPICS on the future E-ELT. One of the biggest limitations
of this technique is the instrumental polarization (IP) generated
in the telescope optical path, which can often be larger than the
signal to be measured. In most cases this instrumental polarization
changes over time and is dependent on the errors affecting the optical
elements of the system. We have modeled the VLT and E-ELT telescope
layouts to characterize the instrumental polarization generated
on their optical paths using the M&m's code, an error budget
and performance simulator for polarimetric systems. In this study
we present the realistic Mueller matrices calculated with M&m's
for both systems, with and without the setups to correct for the IP,
showing that correction can be achieved, allowing for an accurate
polarimetric performance.
---------------------------------------------------------
Title: Potential of phase-diversity for metrology of active
instruments
Authors: Korkiakoski, Visa; Venema, Lars; Agocs, Tibor; Keller,
Christoph U.; Doelman, Niek; Fraanje, Rufus; Andrei, Raluca; Verhaegen,
Michel
2012SPIE.8450E..5DK Altcode:
We investigate the potential of phase-diversity (PD) and
Gerchberg-Saxton (GS) algorithms in the calibration of active
instruments. A set of images is recorded with the focal-plane
scientific camera, each image having a known and unique defocus. The
phase-retrieval algorithms are used, with those images, to estimate
the non-common path aberration that needs to be compensated by correct
alignment of the instrument. We demonstrate by numerical simulations
that such algorithms, in particular GS, are sufficient detection
methods to fully correct wavefronts with an rms error at least up to
6 rad — but this requires several iterative correction stages.
---------------------------------------------------------
Title: Constraining the circumbinary envelope of Z Canis Majoris
via imaging polarimetry
Authors: Canovas, H.; Min, M.; Jeffers, S. V.; Rodenhuis, M.; Keller,
C. U.
2012A&A...543A..70C Altcode: 2012arXiv1205.3784C
Context. Z CMa is a complex binary system composed of a Herbig Be
and an FU Ori star. The Herbig star is surrounded by a dust cocoon of
variable geometry, and the whole system is surrounded by an infalling
envelope. Previous spectropolarimetric observations have reported a
preferred orientation of the polarization angle, perpendicular to the
direction of a very extended, parsec-sized jet associated with the
Herbig star. <BR /> Aims: The variability in the amount of polarized
light has been associated to changes in the geometry of the dust cocoon
that surrounds the Herbig star. We aim to constrain the properties of
Z CMa by means of imaging polarimetry at optical wavelengths. <BR />
Methods: Using ExPo, a dual-beam imaging polarimeter that operates at
optical wavelengths, we have obtained imaging (linear) polarimetric
data of Z CMa. Our observations were secured during the return to
quiescence after the 2008 outburst. <BR /> Results: We detect three
polarized features over Z CMa. Two of these features are related to
the two jets reported in this system: the large jet associated to the
Herbig star, and the micro-jet associated to the FU Ori star. Our
results suggest that the micro-jet extends to a distance ten times
longer than reported in previous studies. The third feature suggests the
presence of a hole in the dust cocoon that surrounds the Herbig star
of this system. According to our simulations, this hole can produce
a pencil beam of light that we see scattered off the low-density
envelope surrounding the system. <P />Based on observations made with
the William Herschel Telescope operated on the island of La Palma by
the Isaac Newton Group in the Spanish Observatorio del Roque de los
Muchachos of the Instituto de Astrofísica de Canarias.
---------------------------------------------------------
Title: Linear analytical solution to the phase diversity problem
for extended objects based on the Born approximation
Authors: Andrei, Raluca M.; Smith, Carlas S.; Fraanje, Rufus;
Verhaegen, Michel; Korkiakoski, Visa A.; Keller, Christoph U.;
Doelman, Niek
2012SPIE.8447E..6UA Altcode:
In this paper we give a new wavefront estimation technique that
overcomes the main disadvantages of the phase diversity (PD)
algorithms, namely the large computational complexity and the fact
that the solutions can get stuck in a local minima. Our approach gives
a good starting point for an iterative algorithm based on solving a
linear system, but it can also be used as a new wavefront estimation
method. The method is based on the Born approximation of the wavefront
for small phase aberrations which leads to a quadratic point-spread
function (PSF), and it requires two diversity images. First we take the
differences between the focal plane image and each of the two diversity
images, and then we eliminate the constant object, element-wise,
from the two equations. The result is an overdetermined set of linear
equations for which we give three solutions using linear least squares
(LS), truncated total least squares (TTLS) and bounded data uncertainty
(BDU). The last two approaches are suited when considering measurements
affected by noise. Simulation results show that the estimation is
faster than conventional PD algorithms.
---------------------------------------------------------
Title: The Majorana Demonstrator: A Search for Neutrinoless
Double-beta Decay of Germanium-76
Authors: Wilkerson, J. F.; Aguayo, E.; Avignone, F. T., Iii; Back,
H. O.; Barabash, A. S.; Beene, J. R.; Bergevin, M.; Bertrand, F. E.;
Boswell, M.; Brudanin, V.; Busch, M.; Chan, Y. -D.; Christofferson,
C. D.; Collar, J. I.; Combs, D. C.; Cooper, R. J.; Detwiler, J. A.;
Doe, P. J.; Efremenko, Yu; Egorov, V.; Ejiri, H.; Elliott, S. R.;
Esterline, J.; Fast, J. E.; Fields, N.; Finnerty, P.; Fraenkle, F. M.;
Gehman, V. M.; Giovanetti, G. K.; Green, M. P.; Guiseppe, V. E.;
Gusey, K.; Hallin, A. L.; Hazama, R.; Henning, R.; Hoppe, E. W.;
Horton, M.; Howard, S.; Howe, M. A.; Johnson, R. A.; Keeter, K. J.;
Keller, C.; Kidd, M. F.; Knecht, A.; Kochetov, O.; Konovalov, S. I.;
Kouzes, R. T.; LaFerriere, B. D.; LaRoque, B. H.; Leon, J.; Leviner,
L. E.; Loach, J. C.; MacMullin, S.; Marino, M. G.; Martin, R. D.; Mei,
D. -M.; Merriman, J. H.; Miller, M. L.; Mizouni, L.; Nomachi, M.;
Orrell, J. L.; Overman, N. R.; Phillips, D. G., Ii; Poon, A. W. P.;
Perumpilly, G.; Prior, G.; Radford, D. C.; Rielage, K.; Robertson,
R. G. H.; Ronquest, M. C.; Schubert, A. G.; Shima, T.; Shirchenko,
M.; Snavely, K. J.; Steele, D.; Strain, J.; Thomas, K.; Timkin, V.;
Tornow, W.; Vanyushin, I.; Varner, R. L.; Vetter, K.; Vorren, K.;
Yakushev, E.; Young, A. R.; Yu, C. -H.; Yumatov, V.; Zhang, C.
2012JPhCS.375d2010W Altcode:
The observation of neutrino less double-beta decay would show that
neutrinos are Majorana particles and provide information on neutrino
mass. Attaining sensitivities for neutrino masses in the inverted
hierarchy region requires large, tonne scale detectors with extremely
low backgrounds, at the level of 10<SUP>-3</SUP> counts keV<SUP>-1</SUP>
t<SUP>-1</SUP> y<SUP>-1</SUP> or lower in the region of the signal. The
MAJORANA collaboration is constructing the DEMONSTRATOR, an array
consisting of 40 kg of p-type point contact germanium detectors, at
least half of which will be enriched to 86% in <SUP>76</SUP>Ge. The
primary aim is to show the feasibility for a future tonne scale
measurement. With a sub-keV energy threshold, the array should also
be able to search for light WIMP dark matter. This paper presents a
brief update on the status of constructing the DEMONSTRATOR including
an electroforming facility that is now operating underground at the
Sanford Underground Research Facility.
---------------------------------------------------------
Title: Experimental validation of optimization concepts for
focal-plane image processing with adaptive optics
Authors: Korkiakoski, Visa; Keller, Christoph U.; Doelman, Niek;
Fraanje, Rufus; Andrei, Raluca; Verhaegen, Michel
2012SPIE.8447E..5ZK Altcode:
We show experimental results demonstrating the feasibility of an
extremely fast sequential phase-diversity (SPD) algorithm for point
sources. The algorithm can be implemented on a typical adaptive
optics (AO) system to improve the wavefront reconstruction beyond
the capabilities of a wavefront sensor by using the information
from the imaging camera. The algorithm is based on a small-phase
approximation enabling fast numerical implementation, and it finds the
optimal wavefront correction by iteratively updating the deformable
mirror. Our experiments were made at an AO-setup with a 37 actuator
membrane mirror, and the results show that the algorithm finds an
optimal image quality in 5-10 iterations, when the initial wavefront
errors are typical non-common path aberrations having a magnitude of
1-1.5 rad rms. The results are in excellent agreement with corresponding
numerical simulations.
---------------------------------------------------------
Title: Dark matter sensitivities of the Majorana Demonstrator
Authors: Giovanetti, G. K.; Aguayo, E.; Avignone, F. T., Iii; Back,
H. O.; Barabash, A. S.; Beene, J. R.; Bergevin, M.; Bertrand, F. E.;
Boswell, M.; Brudanin, V.; Busch, M.; Chan, Y. -D.; Christofferson,
C. D.; Collar, J. I.; Combs, D. C.; Cooper, R. J.; Detwiler, J. A.;
Doe, P. J.; Efremenko, Yu; Egorov, V.; Ejiri, H.; Elliott, S. R.;
Esterline, J.; Fast, J. E.; Fields, N.; Finnerty, P.; Fraenkle, F. M.;
Gehman, V. M.; Green, M. P.; Guiseppe, V. E.; Gusey, K.; Hallin, A. L.;
Hazama, R.; Henning, R.; Hoppe, E. W.; Horton, M.; Howard, S.; Howe,
M. A.; Johnson, R. A.; Keeter, K. J.; Keller, C.; Kidd, M. F.; Knecht,
A.; Kochetov, O.; Konovalov, S. I.; Kouzes, R. T.; LaFerriere, B. D.;
LaRoque, B. H.; Leon, J.; Leviner, L. E.; Loach, J. C.; MacMullin, S.;
Marino, M. G.; Martin, R. D.; Mei, D. -M.; Merriman, J. H.; Miller,
M. L.; Mizouni, L.; Nomachi, M.; Orrell, J. L.; Overman, N. R.;
Phillips, D. G., Ii; Poon, A. W. P.; Perumpilly, G.; Prior, G.;
Radford, D. C.; Rielage, K.; Robertson, R. G. H.; Ronquest, M. C.;
Schubert, A. G.; Shima, T.; Shirchenko, M.; Snavely, K. J.; Steele,
D.; Strain, J.; Thomas, K.; Timkin, V.; Tornow, W.; Vanyushin, I.;
Varner, R. L.; Vetter, K.; Vorren, K.; Wilkerson, J. F.; Yakushev,
E.; Young, A. R.; Yu, C. -H.; Yumatov, V.; Zhang, C.
2012JPhCS.375a2014G Altcode:
The Majorana Demonstrator is an array of natural and enriched high
purity germanium detectors that will search for the neutrinoless
double-beta decay of Germanium-76 and perform a search for weakly
interacting massive particles with masses below 10 GeV. To reach the
background rate goal in the neutrinoless double-beta decay region of
interest of 4 counts/keV/t/y, the DEMONSTRATOR will utilize a number of
background reduction strategies, including a time-correlated event cut
for <SUP>68</SUP>Ge that requires a sub-keV energy threshold. This low
energy threshold allows the DEMONSTRATOR to extend its physics reach to
include a search for light WIMPs. We will discuss the detector systems
and data analysis techniques required to achieve sub-keV thresholds
as well as present the projected dark matter sensitivities of the
Majorana Demonstrator.
---------------------------------------------------------
Title: Extremely fast focal-plane wavefront sensing for extreme
adaptive optics
Authors: Keller, Christoph U.; Korkiakoski, Visa; Doelman, Niek;
Fraanje, Rufus; Andrei, Raluca; Verhaegen, Michel
2012SPIE.8447E..21K Altcode: 2012arXiv1207.3273K
We present a promising approach to the extremely fast sensing and
correction of small wavefront errors in adaptive optics systems. As
our algorithm's computational complexity is roughly proportional
to the number of actuators, it is particularly suitable to
systems with 10,000 to 100,000 actuators. Our approach is based
on sequential phase diversity and simple relations between the
point-spread function and the wavefront error in the case of small
aberrations. The particular choice of phase diversity, introduced
by the deformable mirror itself, minimizes the wavefront error as
well as the computational complexity. The method is well suited for
high contrast astronomical imaging of point sources such as the
direct detection and characterization of exoplanets around stars, and
it works even in the presence of a coronagraph that suppresses the
diffraction pattern. The accompanying paper in these proceedings by
Korkiakoski et al. describes the performance of the algorithm using
numerical simulations and laboratory tests.
---------------------------------------------------------
Title: FOAM: the modular adaptive optics framework
Authors: van Werkhoven, T. I. M.; Homs, L.; Sliepen, G.; Rodenhuis,
M.; Keller, C. U.
2012SPIE.8447E..2VV Altcode:
Control software for adaptive optics systems is mostly custom built
and very specific in nature. We have developed FOAM, a modular adaptive
optics framework for controlling and simulating adaptive optics systems
in various environments. Portability is provided both for different
control hardware and adaptive optics setups. To achieve this, FOAM is
written in C++ and runs on standard CPUs. Furthermore we use standard
Unix libraries and compilation procedures and implemented a hardware
abstraction layer in FOAM. We have successfully implemented FOAM on the
adaptive optics system of ExPo - a high-contrast imaging polarimeter
developed at our institute - in the lab and will test it on-sky late
June 2012. We also plan to implement FOAM on adaptive optics systems
for microscopy and solar adaptive optics. FOAM is available* under
the GNU GPL license and is free to be used by anyone.
---------------------------------------------------------
Title: Bilinear solution to the phase diversity problem for extended
objects based on the Born approximation
Authors: Andrei, Raluca M.; Fraanje, Rufus; Verhaegen, Michel;
Korkiakoski, Visa A.; Keller, Christoph U.; Doelman, Niek
2012SPIE.8447E..6TA Altcode:
We propose a new approach for the joint estimation of aberration
parameters and unknown object from diversity images with applications
in imaging systems with extended objects as astronomical ground-based
observations or solar telescopes. The motivation behind our idea is
to decrease the computational complexity of the conventional phase
diversity (PD) algorithm and avoid the convergence to local minima
due to the use of nonlinear estimation algorithms. Our approach is
able to give a good starting point for an iterative algorithm or it
can be used as a new wavefront estimation method. When the wavefront
aberrations are small, the wavefront can be approximated with a linear
term which leads to a quadratic point-spread function (PSF) in the
aberration parameters. The presented approach involves recording two or
more diversity images and, based on the before mentioned approximation
estimates the aberration parameters and the object by solving a system
of bilinear equations, which is obtained by subtracting from each
diversity image the focal plane image. Moreover, using the quadratic
PSFs gives improved performance to the conventional PD algorithm through
the fact that the gradients of the PSFs have simple analytical formulas.
---------------------------------------------------------
Title: Simulation of automotive EMC emission test procedures based
on cable bundle measurements
Authors: Gonser, M.; Keller, C.; Hansen, J.; Khillkevich, V.;
Radchenko, A.; Pommerenke, D.; Weigel, R.
2012imsd.conf59432G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: HARPS spectropolarimetry of classical T Tauri stars
Authors: Johns-Krull, C. M.; Valenti, J. A.; Jeffers, S. V.; Piskunov,
N. E.; Kochukhov, O.; Keller, C.; Snik, F.; Rodenhuis, M.; Makaganiuk,
V.; Stempels, H.
2012AIPC.1429...43J Altcode:
We present high spectral resolution Stokes V polarimetery of the
Classical T Tauri stars (CTTSs) GQ Lup and TW Hya obtained with
the polarimetric upgrade to the HARPS spectrometer on the ESO 3.6 m
telescope. We present data on both photospheric lines and emission
lines, concentrating our discussion on the polarization properties
of the He I emission lines at 5876 A and 6678 A. The He I lines
in both these CTTS contain both narrow emission cores, believed to
come from near the accretion shock region on these stars, and broad
emission components which may come from either a wind or the large
scale magnetospheric accretion flow. We detect strong polarization in
the narrow component of both the He I emission lines in both stars. We
observe a maximum implied field strength of 5.8 +/- 0.3 kG in the 5876
A˚ line of GQ Lup, the highest field strength measured to date in
this line for a CTTS. We find field strengths in the two He I lines
that are consistent with each other, unlike what has been reported in
the literature on at least one star. We do not detect any polarization
in the broad component of the He I lines on these stars, strengthening
the conclusion that they form over a substantially different volume
relative the formation region of the narrow component of the He I lines.
---------------------------------------------------------
Title: SPEX2Earth, a novel spectropolarimeter for remote sensing of
aerosols and clouds
Authors: Smit, J. M.; Rietjens, J. H. H.; Hasekamp, O.; Stam, D. M.;
Snik, F.; van Harten, G.; Keller, C. U.; van der Togt, O.; Verlaan,
A. L.; Moddemeijer, K.; Beijersbergen, M.; Voors, R.; Wielinga, K.;
Vollmuller, B. -J.
2012EGUGA..1414166S Altcode:
Multi-angle spectro-polarimetry is the tool for the remote detection and
characterization of aerosol and clouds in the Earth's atmosphere. Using
a novel technique to measure polarization, we have developed a 30 kg
instrument design to simultaneously measure the intensity and state of
linear polarization of scattered sunlight, from 400 to 800 nm and 1200
to 1600 nm, for 30 viewing directions, each with a 30° swath. Aerosols
affect the climate directly by scattering and absorption of solar
radiation, and by scattering, absorption, and emission of thermal
radiation. Aerosols also affect the climate by changing the macro-
and microphysical properties of clouds (the so-called indirect and
semi-direct effects). Estimates of aerosol effects on the climate
are hampered by insufficient knowledge of aerosol properties (size
distribution, shape, and single scattering albedo) at a global
scale. From several studies we know that these properties can only be
determined with sufficient accuracy and unambiguously with satellite
instruments that measure both intensity and polarization at multiple
wavelengths and multiple viewing angles1,2.Polarization measurements
must have a high accuracy, typically better than 0.1%. Achieving global
coverage requires a large instantaneous field of view. Developing
an instrument that combines all of these specifications can be
considered as the most important challenge in polarimetric aerosol
remote sensing. SPEX2Earth is such an instrument. It has been derived
from the prototype spectropolarimeter SPEX (Spectro-polarimeter
for Planetary Exploration), that was originally developed for a Mars
orbiter. Possible target platforms for SPEX2Earth are the International
Space Station, or a low-Earth orbit platform. SPEX2Earth uses a
novel technique for its radiance and polarization measurements:
through a series of carefully selected birefringent crystals,
the radiance of scattered sunlight is spectrally modulated3. The
modulation amplitude and phase are proportional to the degree and
angle of linear polarization respectively. Two modulated spectra are
produced per ground pixel, with a 180° degree phase shift between
their modulations. The sum of the two spectra yields a modulation-free
high resolution radiance spectrum of the scattered sunlight. The
birefringent crystals determine the modulation frequency and thereby
the resolution of the polarization spectrum. The technique is entirely
passive, i.e. the polarization modulation is established without moving
parts or active components.. SPEX2Earth's novel polarimetric technique
allows for achieving the extremely high polarimetric accuracy (~0.001
in linear polarization) needed to derive properties of aerosol (size,
shape, refractive index, optical thickness, single scattering albedo)
and clouds (droplet size, number concentration, optical thickness,
phase, top/base height, cloud cover) with sufficient accuracy for
climate research. With its relatively high spectral resolution,
SPEX2Earth resolves the O2-A absorption band, which is important
for deriving aerosol and cloud height. The viewing angles sample the
scattering phase functions of aerosol and cloud particles, resolving
characteristic angular features, and allowing to distinguish different
types of particles. We will present the SPEX2Earth instrument, outline
its spectral modulation principle and discuss its advantages compared
with traditional polarimetric techniques. Expected performances are
discussed, and recent performance results of the SPEX prototype are
presented.
---------------------------------------------------------
Title: Multiwavelength imaging polarimetry of Venus at various
phase angles
Authors: Einarsen, L. J.; Rodenhuis, M.; Snik, F.; Keller, C. U.;
Stam, D. M.; de Kok, R. J.; Bianda, M.; Ramelli, R.
2012EGUGA..14.8670E Altcode:
Venus is the only planet with an atmosphere that we can observe from the
ground at a large range of phase angles. Therefore it constitutes an
important benchmark for direct observations of exoplanets, which will
soon become available. Moreover, polarimetric observations at various
phase angles and wavelengths provide a unique way to characterize any
(exo-)planetary atmosphere. For instance, the famous study by Hansen
& Hovenier (1974) which combines disk-integrated polarimetric
observations and modeling has unambiguously shown that Venus' upper
atmosphere consists of sulphuric acid droplets of ~1 um in diameter. We
present new spatially resolved observations of Venus using the imaging
polarimeters ExPo at the William Herschel Telescope and ZIMPOL at the
IRSOL telescope. These observations are taken in narrow-band filters
from 364--648 nm, and span phase angles from 10--49 degrees. We
find that the degree of polarization varies strongly with wavelength
and phase angle, as generally predicted by the model by Hansen &
Hovenier. However, the polarization behaviour near the equator differs
considerably from that at the poles, hinting at different atmospheric
compositions and/or stratifications. In the intensity images we
detect a significant shift of the location of maximum intensity with
wavelength. These observations allow us to refine the model by Hansen
& Hovenier, and we present the preliminary results of our efforts
to do so.
---------------------------------------------------------
Title: Observing the Earth as an exoplanet
Authors: Karalidi, T.; Stam, D. M.; Snik, F.; Keller, C. U.; Sparks,
W. B.; Bagnulo, S.
2012EGUGA..1410571K Altcode:
Observations of Solar System planets, including the Earth, have
shown the power of polarimetry for the characterization of planetary
atmospheres and surfaces, and its ability to break degeneracies in
retrievals from flux observations only and is thus essential for
the full characterization of atmospheres and surfaces of (exo-)
planets. With the discoveries of the first rocky exoplanets, the
quest for Earth-like exoplanets and signs of their habitability has
started. Since exoplanet observations will yield a signal that is
integrated over the illuminated and visible part of the planet's
disk, the main challenge for the interpretation of future exoplanet
observations in terms of habitability will be disentangling the
contributions from the different surface types and clouds. Numerical
codes have been developed to model the spectral signals of oceans,
continents, atmospheric gases, aerosols and clouds, but neither
these codes nor retrieval algorithms can be validated by lack of
disk-integrated observations of the Earth at a range of phase angles
and wavelengths. We present LOUPE (Lunar Observatory for Unresolved
Polarimetry of the Earth) as an instrument for a lunar lander. LOUPE
will measure the disk-integrated flux and state of polarization of
sunlight that is reflected by the Earth. LOUPE will offer a unique
opportunity to observe the Earth as if it were an exoplanet. Thanks to
the characteristics of the Moon's orbit around our planet, such a lunar
observatory will witness the daily rotation of the Earth, with various
surface types rotating in and out of view. During a month, it will also
see the Earth through all phase angles, ranging from a full Earth to a
new or almost new Earth, just as we can expect for observations of an
exoplanet (depending on its orbital inclination angle). Finally, during
the year, seasonal variations will become apparent. Such observations
cannot be obtained by integrating spatially resolved observations
by Earth remote-sensing satellites, nor by so-called Earthshine
measurements, which capture sunlight that has first been reflected by
the Earth and then by the lunar surface, because these can only be done
when the moon is seen at large phase angles (thus when a large fraction
of the lunar nightside and hence a large fraction of the Earth dayside
are in view). Apart from a description of the LOUPE instrument, we will
show numerically simulated flux and polarization spectra of Earth-like
exoplanets to 1. illustrate the spectral and temporal variations that
we can expect to observe from the moon, 2. point out the information
that could be retrieved from such observations
---------------------------------------------------------
Title: Planetary science: In search of biosignatures
Authors: Keller, Christoph U.; Stam, Daphne M.
2012Natur.483...38K Altcode:
An analysis of the intensity and polarization of sunlight reflected
by Earth reveals signatures of life on our planet. What prospects are
there for using similar measurements to find life on planets outside the
Solar System? Planetary scientists offer some answers. See Letter p.64
---------------------------------------------------------
Title: Magnetism, chemical spots, and stratification in the HgMn
star ϕ Phoenicis
Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels,
H. C.; Valenti, J. A.
2012A&A...539A.142M Altcode: 2011arXiv1111.6065M
Context. Mercury-manganese (HgMn) stars have been considered as
non-magnetic and non-variable chemically peculiar (CP) stars for a
long time. However, recent discoveries of the variability in spectral
line profiles have suggested an inhomogeneous surface distribution
of chemical elements in some HgMn stars. From the studies of other
CP stars it is known that magnetic field plays a key role in the
formation of surface spots. All attempts to find magnetic fields in
HgMn stars have yielded negative results. <BR /> Aims: In this study,
we investigate the possible presence of a magnetic field in ϕ Phe
(HD 11753) and reconstruct surface distribution of chemical elements
that show variability in spectral lines. We also test a hypothesis
that a magnetic field is concentrated in chemical spots and look into
the possibility that some chemical elements are stratified with depth
in the stellar atmosphere. <BR /> Methods: Our analysis is based on
high-quality spectropolarimetric time-series observations, covering
a full rotational period of the star. Spectra were obtained with the
HARPSpol at the ESO 3.6-m telescope. To increase the sensitivity of
the magnetic field search, we employed the least-squares deconvolution
(LSD) technique. Using Doppler imaging code INVERS10, we reconstructed
surface chemical distributions by utilising information from
multiple spectral lines. The vertical stratification of chemical
elements was calculated with the DDAFit program. <BR /> Results:
Combining information from all suitable spectral lines, we set an
upper limit of 4 G on the mean longitudinal magnetic field. For
chemical spots, an upper limit on the longitudinal field varies
between 8 and 15 G. We confirmed the variability of Y, Sr, and Ti
and detected variability in Cr lines. Stratification analysis showed
that Y and Ti are not concentrated in the uppermost atmospheric
layers. <BR /> Conclusions: Our spectropolarimetric observations
rule out the presence of a strong, globally-organised magnetic field
in ϕ Phe. This implies an alternative mechanism of spot formation,
which could be related to a non-equilibrium atomic diffusion. However,
the typical time scales of the variation in stratification predicted
by the recent time-dependent diffusion models exceed significantly
the spot evolution time-scale reported for ϕ Phe. <P />Based on
observations collected at the European Southern Observatory, Chile
(ESO programme 084.D-0338). Figures 9-12 are available in electronic
form at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Direct imaging of a massive dust cloud around R Coronae
Borealis
Authors: Jeffers, S. V.; Min, M.; Waters, L. B. F. M.; Canovas, H.;
Rodenhuis, M.; de Juan Ovelar, M.; Chies-Santos, A. L.; Keller, C. U.
2012A&A...539A..56J Altcode: 2012arXiv1203.1265J
We present recent polarimetric images of the highly variable star R CrB
using ExPo and archival WFPC2 images from the HST. We observed R CrB
during its current dramatic minimum where it decreased more than 9 mag
due to the formation of an obscuring dust cloud. Since the dust cloud is
only in the line-of-sight, it mimics a coronograph allowing the imaging
of the star's circumstellar environment. Our polarimetric observations
surprisingly show another scattering dust cloud at approximately 1.3”
or 2000 AU from the star. We find that to obtain a decrease in the
stellar light of 9 mag and with 30% of the light being reemitted
at infrared wavelengths (from R CrB's SED) the grains in R CrB's
circumstellar environment must have a very low albedo of approximately
0.07%. We show that the properties of the dust clouds formed around R
CrB are best fitted using a combination of two distinct populations of
grains size. The first are the extremely small 5 nm grains, formed in
the low density continuous wind, and the second population of large
grains (~0.14 μm) which are found in the ejected dust clouds. The
observed scattering cloud, not only contains such large grains, but
is exceptionally massive compared to the average cloud. <P />Based on
observations made with the William Herschel Telescope operated on the
island of La Palma by the Isaac Newton Group in the Spanish Observatorio
del Roque de los Muchachos of the Instituto de Astrofisica de Canarias.
---------------------------------------------------------
Title: The effects of disk and dust structure on observed polarimetric
images of protoplanetary disks
Authors: Min, M.; Canovas, H.; Mulders, G. D.; Keller, C. U.
2012A&A...537A..75M Altcode: 2011arXiv1111.4348M
Context. Imaging polarimetry is a powerful tool for imaging faint
circumstellar material. It is a rapidly developing field with great
promise for diagnostics of both the large-scale structures and the
small-scale details of the scattering particles. <BR /> Aims: For a
correct analysis of observations we need to fully understand the effects
of dust particle parameters, as well as the effects of the telescope,
atmospheric seeing, and assumptions about the data reduction and
processing of the observed signal. Here we study the major effects of
dust particle structure, size-dependent grain settling, and instrumental
properties. <BR /> Methods: We performed radiative transfer modeling
using different dust particle models and disk structures. To study the
influence of seeing and telescope diffraction we ran the models through
an instrument simulator for the ExPo dual-beam imaging polarimeter
mounted at the 4.2 m William Herschel Telescope (WHT). <BR /> Results:
Particle shape and size have a strong influence on the brightness and
detectability of the disks. In the simulated observations, the central
resolution element also contains contributions from the inner regions
of the protoplanetary disk besides the unpolarized central star. This
causes the central resolution element to be polarized, making simple
corrections for instrumental polarization difficult. This effect
strongly depends on the spatial resolution, so adaptive optics
systems are needed for proper polarization calibration. <BR />
Conclusions: We find that the commonly employed homogeneous sphere
model gives results that differ significantly from more realistic
models. For a proper analysis of the wealth of data available now or
in the near future, one must properly take the effects of particle
types and disk structure into account. The observed signal depends
strongly on the properties of these more realistic models, thus
providing a potentially powerful diagnostic. We conclude that it
is important to correctly understand telescope depolarization and
calibration effects for a correct interpretation of the degree of
polarization. <P />Appendix A is available in electronic form at <A
href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Signatures of Water Clouds on Exoplanets: Numerical
Simulations.
Authors: Karalidi, T.; Stam, D. M.; Keller, C. U.
2011ASPC..450..101K Altcode:
Clouds are of crucial importance for a planetary climate, because
they store atmospheric volatiles, and because they scatter and
absorb incident starlight and absorb, emit and scatter thermal
radiation. Consequently, the detection and characterization of
clouds on a planet can provide us with a wealth of information on
the conditions on the surface. Here, we present numerically simulated
flux and polarization spectra, from 0.3 μm to 1.0 μm, of starlight
reflected by Earth-like exoplanets that are covered by horizontally
homogeneous water clouds, for different cloud altitudes and particle
sizes. Our results show that the degree of polarization P is sensitive
to the particle size, in particular at phase angles between 30° and
∼50° and around 90°, and to the cloud top altitude, in particular
at wavelengths between 0.35 μm and 0.7 μm. The information in P
should be easier to retrieve than that in F.
---------------------------------------------------------
Title: Innovative Imaging of Young Stars: First Light ExPo
Observations
Authors: Jeffers, S. V.; Canovas, H.; Keller, C. U.; Min, M.;
Rodenhuis, M.
2011ASPC..448...15J Altcode: 2011csss...16...15J
We have developed an innovative imaging polariemter, ExPo, that excels
in the imaging of the circumstellar environments of young stars. The
basic physics that ExPo exploits is that starlight reflected from a
star's circumstellar environment becomes linearly polarised, making
it easily separable from unpolarised starlight. Our preliminary
results, from the William Herschel Telescope in La Palma, show that
ExPo has successfully detected several known protoplanetary disks out
to a much larger distance and at a finer resolution than previously
observed. ExPo has also made a significant number of new detections
of protoplanetary disks and stellar outflows. We use innovative data
analysis techniques, related to speckle interferometry, to detect the
innermost parts of the disk to much closer than any other techniques
operating at visible wavelengths. In this paper I present highlights
of ExPo's first light observations.
---------------------------------------------------------
Title: New Insights into Stellar Magnetism from the Spectropolarimetry
in All Four Stokes Parameters
Authors: Kochukhov, O.; Snik, F.; Piskunov, N.; Jeffers, S. V.;
Keller, C. U.; Makaganiuk, V.; Valenti, J. A.; Johns-Krull, C. M.;
Rodenhuis, M.; Stempels, H. C.
2011ASPC..448..245K Altcode: 2011csss...16..245K
Development of high-resolution spectropolarimetry has stimulated a
major progress in our understanding of the magnetism and activity of
late-type stars. During the last decade magnetic fields were discovered
and mapped for various types of active stars using spectropolarimetric
methods. However, these observations and modeling attempts are
inherently incomplete since they are based on the interpretation of the
stellar circular polarization alone. Taking advantage of the recently
commissioned HARPS polarimeter, we obtained the first systematic
observations of cool active stars in all four Stokes parameters. Here we
report detection of the magnetically induced linear polarization in the
RS CVn binary HR 1099 and phase-resolved full Stokes vector observations
of varepsilon Eri. For the latter star we measured the field strength
with the precision of ∼0.1 G over a complete rotation cycle and
reconstructed the global field topology with the help of magnetic
Doppler imaging. Our observations of the inactive solar-like star α
Cen A indicate the absence of the global field stronger than 0.2 G.
---------------------------------------------------------
Title: Spectropolarimeter for planetary exploration (SPEX):
performance measurements with a prototype
Authors: Voors, Robert; Moon, Scott G.; Hannemann, Sandro; Rietjens,
Jeroen H. H.; van Harten, Gerard; Snik, Frans; Smit, Martijn; Stam,
Daphne M.; Keller, Christoph U.; Laan, Erik C.; Verlaan, Adrianus L.;
Vliegenthart, Willem A.; ter Horst, Rik; Navarro, Ramón; Wielenga,
Klaas
2011SPIE.8176E..0DV Altcode: 2011SPIE.8176E...9V
SPEX (Spectropolarimeter for Planetary Exploration) was developed
in close cooperation between scientific institutes and space
technological industries in the Netherlands. It is used for measuring
microphysical properties of aerosols and cloud particles in planetary
atmospheres. SPEX utilizes a number of novel ideas. The key feature
is that full linear spectropolarimetry can be performed without the
use of moving parts, using an instrument of approximately 1 liter
in volume. This is done by encoding the degree and angle of linear
polarization (DoLP and AoLP) of the incoming light in a sinusoidal
modulation of the intensity spectrum. Based on this principle,
and after gaining experience from breadboard measurements using the
same principle, a fully functional prototype was constructed. The
functionality and the performance of the prototype were shown by
extensive testing. The simulated results and the laboratory measurements
show striking agreement. SPEX would be a valuable addition to any
mission that aims to study the composition and structure of planetary
atmospheres, for example, missions to Mars, Venus, Jupiter, Saturn
and Titan. In addition, on an Earth-orbiting satellite, SPEX could
give unique information on particles in our own atmosphere.
---------------------------------------------------------
Title: Data Reduction Approach for the Extreme Polarimeter
Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Keller, C. U.
2011ASPC..449...79C Altcode:
ExPo (Extreme Polarimeter) is an imaging polarimeter that we are
building at Utrecht University, The Netherlands. It will detect
polarized light from circumstellar disks and extrasolar planets,
initially at the 4.2 m WHT and later at other telescopes. We have
developed a data reduction approach that minimizes the influence of
instrumental and atmospherical effects by using a partially transmitting
coronagraph focal-plane mask. The approach has been tested with a
laboratory simulator and an ExPo prototype.
---------------------------------------------------------
Title: Simulating Polarized Light from Exoplanets
Authors: Jeffers, S. V.; Miesen, N.; Rodenhuis, M.; Keller, C. U.;
Canovas, H.
2011ASPC..449..391J Altcode:
In Utrecht we are building an imaging polarimeter, ExPo (Extreme
Polarimeter), to image circumstellar disk and characterize extra-solar
planets. To test and calibrate ExPo, we have built a laboratory-based
simulator that mimicks a star with a Jupiter-like exoplanet as seen by
the 4.2 m William-Herschel Telescope. The star and planet are simulated
using two single-mode fibres in close proximity that are fed with a
broadband arc lamp. The unpolarized star has a flux of 10<SUP>11</SUP>
photons s<SUP>-1</SUP>, to simulate a mv=0 star, and the planet is
partially linearly polarized, with a flux of as little as 10<SUP>2</SUP>
photons s<SUP>-1</SUP> to simulate reflected star light with a contrast
ratio of as much as 10<SUP>-9</SUP>. The telescope is simulated with
two lenses, and seeing can be included with a rotating glass plate
covered with hairspray, while dispersion is approximated with a wedge
prism. These are the first realistic laboratory simulations of imaging
polarimetry for exoplanet detection and characterization.
---------------------------------------------------------
Title: Design and Prototype Results of the ExPo Imaging Polarimeter
Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S.; Keller, C.
2011ASPC..449...33R Altcode:
We present the design and prototype laboratory results of ExPo, an
imaging polarimeter for the study of circumstellar disks and possibly
exoplanet detection currently under development at the University of
Utrecht. The instrument is designed to achieve a contrast ratio of
10<SUP>-9</SUP> between the unpolarized starlight and the polarized
source. First light is scheduled for the second half of 2008 at the
4.2 m William Herschel telescope at La Palma. The instrument is based
on the dual beam-exchange technique, simultaneously imaging the two
orthogonal polarization states. It employs a ferro-electric liquid
crystal retarder and a single electron-multiplying camera for fast
modulation of the polarization. The instrument operates in the visible
and has a field of view of (20″ × 20″).
---------------------------------------------------------
Title: M&m's: an error budget and performance simulator code
for polarimetric systems
Authors: de Juan Ovelar, Maria; Snik, Frans; Keller, Christoph U.
2011SPIE.8160E..0CD Altcode: 2011SPIE.8160E...8D; 2012arXiv1207.4241O
Although different approaches to model a polarimeter's accuracy have
been described before, a complete error budgeting tool for polarimetric
systems has not been yet developed. Based on the framework introduced
by Keller & Snik, in 2009, we have developed the M&m's code
as a first attempt to obtain a generic tool to model the performance
and accuracy of a given polarimeter, including all the potential error
contributions and their dependencies on physical parameters. The main
goal of the code is to provide insight on the combined influence of many
polarization errors on the polarimetric accuracy of any polarimetric
instrument. In this work we present the mathematics and physics based
on which the code is developed as well as its general structure and
operational scheme. Discussion of the advantages of the M&m's
approach to error budgeting and polarimetric performance simulation
is carried out and a brief outlook of further development of the code
is also given.
---------------------------------------------------------
Title: Prototyping for the Spectropolarimeter for Planetary
EXploration (SPEX): calibration and sky measurements
Authors: van Harten, Gerard; Snik, Frans; Rietjens, Jeroen H. H.; Smit,
J. Martijn; de Boer, Jozua; Diamantopoulou, Renia; Hasekamp, Otto P.;
Stam, Daphne M.; Keller, Christoph U.; Laan, Erik C.; Verlaan, Ad L.;
Vliegenthart, Willem A.; ter Horst, Rik; Navarro, Ramón; Wielinga,
Klaas; Hannemann, Sandro; Moon, Scott G.; Voors, Robert
2011SPIE.8160E..0ZV Altcode: 2011SPIE.8160E..28V
We present the Spectropolarimeter for Planetary EXploration (SPEX),
a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm
(with 2 nm intensity resolution), that is compact (~ 1 liter), robust
and lightweight. This is achieved by employing the unconventional
spectral polarization modulation technique, optimized for linear
polarimetry. The polarization modulator consists of an achromatic
quarter-wave retarder and a multiple-order retarder, followed by a
polarizing beamsplitter, such that the incoming polarization state
is encoded as a sinusoidal modulation in the intensity spectrum,
where the amplitude scales with the degree of linear polarization,
and the phase is determined by the angle of linear polarization. An
optimized combination of birefringent crystals creates an athermal
multiple-order retarder, with a uniform retardance across the field
of view. Based on these specifications, SPEX is an ideal, passive
remote sensing instrument for characterizing planetary atmospheres
from an orbiting, air-borne or ground-based platform. By measuring the
intensity and polarization spectra of sunlight that is scattered in
the planetary atmosphere as a function of the single scattering angle,
aerosol microphysical properties (size, shape, composition), vertical
distribution and optical thickness can be derived. Such information is
essential to fully understand the climate of a planet. A functional
SPEX prototype has been developed and calibrated, showing excellent
agreement with end-to-end performance simulations. Calibration tests
show that the precision of the polarization measurements is at least
2 • 10<SUP>-4</SUP>. We performed multi-angle spectropolarimetric
measurements of the Earth's atmosphere from the ground in conjunction
with one of AERONET's sun photometers. Several applications exist for
SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter
and the Earth, and SPEX can also be part of a ground-based aerosol
monitoring network.
---------------------------------------------------------
Title: The ZIMPOL high contrast imaging polarimeter for SPHERE:
sub-system test results
Authors: Roelfsema, Ronald; Gisler, Daniel; Pragt, Johan; Schmid,
Hans Martin; Bazzon, Andreas; Dominik, Carsten; Baruffolo, Andrea;
Beuzit, Jean-Luc; Charton, Julien; Dohlen, Kjetil; Downing, Mark;
Elswijk, Eddy; Feldt, Markus; de Haan, Menno; Hubin, Norbert; Kasper,
Markus; Keller, Christoph; Lizon, Jean-Louis; Mouillet, David; Pavlov,
Alexey; Puget, Pascal; Rochat, Sylvain; Salasnich, Bernardo; Steiner,
Peter; Thalmann, Christian; Waters, Rens; Wildi, François
2011SPIE.8151E..0NR Altcode: 2011SPIE.8151E..21R
SPHERE (Spectro-Polarimetric High Contrast Exoplanet Research) is
one of the first instruments which aim for the direct detection from
extra-solar planets. The instrument will search for direct light from
old planets with orbital periods of several months to several years
as we know them from our solar system. These are planets which are in
or close to the habitable zone. ZIMPOL (Zurich Imaging Polarimeter)
is the high contrast imaging polarimeter subsystem of the ESO SPHERE
instrument. ZIMPOL is dedicated to detect the very faint reflected and
hence polarized visible light from extrasolar planets. The search for
reflected light from extra-solar planets is very demanding because
the signal decreases rapidly with the orbital separation. For a
Jupiter-sized object and a separation of 1 AU the planet/star contrast
to be achieved is on the order of 10<SUP>-8</SUP> for a successful
detection. This is much more demanding than the direct imaging of young
self-luminous planets. ZIMPOL is located behind an extreme AO system
(SAXO) and a stellar coronagraph. SPHERE is foreseen to have first light
at the VLT at the end of 2012. ZIMPOL is currently in the subsystem
testing phase. We describe the results of verification and performance
testing done at the NOVA-ASTRON lab. We will give an overview of
the system noise performance, the polarimetric accuracy and the high
contrast testing. For the high contrast testing we will describe the
impact of crucial system parameters on the contrast performance. SPHERE
is an instrument designed and built by a consortium consisting of IPAG,
MPIA, LAM, LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA,
ONERA and ASTRON in collaboration with ESO.
---------------------------------------------------------
Title: No magnetic field in the spotted HgMn star μ Leporis
Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels,
H. C.; Valenti, J. A.
2011A&A...534L..13K Altcode: 2011arXiv1110.0829K
Context. Chemically peculiar stars of the mercury-manganese (HgMn)
type represent a new class of spotted late-B stars, in which evolving
surface chemical inhomogeneities are apparently unrelated to the
presence of strong magnetic fields but are produced by some hitherto
unknown astrophysical mechanism. <BR /> Aims: The goal of this study
is to perform a detailed line profile variability analysis and carry
out a sensitive magnetic field search for one of the brightest HgMn
stars - μ Lep. <BR /> Methods: We acquired a set of very high-quality
intensity and polarization spectra of μ Lep with the HARPSpol
polarimeter. These data were analyzed with the multiline technique
of least-squares deconvolution in order to extract information on
the magnetic field and line profile variability. <BR /> Results:
Our spectra show very weak but definite variability in the lines
of Sc, all Fe-peak elements represented in the spectrum of μ Lep,
as well as Y, Sr, and Hg. Variability might also be present in the
lines of Si and Mg. Anomalous profile shapes of Ti ii and Y ii lines
suggest a dominant axisymmetric distribution of these elements. At
the same time, we found no evidence of the magnetic field in μ Lep,
with the 3σ upper limit of only 3 G for the mean longitudinal magnetic
field. This is the most stringent upper limit on the possible magnetic
field derived for a spotted HgMn star. <BR /> Conclusions: The very
weak variability detected for many elements in the spectrum μ Lep
suggests that low-contrast chemical inhomogeneities may be common in
HgMn stars and that they have not been recognized until now due to the
limited precision of previous spectroscopic observations and a lack
of time-series data. The null result of the magnetic field search
reinforces the conclusion that formation of chemical spots in HgMn
stars is not magnetically driven. <P />Based on observations collected
at the European Southern Observatory, Chile (ESO programs 084.D-0338,
086.D-0240).
---------------------------------------------------------
Title: Fast horizontal flows in a quiet sun MHD simulation and their
spectroscopic signatures
Authors: Vitas, N.; Fischer, C. E.; Vögler, A.; Keller, C. U.
2011A&A...532A.110V Altcode:
Numerical simulations of solar surface convection have predicted
the existence of supersonic horizontal flows in the photospheric
granulation. Recently, the detection of such flows in data from the
Hinode satellite was reported. We study supersonic granular flows in
detail to understand their signatures in spectral lines and to test
the observational detection method used to identify these flows in
the Hinode observations. We perform time-dependent 3D radiative MHD
numerical simulations and synthesize the Fe i 6302 Å spectral lines at
the resolution of the Hinode data for different viewing angles covering
the center-limb variation. There is very large variation in the detailed
shape of the emergent line profiles depending on the viewing angle and
the particular flow properties and orientation. At the full simulation
resolution the supersonic flows can even produce distinct satellite
lines. After smearing to the Hinode resolution sufficient signature
of supersonic motion remains. Our analysis shows that the detection
method used to analyze the Hinode data is indeed applicable. However,
the detection is very sensitive to ad hoc parameter choices and can
also misidentify supersonic flows.
---------------------------------------------------------
Title: The search for magnetic fields in mercury-manganese stars
Authors: Makaganiuk, Vitalii; Kochukhov, Oleg; Piskunov, Nikolai;
Jeffers, Sandra V.; Johns-Krull, Christopher M.; Keller, Christoph
U.; Rodenhuis, Michiel; Snik, Frans; Stempels, Henricus C.; Valenti,
Jeff A.
2011IAUS..272..202M Altcode:
Mercury-manganese (HgMn) stars were considered to be non-magnetic,
showing no evidence of surface spots. However, recent investigations
revealed that some stars in this class possess an inhomogeneous
distribution of chemical elements on their surfaces. According to
our current understanding, the most probable mechanism of spot
formation involves magnetic fields. Taking the advantage of a
newly-built polarimeter attached to the HARPS spectrometer at the
ESO 3.6m-telescope, we performed a high-precision spectropolarimetric
survey of a large group of HgMn stars. The main purpose of this study
was to find out how typical it is for HgMn stars to have weak magnetic
fields. We report no magnetic field detection for any of the studied
objects, with a typical precision of the longitudinal field measurements
of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn
stars lack large-scale magnetic fields typical of spotted magnetic Ap
stars and probably lack any fields capable of creating and sustaining
chemical spots. Our study confirms that alongside the magnetically
altered atomic diffusion, there must be other structure formation
mechanism operating in the atmospheres of late-B main sequence stars.
---------------------------------------------------------
Title: Data-reduction techniques for high-contrast imaging
polarimetry. Applications to ExPo
Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Min, M.; Keller,
C. U.
2011A&A...531A.102C Altcode: 2011arXiv1105.2961C
Context. Imaging polarimetry is a powerful tool for detecting and
characterizing exoplanets and circumstellar environments. Polarimetry
allows a separation of the light coming from an unpolarized source
such as a star and the polarized source such as a planet or a
protoplanetary disk. Future facilities like SPHERE at the VLT or
EPICS at the E-ELT will incorporate imaging polarimetry to detect
exoplanets. The Extreme Polarimeter (ExPo) is a dual-beam imaging
polarimeter that can currently reach contrast ratios of 10<SUP>5</SUP>,
enough to characterize circumstellar environments. <BR /> Aims: We
present the data-reduction steps for a dual-beam imaging polarimeter
that can reach contrast ratios of 10<SUP>5</SUP>. <BR /> Methods: The
data obtained with ExPo at the William Herschel Telescope (WHT) are
analyzed. Instrumental artifacts and noise sources are discussed for
an unpolarized star and for a protoplanetary disk (AB Aurigae). <BR />
Results: The combination of fast modulation and dual-beam techniques
allows us to minimize instrumental artifacts. A proper data processing
and alignment of the images is fundamental when dealing with high
contrasts. Imaging polarimetry proves to be a powerful method to
resolve circumstellar environments even without a coronagraph mask or
an adaptive optics system.
---------------------------------------------------------
Title: Spectral and polarimetric characterization of gazeous and
telluric planets with SEE COAST
Authors: Boccaletti, A.; Baudoz, P.; Mawet, D.; Schneider, J.;
Tinetti, G.; Galicher, R.; Stam, D.; Cavarroc, C.; Hough, J.; Doel,
P.; Pinfield, D.; Keller, C. -U.; Beuzit, J. -L.; Udry, S.; Ferrari,
A.; Martin, E.; Ménard, F.; Sein, E.
2011EPJWC..1607002B Altcode:
SEE COAST stands for Super Earth Explorer - Coronagraphic Off-Axis
Space Telescope. The concept was initially proposed to ESA for
Cosmic Vision. None of the direct detection exoplanet proposals were
selected in 2007 and we are now pursuing our efforts to consolidate
the astrophysical program and the technical developments for the next
call for proposal. The prime objective of SEE COAST is to contribute
to the understanding of the formation and evolution of planetary
systems. Exploring the diversity of these objects is therefore the
main driver to define the instrumentation. In the next decade the
improvement of radial velocity instruments and obviously temporal
coverage will provide us with a large numbers of long period giants as
well as telluric planets, namely Super Earths. Obtaining the spectral
and polarimetric signatures of these objects in the visible range
to measure atmospheric parameters (molecular composition, clouds,
soils, …) will be unique and with important scientific returns. A
space mission complementary to near IR instruments like SPHERE, GPI,
JWST and later ELTs for the full characterization of giants and Super
Earths is a first secure step towards the longer term goal that is
the characterization of telluric planets with mass and atmosphere
comparable to that of the Earth. An overview of the astrophysical
motivation and the trade-off that lead to a simple integrated concept
of a space-based high contrast imaging instrument are given here.
---------------------------------------------------------
Title: Chemical spots in the absence of magnetic field in the binary
HgMn star 66 Eridani
Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels,
H. C.; Valenti, J. A.
2011A&A...529A.160M Altcode: 2011arXiv1102.4661M
Context. According to our current understanding, a subclass of the
upper main-sequence chemically peculiar stars, called mercury-manganese
(HgMn), is non-magnetic. Nevertheless, chemical inhomogeneities were
recently discovered on their surfaces. At the same time, no global
magnetic fields stronger than 1-100 G are detected by systematic
studies. <BR /> Aims: The goals of our study are to search for a
magnetic field in the HgMn binary system 66 Eri and to investigate
chemical spots on the stellar surfaces of both components. <BR />
Methods: Our analysis is based on high-quality spectropolarimetric
time-series observations obtained during 10 consecutive nights with
the HARPSpol instrument at the ESO 3.6-m telescope. To increase the
sensitivity of the magnetic field search we employed a least-squares
deconvolution (LSD). We used spectral disentangling to measure radial
velocities and study the line profile variability. Chemical spot
geometry was reconstructed using multi-line Doppler imaging. <BR />
Results: We report a non-detection of magnetic field in 66 Eri, with
error bars 10-24 G for the longitudinal field. Circular polarization
profiles also do not indicate any signatures of complex surface
magnetic fields. For a simple dipolar field configuration we estimated
an upper limit of the polar field strength to be 60-70 G. For the
HgMn component we found variability in spectral lines of Ti, Ba,
Y, and Sr with the rotational period equal to the orbital one. The
surface maps of these elements reconstructed with the Doppler imaging
technique show a relative underabundance on the hemisphere facing the
secondary component. The contrast of chemical inhomogeneities ranges
from 0.4 for Ti to 0.8 for Ba. <P />Based on observations collected
at the European Southern Observatory, Chile (ESO program 084.D-0338).
---------------------------------------------------------
Title: First Detection of Linear Polarization in the Line Profiles
of Active Cool Stars
Authors: Kochukhov, O.; Makaganiuk, V.; Piskunov, N.; Snik, F.;
Jeffers, S. V.; Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.;
Valenti, J. A.
2011ApJ...732L..19K Altcode: 2011arXiv1103.6028K
The application of high-resolution spectropolarimetry has led to major
progress in understanding the magnetism and activity of late-type
stars. During the last decade, magnetic fields have been discovered and
mapped for many types of active cool stars using spectropolarimetric
data. However, these observations and modeling attempts are
fundamentally incomplete since they are based on the interpretation of
the circular polarization alone. Taking advantage of the newly built
HARPS polarimeter, we have obtained the first systematic observations
of several cool active stars in all four Stokes parameters. Here we
report the detection of magnetically induced linear polarization for
the primary component of the very active RS CVn binary HR 1099 and
for the moderately active K dwarf ɛ Eri. For both stars the amplitude
of linear polarization signatures is measured to be ~10<SUP>-4</SUP>
of the unpolarized continuum, which is approximately a factor of 10
lower than for circular polarization. This is the first detection of
the linear polarization in line profiles of cool active stars. Our
observations of the inactive solar-like star α Cen A show neither
circular nor linear polarization above the level of ~10<SUP>-5</SUP>,
indicating the absence of a net longitudinal magnetic field stronger
than 0.2 G. <P />Based on observations obtained at the European Southern
Observatory (ESO programs 083.D-1000(A) and 084.D-0338(A)).
---------------------------------------------------------
Title: The Polarization Optics for the European Solar Telescope
Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.;
Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.;
Snik, F.; Socas-Navarro, H.
2011ASPC..437..329B Altcode:
EST, the European Solar Telescope, is a 4-m class solar telescope,
which will be located at the Canary Islands. It is currently in the
conceptual design phase as a European funded project. In order to
fulfill the stringent requirements for polarimetric sensitivity
and accuracy, the polarimetry has been included in the design
work from the very beginning. The overall philosophy has been to
use a combination of techniques, which includes a telescope with
low (and stable) instrumental polarization, optimal full Stokes
polarimeters, differential measurement schemes, fast modulation
and demodulation, and accurate calibration, and at the same time not
giving up flexibility. The current baseline optical layout consists of a
14-mirror layout, which is polarimetrically compensated and non-varying
in time. In the polarization free F2 focus ample space is reserved for
calibration and modulators and a polarimetric switch. At instrument
level the s-, and p-planes of individual components are aligned,
resulting in a system in which eigenvectors can travel undisturbed
through the system.
---------------------------------------------------------
Title: The HARPS Polarimeter
Authors: Snik, F.; Kochukhov, O.; Piskunov, N.; Rodenhuis, M.; Jeffers,
S.; Keller, C.; Dolgopolov, A.; Stempels, E.; Makaganiuk, V.; Valenti,
J.; Johns-Krull, C.
2011ASPC..437..237S Altcode: 2010arXiv1010.0397S
We recently commissioned the polarimetric upgrade of the HARPS
spectrograph at ESO's 3.6-m telescope at La Silla, Chile. The HARPS
polarimeter is capable of full Stokes spectropolarimetry with large
sensitivity and accuracy, taking advantage of the large spectral
resolution and stability of HARPS. In this paper we present the
instrument design and its polarimetric performance. The first HARPSpol
observations show that it can attain a polarimetric sensitivity
of ∼10<SUP>-5</SUP> (after addition of many lines) and that no
significant instrumental polarization effects are present.
---------------------------------------------------------
Title: HARPSpol — The New Polarimetric Mode for HARPS
Authors: Piskunov, N.; Snik, F.; Dolgopolov, A.; Kochukhov, O.;
Rodenhuis, M.; Valenti, J.; Jeffers, S.; Makaganiuk, V.; Johns-Krull,
C.; Stempels, E.; Keller, C.
2011Msngr.143....7P Altcode:
The HARPS spectrograph can now perform a full polarisation analysis
of spectra. It has been equipped with a polarimetric unit, HARPSpol,
which was jointly designed and produced by Uppsala, Utrecht and Rice
Universities and by the STScI. Here we present the new instrument,
demonstrate its polarisation capabilities and show the first scientific
results.
---------------------------------------------------------
Title: The search for magnetic fields in mercury-manganese stars
Authors: Makaganiuk, V.; Kochukhov, O.; Piskunov, N.; Jeffers, S. V.;
Johns-Krull, C. M.; Keller, C. U.; Rodenhuis, M.; Snik, F.; Stempels,
H. C.; Valenti, J. A.
2011A&A...525A..97M Altcode: 2010arXiv1010.3931M
Context. A subclass of the upper main-sequence chemically peculiar
stars, mercury-manganese (HgMn) stars were traditionally considered to
be non-magnetic, showing no evidence of variability in their spectral
line profiles. However, discoveries of chemical inhomogeneities on
their surfaces imply that this assumption should be investigated. In
particular, spectroscopic time-series of AR Aur, α And, and five other
HgMn stars indicate the presence of chemical spots. At the same time,
no signatures of global magnetic fields have been detected. <BR />
Aims: We attempt to understand the physical mechanism that causes the
formation of chemical spots in HgMn stars and gain insight into the
potential magnetic field properties at their surfaces; we performed a
highly sensitive search for magnetic fields for a large set of HgMn
stars. <BR /> Methods: With the aid of a new polarimeter attached
to the HARPS spectrometer at the ESO 3.6 m-telescope, we obtained
high-quality circular polarization spectra of 41 single and double HgMn
stars. Using a multi-line analysis technique on each star, we co-added
information from hundreds of spectral lines to ensure significantly
greater sensitivity to the presence of magnetic fields, including very
weak fields. <BR /> Results: For the 47 individual objects studied,
including six components of SB2 systems, we do not detect any magnetic
fields at greater than the 3σ level. The lack of detection in the
circular polarization profiles indicates that if strong fields are
present on these stars, they must have complex surface topologies. For
simple global fields, our detection limits imply upper limits to the
fields present of 2-10 Gauss in the best cases. <BR /> Conclusions:
We conclude that HgMn stars lack large-scale magnetic fields, which
is typical of spotted magnetic Ap stars, of sufficient strength to
form and sustain the chemical spots observed on HgMn stars. Our study
confirms that in addition to magnetically altered atomic diffusion,
there exists another differentiation mechanism operating in the
atmospheres of late-B main sequence stars that can produce compositional
inhomogeneities on their surfaces. <P />Based on observations collected
at the European Southern Observatory, Chile (ESO programs 083.D-1000,
084.D-0338, 085.D-0296).Figure 5 is only available in electronic form
at <A href="http://www.aanda.org">http://www.aanda.org</A>
---------------------------------------------------------
Title: Imaging polarimetry of circumstellar environments with the
Extreme Polarimeter
Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Min, M.; Keller,
C. U.
2010lyot.confE..20R Altcode:
Three successful observation campaigns have been conducted with
the Extreme Polarimeter, an imaging polarimeter for the study of
circumstellar environments in scattered light at visible wavelengths. A
contrast ratio between the central star and the circumstellar source
of 10-5 can be achieved with polarimetry, with a Lyot coronograph
capable of increasing this contrast by several orders of magnitude. The
instrument currently operates without an adaptive optics system. An
Adaptive Optics system under development for ExPo is expected to
increase the contrast further. The polarimeter uses the dual-beam
exchange technique, in which the two orthogonal polarisation states
are imaged simultaneously after which a polarisation modulator is used
to swap the polarisation states of the two beams before the next image
is taken. The imaging polarimetry technique developed with ExPo will
be used in the polarimetry arm of the EPICS exoplanet characterisation
instrument proposed for the E-ELT. Here we present the results from the
first observation campaigns, highlighting observations of protoplanetary
disks around several young stars. Systematic effects that limit the
polarimetric sensitivity, and the strategies we employ to overcome
them, are discussed in detail. In particular, the advantages of the
dual-beam exchange polarimetry method are demonstrated.
---------------------------------------------------------
Title: EPOL: the exoplanet polarimeter for EPICS at the E-ELT
Authors: Snik, F.; Keller, C.; Ovelar, M. J.; Rodenhuis, M.;
Korkiakoski, V.; Venema, L.; Jager, R.; Rigal, F.; Hanenburg, H.;
Roelfsema, R.; Schmidt, H. M.; Verinaud, C.; Kasper, M.; Martinez,
P.; Yaitskova, N.
2010lyot.confE..82S Altcode:
EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging
Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive
imaging polarimetry to differentiate between linearly polarized
light from exoplanets and unpolarized, scattered starlight and to
characterize properties of exoplanet atmospheres and surfaces that
cannot be determined from intensity observations alone. EPOL consists
of a coronagraph and a dual-beam polarimeter with a liquid-crystal
retarder to exchange the polarization of the two beams. The polarimetry
thereby increases the contrast between star and exoplanet by 3 to 5
orders of magnitude over what the extreme adaptive optics and the EPOL
coronagraph alone can achieve. EPOL operates between 600 and 900 nm,
can select more specific wavelength bands with filters and aims at
having an integral field unit to obtain linearly polarized spectra of
known exoplanets. We present the conceptual design of EPOL along with
an analysis of its performance.
---------------------------------------------------------
Title: Imaging polarimetry of protoplanetary disks: feasibility
and usability
Authors: Min, M.; Jeffers, S. V.; Rodenhuis, M.; Canovas, H.; Buenzli,
E.; Keller, C. U.; Waters, L. B. F. M.; Dominik, C.
2010lyot.confE..34M Altcode:
Imaging polarimetry is one of the most promising tools to map the
structure of faint protoplanetary disks. In this contribution we
discuss the feasibility of imaging polarimetry of protoplanetary
disks and the usability to answer the scientific questions in the
field. From the theoretical side we do this by simulations of disks of
various geometries and dust properties. We model the expected signal and
detailed predictions for current and upcoming imaging polarimeters. This
way we can address the question what the diagnostic value of polarimetry
is for the structure of the disk and the characteristics of the grains
in it. We compare extremely fluffy aggregated grains and compact
homogeneous grains and show that their expected signal is significantly
different. In combination with infrared/mm observations this could
allow us to obtain grain properties in addition to mapping of the
disk geometry. From the observational side we address the issues by
discussing some of the early results from the Extreme Polarimeter
(ExPo). ExPo is a sensitive imaging polarimeter designed to be a
pathfinding instrument for the large imaging polarimetry projects
planned for the VLT and the ELT. Already it proves to be a pioneering
instrument in the field of imaging polarimetry of circumstellar matter.
---------------------------------------------------------
Title: Observations of solar scattering polarization at high spatial
resolution
Authors: Snik, F.; de Wijn, A. G.; Ichimoto, K.; Fischer, C. E.;
Keller, C. U.; Lites, B. W.
2010A&A...519A..18S Altcode: 2010arXiv1005.5042S
Context. The weak, turbulent magnetic fields that supposedly
permeate most of the solar photosphere are difficult to observe,
because the Zeeman effect is virtually blind to them. The Hanle
effect, acting on the scattering polarization in suitable lines,
can in principle be used as a diagnostic for these fields. However,
the prediction that the majority of the weak, turbulent field resides
in intergranular lanes also poses significant challenges to scattering
polarization observations because high spatial resolution is usually
difficult to attain. <BR /> Aims: We aim to measure the difference
in scattering polarization between granules and intergranules. We
present the respective center-to-limb variations, which may serve as
input for future models. <BR /> Methods: We perform full Stokes filter
polarimetry at different solar limb positions with the CN band filter
of the Hinode-SOT Broadband Filter Imager, which represents the first
scattering polarization observations with sufficient spatial resolution
to discern the granulation. Hinode-SOT offers unprecedented spatial
resolution in combination with high polarimetric sensitivity. The CN
band is known to have a significant scattering polarization signal,
and is sensitive to the Hanle effect. We extend the instrumental
polarization calibration routine to the observing wavelength,
and correct for various systematic effects. <BR /> Results: The
scattering polarization for granules (i.e., regions brighter than
the median intensity of non-magnetic pixels) is significantly larger
than for intergranules. We derive that the intergranules (i.e., the
remaining non-magnetic pixels) exhibit (9.8±3.0)% less scattering
polarization for 0.2 < μ ≤ 0.3, although systematic effects cannot
be completely excluded. <BR /> Conclusions: These observations constrain
MHD models in combination with (polarized) radiative transfer in terms
of CN band line formation, radiation anisotropy, and magnetic fields.
---------------------------------------------------------
Title: The ZIMPOL high-contrast imaging polarimeter for SPHERE:
design, manufacturing, and testing
Authors: Roelfsema, Ronald; Schmid, Hans Martin; Pragt, Johannes;
Gisler, Daniel; Waters, Rens; Bazzon, Andreas; Baruffolo, Andrea;
Beuzit, Jean-Luc; Boccaletti, Anthony; Charton, Julien; Cumani,
Claudio; Dohlen, Kjetil; Downing, Mark; Elswijk, Eddy; Feldt, Markus;
Groothuis, Charlotte; de Haan, Menno; Hanenburg, Hiddo; Hubin, Norbert;
Joos, Franco; Kasper, Markus; Keller, Christoph; Kragt, Jan; Lizon,
Jean-Louis; Mouillet, David; Pavlov, Aleksej; Rigal, Florence; Rochat,
Sylvain; Salasnich, Bernardo; Steiner, Peter; Thalmann, Christian;
Venema, Lars; Wildi, François
2010SPIE.7735E..4BR Altcode: 2010SPIE.7735E.144R
ZIMPOL is the high contrast imaging polarimeter subsystem of the
ESO SPHERE instrument. ZIMPOL is dedicated to detect the very
faint reflected and hence polarized visible light from extrasolar
planets. ZIMPOL is located behind an extreme AO system (SAXO) and a
stellar coronagraph. SPHERE is foreseen to have first light at the
VLT at the end of 2011. ZIMPOL is currently in the manufacturing,
integration and testing phase. We describe the optical, polarimetric,
mechanical, thermal and electronic design as well as the design
trade offs. Specifically emphasized is the optical quality of the key
performance component: the Ferro-electric Liquid Crystal polarization
modulator (FLC). Furthermore, we describe the ZIMPOL test setup and
the first test results on the achieved polarimetric sensitivity and
accuracy. These results will give first indications for the expected
overall high contrast system performance. SPHERE is an instrument
designed and built by a consortium consisting of LAOG, MPIA, LAM,
LESIA, Fizeau, INAF, Observatoire de Genève, ETH, NOVA, ONERA and
ASTRON in collaboration with ESO.
---------------------------------------------------------
Title: SPEX: the spectropolarimeter for planetary exploration
Authors: Snik, Frans; Rietjens, Jeroen H. H.; van Harten, Gerard;
Stam, Daphne M.; Keller, Christoph U.; Smit, J. Martijn; Laan, Erik
C.; Verlaan, Ad L.; Ter Horst, Rik; Navarro, Ramón; Wielinga, Klaas;
Moon, Scott G.; Voors, Robert
2010SPIE.7731E..1BS Altcode: 2010SPIE.7731E..34S
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative,
compact instrument for spectropolarimetry, and in particular for
detecting and characterizing aerosols in planetary atmospheres. With
its ~1-liter volume it is capable of full linear spectropolarimetry,
without moving parts. The degree and angle of linear polarization
of the incoming light is encoded in a sinusoidal modulation of the
intensity spectrum by an achromatic quarter-wave retarder, an athermal
multiple-order retarder and a polarizing beam-splitter in the entrance
pupil. A single intensity spectrum thus provides the spectral dependence
of the degree and angle of linear polarization. Polarimetry has proven
to be an excellent tool to study microphysical properties (size, shape,
composition) of atmospheric particles. Such information is essential
to better understand the weather and climate of a planet. The current
design of SPEX is tailored to study Martian dust and ice clouds
from an orbiting platform: a compact module with 9 entrance pupils
to simultaneously measure intensity spectra from 400 to 800 nm, in
different directions along the flight direction (including two limb
viewing directions). This way, both the intensity and polarization
scattering phase functions of dust and cloud particles within a ground
pixel are sampled while flying over it. We describe the optical and
mechanical design of SPEX, and present performance simulations and
initial breadboard measurements. Several flight opportunities exist
for SPEX throughout the solar system: in orbit around Mars, Jupiter
and its moons, Saturn and Titan, and the Earth.
---------------------------------------------------------
Title: EPICS: direct imaging of exoplanets with the E-ELT
Authors: Kasper, Markus; Beuzit, Jean-Luc; Verinaud, Christophe;
Gratton, Raffaele G.; Kerber, Florian; Yaitskova, Natalia; Boccaletti,
Anthony; Thatte, Niranjan; Schmid, Hans Martin; Keller, Christoph;
Baudoz, Pierre; Abe, Lyu; Aller-Carpentier, Emmanuel; Antichi, Jacopo;
Bonavita, Mariangela; Dohlen, Kjetil; Fedrigo, Enrico; Hanenburg,
Hiddo; Hubin, Norbert; Jager, Rieks; Korkiakoski, Visa; Martinez,
Patrice; Mesa, Dino; Preis, Olivier; Rabou, Patrick; Roelfsema,
Ronald; Salter, Graeme; Tecza, Mathias; Venema, Lars
2010SPIE.7735E..2EK Altcode: 2010SPIE.7735E..81K
Presently, dedicated instruments at large telescopes (SPHERE for the
VLT, GPI for Gemini) are about to discover and explore self-luminous
giant planets by direct imaging and spectroscopy. The next generation
of 30m-40m ground-based telescopes, the Extremely Large Telescopes
(ELTs), have the potential to dramatically enlarge the discovery space
towards older giant planets seen in reflected light and ultimately
even a small number of rocky planets. EPICS is a proposed instrument
for the European ELT, dedicated to the detection and characterization
of Exoplanets by direct imaging, spectroscopy and polarimetry. ESO
completed a phase-A study for EPICS with a large European consortium
which - by simulations and demonstration experiments - investigated
state-of-the-art diffraction and speckle suppression techniques to
deliver highest contrasts. The paper presents the instrument concept and
analysis as well as its main innovations and science capabilities. EPICS
is capable of discovering hundreds of giant planets, and dozens of
lower mass planets down to the rocky planets domain.
---------------------------------------------------------
Title: EPOL: the exoplanet polarimeter for EPICS at the E-ELT
Authors: Keller, Christoph U.; Schmid, Hans Martin; Venema, Lars B.;
Hanenburg, Hiddo; Jager, Rieks; Kasper, Markus; Martinez, Patrice;
Rigal, Florence; Rodenhuis, Michiel; Roelfsema, Ronald; Snik, Frans;
Verinaud, Christophe; Yaitskova, Natalia
2010SPIE.7735E..6GK Altcode: 2010SPIE.7735E.212K
EPOL is the imaging polarimeter part of EPICS (Exoplanet Imaging
Camera and Spectrograph) for the 42-m E-ELT. It is based on sensitive
imaging polarimetry to differentiate between linearly polarized
light from exoplanets and unpolarized, scattered starlight and to
characterize properties of exoplanet atmospheres and surfaces that
cannot be determined from intensity observations alone. EPOL consists
of a coronagraph and a dual-beam polarimeter with a liquid-crystal
retarder to exchange the polarization of the two beams. The polarimetry
thereby increases the contrast between star and exoplanet by 3 to 5
orders of magnitude over what the extreme adaptive optics and the EPOL
coronagraph alone can achieve. EPOL operates between 600 and 900 nm,
can select more specific wavelength bands with filters and aims at
having an integral field unit to obtain linearly polarized spectra of
known exoplanets. We present the conceptual design of EPOL along with
an analysis of its performance.
---------------------------------------------------------
Title: The polarization optics for the European Solar Telescope (EST)
Authors: Bettonvil, F. C. M.; Collados, M.; Feller, A.; Gelly, B. F.;
Keller, C. U.; Kentischer, T. J.; López Ariste, A.; Pleier, O.;
Snik, F.; Socas-Navarro, H.
2010SPIE.7735E..6IB Altcode: 2010SPIE.7735E.214B
EST (European Solar Telescope) is a 4-m class solar telescope, which
is currently in the conceptual design phase. EST will be located at
the Canary Islands and aims at observations with the best possible
spectral, spatial and temporal resolution and best polarimetric
performance, of the solar photosphere and chromosphere, using a
suite of instruments that can efficiently produce two-dimensional
spectropolarimetric information of the thermal, dynamic and magnetic
properties of the plasma over many scale heights, and ranging from
λ=350 until 2300 nm. In order to be able to fulfill the stringent
requirements for polarimetric sensitivity and accuracy, from the very
beginning the polarimetry has been included in the design work. The
overall philosophy has been to use a combination of techniques, which
includes a telescope with low (and stable) instrumental polarization,
optimal full Stokes polarimeters, differential measurement schemes,
fast modulation and demodulation, and accurate calibration. The
current baseline optical layout consists of a 14-mirror layout,
which is polarimetrically compensated and nonvarying in time. In the
polarization free F2 focus ample space is reserved for calibration and
modulators and a polarimetric switch. At instrument level the s-, and
p-planes of individual components are aligned, resulting in a system
in which eigenvectors can travel undisturbed through the system.
---------------------------------------------------------
Title: Prediction of underground argon content for dark matter
experiments
Authors: Mei, D. -M.; Yin, Z. -B.; Spaans, J.; Koppang, M.; Hime,
A.; Keller, C.; Gehman, V. M.
2010PhRvC..81e5802M Altcode: 2009arXiv0912.5368M
In this paper, we demonstrate the use of physical models to evaluate the
production of Ar39 and Ar40 underground. Considering both cosmogenic
Ar39 production and radiogenic Ar40 production in situ and from
external sources, we can derive the ratio of Ar39 to Ar40 in underground
sources. We show for the first time that the Ar39 production underground
is dominated by stopping negative muon capture on K39 and (α,n) induced
subsequent K39(n,p)Ar39 reactions. The production of Ar39 is shown as
a function of depth. We demonstrate that argon depleted in Ar39 can
be obtained only if the depth of the underground resources is greater
than 500 m.w.e. below the surface. Stopping negative muon capture on
K39 dominates over radiogenic production at depths of less than 2000
m.w.e., and that production by muon-induced neutrons is subdominant at
any depth. The depletion factor depends strongly on both radioactivity
level and potassium content in the rock. We measure the radioactivity
concentration and potassium concentration in the rock for a potential
site of an underground argon source in South Dakota. Depending on the
probability of Ar39 and Ar40 produced underground being dissolved in the
water, the upper limit of the concentration of Ar39 in the underground
water at this site is estimated to be in a range of a factor of 1.6 to
155 less than the Ar39 concentration in the atmosphere. The calculation
tools presented in this paper are also critical to the dating method
with Ar39.
---------------------------------------------------------
Title: Inversions of High-Cadence SOLIS-VSM Stokes Observations
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.
2010ASSP...19..515F Altcode: 2010mcia.conf..515F
We have processed full-Stokes observations made with the SOLIS-VSM
using Fe I 630.15 and Fe I 630.25 nm. The data have high spectral and
temporal resolution, moderate spatial resolution, and large polarimetric
sensitivity and accuracy. We use the code LILIA, an LTE inversion code
written by Socas-Navarro (2001), in order to invert the data in vector
magnetic fields. The 180-degree ambiguity in magnetic field orientation
is solved by using the Non-Potential Field Calculation (NPFC) method
of Georgoulis (2005). The output product are maps of the fullmagnetic
field vector at the photospheric level, as illustrated in Fig. 1. We
performed such inversions for observations of active region NOAA 10808
taken during an X-class flare in September 2005. Details of the data
processing and the first results are given in the proceedings of the
Fifth Solar PolarizationWorkshop (ASP Conf. Ser., in press).
---------------------------------------------------------
Title: EPICS, the exoplanet imager for the E-ELT
Authors: Kasper, M.; Beuzit, J. -L.; Verinaud, C.; Baudoz, P.;
Boccaletti, A.; Gratton, R.; Keller, C.; Kerber, F.; Schmid, H. M.;
Thatte, N.; Venema, L.; Yaitskova, N.
2010aoel.confE2009K Altcode:
Very soon, dedicated instruments developments at large telescopes
(SPHERE for the VLT, GPI for Gemini) are about to discover and explore
self-luminous giant planets by direct imaging and spectroscopy in
significant numbers. The next generation of 30m-40m ground-based
telescopes, the Extremely Large Telescopes (ELTs), have the potential
to dramatically enlarge the discovery space towards older giant planets
seen in reflected light and ultimately even a small number of rocky
planets. EPICS is a proposed instrument for the European ELT, dedicated
to the detection and characterization of expolanets by direct imaging
and spectroscopy. EPICS is currently mid-way through a phase-A study
carried out by a large European consortium which - by simulations
and demonstration experiments - will investigate state-of-the-art
diffraction and speckle suppression techniques to deliver highest
contrasts. The final result of the study early 2010 will be a conceptual
design and a development plan for the instrument. We will present the
EPICS concept including the performance analysis and first results from
prototyping experiments and discuss the main challenges and science
capabilities of EPICS.
---------------------------------------------------------
Title: Two Ways of Improving Stokes Inversions
Authors: Becher, H. -M.; Fischer, C.; Keller, C.
2009ASPC..415..160B Altcode:
Performing Stokes inversion on single pixels can be difficult due to
seeing and instrumental smearing. We present two different techniques
to improve Stokes inversions. The first method, REGIONS, is a code that
finds pixels that are likely to contain signal from the same magnetic
element. The second method, mCLEAN, reconstructs a magnetogram if the
point spread function (PSF) of the instrument and seeing is known.
---------------------------------------------------------
Title: Statistics of Convective Collapse Events in the Photosphere
and Chromosphere Observed with the HINODE SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
2009ASPC..415..127F Altcode:
Convective collapse, a theoretically predicted process that
intensifies existing weak magnetic fields in the solar atmosphere,
was first directly observed in a single event by Nagata et al. (2008)
using the high resolution Solar Optical Telescope (SOT) of the Hinode
satellite. Using the same space telescope, we observed 49 such events
and present a statistical analysis of convective collapse events. Our
data sets consist of high resolution time series of polarimetric
spectral scans of two iron lines formed in the lower photosphere and
filter images in Mg I b<SUB>2</SUB> and Ca II H. We were thus able
to study the implication of convective collapse events on the high
photospheric and the chromospheric layers. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, and field strength increase. We found
event durations of about 10 minutes and field strengths of up to
1.65 kG.
---------------------------------------------------------
Title: Statistics of convective collapse events in the photosphere
and chromosphere observed with the Hinode SOT
Authors: Fischer, C. E.; de Wijn, A. G.; Centeno, R.; Lites, B. W.;
Keller, C. U.
2009A&A...504..583F Altcode: 2009arXiv0906.2308F
Convective collapse, a theoretically predicted process that intensifies
existing weak magnetic fields in the solar atmosphere, was first
directly observed in a single event by Nagata et al. (2008, ApJ,
677, L145) using the high resolution Solar Optical Telescope (SOT)
of the Hinode satellite. Using the same space telescope, we observed
49 such events and present a statistical analysis of convective
collapse events. Our data sets consist of high resolution time series
of polarimetric spectral scans of two iron lines formed in the lower
photosphere and filter images in Mg I b{2} and Ca II H, spectral lines
that are formed in the high photosphere and the lower chromosphere,
respectively. We were thus able to study the implication of convective
collapse events on the high photospheric and the chromospheric
layers. We found that in all cases, the event was accompanied by a
continuum bright point and nearly always by a brightening in the Ca
II H images. The magnesium dopplergram exhibits a strong downflow in
about three quarters of the events that took place within the field
of view of the magnesium dopplergram. The physical parameters from
the full Stokes profiles were obtained with the MERLIN Milne-Eddington
inversion code. For each of the 49 events we determined the duration,
maximum photospheric downflow, field strength increase and size. We
found event durations of about 10 min, magnetic element radii of about
0.43 arcsec and 0.35 arcsec, before and after the event, respectively,
and field strengths of up to 1.65 kG.
---------------------------------------------------------
Title: The case for spectropolarimetry with SPEX on EJSM
Authors: Stam, D. M.; Smit, J. M.; Snik, F.; Keller, C. U.
2009epsc.conf..536S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Polarimetric Measurements of Protoplanetary Disks with ExPo
Authors: Canovas, H.; Rodenhuis, M.; Jeffers, S. V.; Keller, C. U.
2009AIPC.1158..381C Altcode:
Polarimetry is a powerful tool for detecting and characterizing
exoplanets and protoplanetary disks as light scattered from
circumstellar material is linearly polarized. We present the first
light results of ExPo (Extreme Polarimeter) [1], a sensitive imaging
polarimeter developed at Utrecht University that works in the
visible part of the spectrum. Our first light observations at the
4.2-meter William Herschel Telescope (WHT) show that ExPo can reach
the high-contrast ratios that are necessary to observe protoplanetary
disks. We present images of the protoplanetary disks around the Herbig
Ae star AB Aurigae and the T Tauri star SU Aurigae. Our results show
the power of polarimetry for future projects e.g. the ZIMPOL arm of
SPHERE, and the EPOL part of EPICS.
---------------------------------------------------------
Title: An IFU for diffraction-limited 3D spectroscopic imaging:
laboratory and on-site tests
Authors: Ren, Deqing; Keller, Christoph; Plymate, Claude
2009SPIE.7438E..18R Altcode: 2009SPIE.7438E..35R
We have developed a state-of-the-art image slicer Integral Field Unit
(IFU) for the McMath-Pierce Solar Telescope (McMP) located at Kitt Peak
National Solar Observatory. The IFU will be used for high-resolution
3-dimensional spectroscopy and polarimetry over a small field of
view that is well corrected by adaptive optics. It consists of 19
effective slices that correspond to a field of view of 6.27"x 7". The
IFU delivers a 152" long slit to an existing spectrograph producing
diffraction-limited 3-dimensional spectroscopy. The 3-D instrument is
being used for highspatial and high-temporal resolution imaging of the
Sun, which is crucial for the magnetic field and spectroscopic studies
of 2-dimensional solar fine structures. We discuss the instrument
construction, laboratory test and on-site trial observations with
the McMP.
---------------------------------------------------------
Title: A Brief History of the Second Solar Spectrum
Authors: Keller, C. U.
2009ASPC..405...29K Altcode:
The development of the ZIMPOL principle at ETH Zurich led to a dramatic
increase in our observational knowledge of the solar scattering
polarization spectrum, now known as the Second Solar Spectrum. This
contribution summarizes the historical developments of this field with
a special focus on the observational events between 1994 and 1996 when
many interesting things happened that cannot be found in the literature.
---------------------------------------------------------
Title: Bioaccumulated manganese and nickel in endemic plants from
New Caledonian mining soils
Authors: Pouschat, P.; Rose, J.; Alliot, I.; Dominici, C.; Keller,
C.; Olivi, L.; Rabier, J.; Ambrosi, J. -P.
2009GeCAS..73R1049P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Polarimetry from the Ground Up
Authors: Keller, C. U.; Snik, F.
2009ASPC..405..371K Altcode: 2008arXiv0809.2772K
Ground-based solar polarimetry has made great progress over the last
decade. Nevertheless, polarimetry is still an afterthought in most
telescope and instrument designs, and most polarimeters are designed
based on experience and rules of thumb rather than using more formal
systems engineering approaches as is common in standard optical design
efforts. Here we present the first steps in creating a set of systems
engineering approaches to the design of polarimeters that makes sure
that the final telescope-instrument-polarimeter system is more than
the sum of its parts.
---------------------------------------------------------
Title: The Prototype of the Small Synoptic Second Solar Spectrum
Telescope (S^5T)
Authors: Snik, F.; Melich, R.; Keller, C. U.
2009ASPC..405..383S Altcode: 2009arXiv0903.2730S
We present the design and the prototype of the Small Synoptic Second
Solar Spectrum Telescope (S^5T), which can autonomously measure
scattering polarization signals on a daily basis with large sensitivity
and accuracy. Its data will be used to investigate the nature of weak,
turbulent magnetic fields through the Hanle effect in many lines. Also
the relation between those fields and the global solar dynamo can be
revealed by spanning the observations over a significant fraction of
a solar cycle. The compact instrument concept is enabled by a radial
polarization converter that allows for “one-shot” polarimetry over the
entire limb of the Sun. A polarimetric sensitivity of ∼10<SUP>-5</SUP>
is achieved by minimizing the instrumental polarization and by
FLC modulation in combination with a fast line-scan camera in the
fiber-fed spectrograph. The first prototype results successfully show
the feasibility of the concept.
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Title: Vector Magnetic Field Inversions of High Cadence SOLIS-VSM Data
Authors: Fischer, C. E.; Keller, C. U.; Snik, F.
2009ASPC..405..311F Altcode:
We have processed full Stokes observations from the SOLIS VSM in the
photospheric lines Fe I 630.15 nm and 630.25 nm. The data sets have
high spectral and temporal resolution, moderate spatial resolution,
and large polarimetric sensitivity and accuracy. We used the LILIA, an
LTE code written by \citet{fischer_Navarro2001} to invert the data. We
also applied the non-potential magnetic field calculation method
of \citet{fischer_Manolis2005} in order to resolve the 180 degree
ambiguity. The output are maps of the full magnetic field vector at
the photospheric level. Here we present the first inversions of the
active region NOAA 10808 during an X-class flare, which occurred on
13 September 2005.
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Title: SOLIS Vector Spectromagnetograph: Status and Science
Authors: Henney, C. J.; Keller, C. U.; Harvey, J. W.; Georgoulis,
M. K.; Hadder, N. L.; Norton, A. A.; Raouafi, N. -E.; Toussaint, R. M.
2009ASPC..405...47H Altcode: 2008arXiv0801.0013H
The Vector Spectromagnetograph (VSM) instrument has recorded
photospheric and chromospheric magnetograms daily since August
2003. Full-disk photospheric vector magnetograms are observed
at least weekly and, since November 2006, area-scans of active
regions daily. Quick-look vector magnetic images, plus X3D and FITS
formated files, are now publicly available daily. In the near future,
Milne-Eddington inversion parameter data will also be available and
a typical observing day will include three full-disk photospheric
vector magnetograms. Besides full-disk observations, the VSM is
capable of high temporal cadence area-scans of both the photosphere
and chromosphere. Carrington rotation and daily synoptic maps are
also available from the photospheric magnetograms and coronal hole
estimate images.
---------------------------------------------------------
Title: Polarization Properties of Real Aluminum Mirrors, I. Influence
of the Aluminum Oxide Layer
Authors: van Harten, G.; Snik, F.; Keller, C. U.
2009PASP..121..377V Altcode: 2009arXiv0903.2740V
In polarimetry, it is important to characterize the polarization
properties of the instrument itself to disentangle real astrophysical
signals from instrumental effects. This article deals with the
accurate measurement and modeling of the polarization properties
of real aluminum mirrors, as used in astronomical telescopes. Main
goals are the characterization of the aluminum oxide layer thickness
at different times after evaporation, and its influence on the
polarization properties of the mirror. The full polarization properties
of an aluminum mirror are measured with Mueller matrix ellipsometry at
different incidence angles and wavelengths. The best fit of theoretical
Mueller matrices to all measurements simultaneously is obtained by
taking into account a model of bulk aluminum with a thin aluminum
oxide film on top of it. Full Mueller matrix measurements of a mirror
are obtained with an absolute accuracy of ∼1% after calibration. The
determined layer thicknesses indicate logarithmic growth in the first
few hours after evaporation, but stability at a value of 4.12 ± 0.08
nm in the long term. Although the aluminum oxide layer is established
to be thin, it is necessary to consider it to accurately describe the
mirror’s polarization properties.
---------------------------------------------------------
Title: Super earth explorer: a coronagraphic off-axis space telescope
Authors: Schneider, J.; Boccaletti, A.; Mawet, D.; Baudoz, P.; Beuzit,
J. -L.; Doyon, R.; Marley, M.; Stam, D.; Tinetti, G.; Traub, W.;
Trauger, J.; Aylward, A.; Cho, J. Y. -K.; Keller, C. -U.; Udry, S.;
SEE-COAST Team
2009ExA....23..357S Altcode: 2008arXiv0811.3908S
The Super-Earth Explorer is an Off-Axis Space Telescope (SEE-COAST)
designed for high contrast imaging. Its scientific objective is to
make the physico-chemical characterization of exoplanets possibly
down to 2 Earth radii. For that purpose it will analyze the spectral
and polarimetric properties of the parent starlight reflected by the
planets, in the wavelength range 400-1,250 nm.
---------------------------------------------------------
Title: An analytical model to demonstrate the reliability of
reconstructed `active longitudes'.
Authors: Jeffers, S. V.; Keller, C. U.
2009AIPC.1094..664J Altcode: 2009csss...15..664J
Photometric light curves show apparent spot concentrations separated
by 180 degrees in longitude that are commonly referred to as `active
longitudes'. These spot concentrations have been observed to change in
strength resulting in the `flip-flop' effect. We use a simple analytical
model to calculate the light curve of a star with an arbitrary spot
pattern to show that `active longitudes' are a likely consequence of
the limited information content contained in a light curve. We also
show that the same effects apply to heavily spotted stars.
---------------------------------------------------------
Title: Characterization of Extra-solar Planets with Direct-Imaging
Techniques
Authors: Tinetti, Giovanna; Cash, Webster; Glassman, Tiffany; Keller,
C.; Oakley, Phil; Snik, F.; Stam, Daphne; Turnbull, Margret
2009astro2010S.296T Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Polarimetry of Mars with SPEX, an Innovative Spectropolarimeter
Authors: Stam, D. M.; Laan, E.; Snik, F.; Karalidi, T.; Keller, C.; Ter
Horst, R.; Navarro, R.; Aas, C.; de Vries, J.; Oomen, G.; Hoogeveen, R.
2008LPICo1447.9078S Altcode:
We present SPEX, an innovative, compact, and robust spectropolarimeter
that measures fluxes and polarization of sunlight reflected by Mars from
400 to 800 nm. With simulations we'll show how with SPEX atmospheric
dust and the surface can be studied.
---------------------------------------------------------
Title: A New Era in Solar Thermal-IR Astronomy: the NSO Array Camera
(NAC) on the McMath-Pierce Telescope
Authors: Ayres, T.; Penn, M.; Plymate, C.; Keller, C.
2008ESPM...12.2.74A Altcode:
The U.S. National Solar Observatory Array Camera (NAC) is a
cryogenically cooled 1Kx1K InSb “Aladdin" array that recently became
operational at the McMath-Pierce facility on Kitt Peak, a high dry site
in the southwest U.S. (Arizona). The new camera is similar to those
already incorporated into instruments on nighttime telescopes, and has
unprecedented sensitivity, low noise, and excellent cosmetics compared
with the Amber Engineering (AE) device it replaces. (The latter was
scavenged from a commercial surveillance camera in the 1990's: only
256X256 format, high noise, and annoying flatfield structure). The
NAC focal plane is maintained at 30 K by a mechanical closed-cycle
helium cooler, dispensing with the cumbersome pumped--solid-N2 40 K
system used previously with the AE camera. The NAC linearity has been
verified for exposures as short as 1 ms, although latency in the data
recording holds the maximum frame rate to about 8 Hz (in "streaming
mode"). The camera is run in tandem with the Infrared Adaptive
Optics (IRAO) system. Utilizing a 37-actuator deformable mirror, IRAO
can--under moderate seeing conditions--correct the telescope image to
the diffraction limit longward of 2.3 mu (if a suitable high contrast
target is available: the IR granulation has proven too bland to reliably
track). IRAO also provides fine control over the solar image for spatial
scanning in long-slit mode with the 14 m vertical "Main" spectrograph
(MS). A 1'X1' area scan, with 0.5" steps orthogonal to the slit
direction, requires less than half a minute, much shorter than p-mode
and granulation evolution time scales. A recent engineering test run,
in April 2008, utilized NAC/IRAO/MS to capture the fundamental (4.6 mu)
and first-overtone (2.3 mu) rovibrational bands of CO, including maps
of quiet regions, drift scans along the equatorial limbs (to measure
the off-limb molecular emissions), and imaging of a fortuitous small
sunspot pair, a final gasp, perhaps, of Cycle 23. Future work with
the NAC will emphasize pathfinding toward the next generation of IR
imaging spectrometers for the Advanced Technology Solar Telescope,
whose 4 m aperture finally will bring sorely needed high spatial
resolution to daytime infrared astronomy. In the meantime, the NAC
is available to qualified solar physicists from around the world to
conduct forefront research in the 1-5 mu region, on the venerable--but
infrared friendly--McMath-Pierce telescope.
---------------------------------------------------------
Title: The upgrade of HARPS to a full-Stokes high-resolution
spectropolarimeter
Authors: Snik, Frans; Jeffers, Sandra; Keller, Christoph; Piskunov,
Nikolai; Kochukhov, Oleg; Valenti, Jeff; Johns-Krull, Christopher
2008SPIE.7014E..0OS Altcode: 2008SPIE.7014E..22S
We present the design of a compact module that converts the HARPS
instrument at the 3.6-m telescope at La Silla to a full-Stokes
high-resolution spectropolarimeter. The polarimeter will replace the
obsolete Iodine cell inside the HARPS Cassegrain adapter. Utilizing
the two fibers going into the spectrograph, two dual-beam systems
can be positioned in the beam: one with a rotating superachromatic
quarter-wave plate for circular polarimetry and one with a rotating
superachromatic half-wave plate for linear polarimetry. A large
polarimetric precision is ensured by the beam-exchange technique
and a minimal amount of instrumental polarization. The polarimeter,
in combination with the ultra-precise HARPS spectrograph, enables
unprecedented observations of stellar magnetic fields and circumstellar
material without compromising the successful planet-finding program.
---------------------------------------------------------
Title: SPHERE ZIMPOL: overview and performance simulation
Authors: Thalmann, Christian; Schmid, Hans M.; Boccaletti, Anthony;
Mouillet, David; Dohlen, Kjetil; Roelfsema, Ronald; Carbillet,
Marcel; Gisler, Daniel; Beuzit, Jean-Luc; Feldt, Markus; Gratton,
Raffaele; Joos, Franco; Keller, Christoph U.; Kragt, Jan, II; Pragt,
Johan H.; Puget, Pascal; Rigal, Florence; Snik, Frans; Waters, Rens;
Wildi, François
2008SPIE.7014E..3FT Altcode: 2008SPIE.7014E.112T
The ESO planet finder instrument SPHERE will search for the
polarimetric signature of the reflected light from extrasolar
planets, using a VLT telescope, an extreme AO system (SAXO),
a stellar coronagraph, and an imaging polarimeter (ZIMPOL). We
present the design concept of the ZIMPOL instrument, a single-beam
polarimeter that achieves very high polarimetric accuracy using fast
polarization modulation and demodulating CCD detectors. Furthermore,
we describe comprehensive performance simulations made with the CAOS
problem-solving environment. We conclude that direct detection of
Jupiter-sized planets in close orbit around the brightest nearby stars
is achievable with imaging polarimetry, signal-switching calibration,
and angular differential imaging.
---------------------------------------------------------
Title: The Extreme Polarimeter (ExPo): design of a sensitive imaging
polarimeter
Authors: Rodenhuis, M.; Canovas, H.; Jeffers, S. V.; Keller, C. U.
2008SPIE.7014E..6TR Altcode: 2008SPIE.7014E.227R
The Extreme Polarimeter (ExPo) is approaching its first deployment
at the 4.2 m William Herschel Telescope at La Palma. This imaging
polarimeter, developed at the Astronomical Institute of Utrecht
University, aims to study circumstellar material at a contrast
ratio with the central star of 10<SUP>-9</SUP>. Working at visible
wavelengths, it will provide an inner working angle down to 0.5
arcsec and a field of view of 20 arcsec diameter. ExPo employs a
dual beam-exchange technique based on polarimeter designs for solar
studies. A partially transmitting coronagraph mask placed in the first
focus reduces the light of the star. The beam is modulated using three
ferro-electric liquid crystals in a Pancharatnam configuration, then
split in a polarizing beamsplitter. Both beams are re-imaged onto
the same Electron-Multiplying CCD camera. We present the design of
the ExPo instrument, highlighting the elements that are critical to
the polarimetric performance. Some prototype laboratory experiments
demonstrating the instrument concept are discussed. These have been
performed using our realistic exoplanet laboratory simulator.
---------------------------------------------------------
Title: Design of a laboratory simulator to test exoplanet imaging
polarimetry
Authors: Jeffers, S. V.; Miesen, N.; Rodenhuis, M.; Keller, C. U.
2008SPIE.7014E..7BJ Altcode: 2008SPIE.7014E.239J
Research on extrasolar planets is one of the most rapidly advancing
fields of astrophysics. In just over a decade since the discovery of
the first extra-solar planet orbiting around 51 Pegasi, 289 extrasolar
planets have been discovered. This breakthrough is the result of
the development of a wide range of new observational techniques
and facilities for the detection and characterisation of extrasolar
planets. In Utrecht we are building the Extreme Polarimeter (ExPo)
to image extra-solar planets and circumstellar environments using
polarimetry at contrast ratio of 10<SUP>-9</SUP>. To test and calibrate
ExPo, we have built a laboratory-based simulator that mimics a star
with a Jupiter-like exoplanet as seen by the 4.2m William Herschel
Telescope. The star and planet are simulated using two single-mode
fibres in close proximity that are fed with a broadband arc lamp with a
contrast ratio down to 10<SUP>-9</SUP>. The planet is partially linearly
polarized. The telescope is simulated with two lenses, and seeing can
be included with a rotating glass plate covered with hairspray. In this
paper we present the scientific requirements and the simulator design.
---------------------------------------------------------
Title: SPEX: an in-orbit spectropolarimeter for planetary exploration
Authors: Snik, Frans; Karalidi, Theodora; Keller, Christoph; Laan,
Erik; ter Horst, Rik; Navarro, Ramon; Stam, Daphne; Aas, Christina;
de Vries, Johan; Oomen, Gijs; Hoogeveen, Ruud
2008SPIE.7010E..15S Altcode: 2008SPIE.7010E..35S
SPEX (Spectropolarimeter for Planetary EXploration) is an innovative,
compact remote-sensing instrument for detecting and characterizing
aerosols. With its 1-liter volume it is capable of full linear
spectropolarimetry, without moving parts. High precision polarimetry is
performed through encoding the degree and angle of linear polarization
of the incoming light in a sinusoidal modulation of the intensity
spectrum. This is achieved by using an achromatic quarter-wave retarder,
an athermal multiple-order retarder and a polarizing beamsplitter
behind each entrance pupil. Measuring a single intensity spectrum thus
provides the spectral dependence of the degree and angle of linear
polarization. Polarimetry has proven to be an excellent tool to study
microphysical properties (size, shape, composition) of atmospheric
particles. Such information is essential to better understand the
weather and climate of a planet. Although SPEX can be used to study
any planetary atmosphere, including the Earth's, the current design of
SPEX is tailored to study Martian dust and ice clouds from an orbiting
platform: a compact module with 9 entrance pupils to simultaneously
measure intensity spectra from 350 to 800 nm, in different directions
along the flight direction (including two limb viewing directions). This
way, both the intensity and polarization scattering phase functions
of dust and cloud particles within a ground pixel are sampled while
flying over it. In the absence of significant amounts of dust and
clouds, the surface properties can be studied. SPEX provides synergy
with instruments on rovers and landers, as it provides a global view
of spatial and temporal variations of the planet.
---------------------------------------------------------
Title: New Observations of the Magnetic Vector Field across the
Solar Disk
Authors: Keller, C. U.; Harvey, J. W.; Henney, C. J.
2008ASPC..384..166K Altcode: 2008csss...14..166K
Full disk solar magnetograms have been available for more than three
decades. However, those maps only show the line-of-sight magnetic
flux. The physical quantity we really want to know is the magnetic
field vector along with the filling factor, i.e. the fractional area
of the resolution element that is occupied by the magnetic field. Since
August 2003, the SOLIS Vector-SpectroMagnetograph has been recording the
photospheric magnetic field vector across the full solar disk with high
sensitivity and resolution. Some of the data are now becoming available
for beta testing, and the first science results are emerging. Here we
provide a brief introduction to the instrument and its data products and
then present some of the data from the first three years of operation.
---------------------------------------------------------
Title: Science Goals of the Extreme Polarimeter (ExPo)
Authors: Jeffers, S. V.; Keller, C. U.; Rodenhuis, M.; Miesen, N.
2007lyot.confE..42J Altcode:
To advance our understanding of the formation, evolution and structure
of extra-solar planetary systems we are building a high-precision
imaging polarimeter (ExPo). ExPo will initially be located at the 4.2m
William Herschel Telescope on La Palma. We will use polarimetric
techniques similar to those developed for high-precision solar
polarimetry to reach a sensitivity of 10^-5, to polarimetrically
image and characterize planets and protoplanetary debris discs. I will
present a review of the proposed data analysis techniques and science
goals that will be achievable using the significant improvement in
polarimetric imaging capabilities.
---------------------------------------------------------
Title: Design Options for the Extreme Polarimeter (ExPo)
Authors: Rodenhuis, M.; Keller, C. U.
2007lyot.confQ..43R Altcode:
The Extreme Polarimeter, ExPo, is being developed for the detailed
study of circumstellar disks and exoplanet characterization at the
4.2-m William Herschel Telescope at La Palma. This imaging polarimeter
is designed to measure linear polarization at the 10-5 level around
bright stars at distances outward of about 0.5 arcsec. We will discuss
the design options for this instrument as well as the advantages and
disadvantages of specific components such as the Atmospheric Dispersion
Compensator, the polarising beamsplitting element, the coronograph mask,
and the polarisation modulator based on either ferroelectric or nematic
liquid crystals. The merits and disadvantages of each of these will
be discussed along with the impact of a certain choice on the overall
instrument performance. Finally, an analysis of several off-the-shelf
scientific cameras for the actual image recording will be presented.
---------------------------------------------------------
Title: Seething Horizontal Magnetic Fields in the Quiet Solar
Photosphere
Authors: Harvey, J. W.; Branston, D.; Henney, C. J.; Keller, C. U.;
SOLIS Team; GONG Team
2007AAS...21010001H Altcode: 2007BAAS...39S.227H
The photospheric magnetic field outside of active regions and the
network has a ubiquitous and dynamic line-of-sight component that
strengthens away from disk center as expected for a nearly horizontal
orientation. This component shows a striking time variation with an
average temporal rms of 1.7 G at 3 arcsec resolution. In our moderate
resolution observations the nearly horizontal component has a frequency
variation power law exponent of -1.4 below 1.5 mHz and is spatially
patchy on scales up to 15 arcsec. <P />The field may be a manifestation
of changing magnetic connections between eruptions and evolution of
small magnetic flux elements in response to convective motions. It shows
no detectable latitude or longitude variations. Preliminary observations
show no evidence of this field in the 854.2 nm chromosphere.
---------------------------------------------------------
Title: Seething Horizontal Magnetic Fields in the Quiet Solar
Photosphere
Authors: Harvey, J. W.; Branston, D.; Henney, C. J.; Keller, C. U.;
SOLIS and GONG Teams
2007ApJ...659L.177H Altcode: 2007astro.ph..2415H
The photospheric magnetic field outside of active regions and the
network has a ubiquitous and dynamic line-of-sight component that
strengthens from disk center to limb as expected for a nearly horizontal
orientation. This component shows a striking time variation with an
average temporal rms near the limb of 1.7 G at ~3" resolution. In our
moderate-resolution observations the nearly horizontal component has
a frequency variation power-law exponent of -1.4 below 1.5 mHz and is
spatially patchy on scales up to ~15". The field may be a manifestation
of changing magnetic connections between eruptions and evolution of
small magnetic flux elements in response to convective motions. It
shows no detectable latitude or longitude variations.
---------------------------------------------------------
Title: The Ba II 4554 / Hβ Imaging Polarimeter for the Dutch Open
Telescope
Authors: Snik, F.; Bettonvil, F. C. M.; Jägers, A. P. L.;
Hammerschlag, R. H.; Rutten, R. J.; Keller, C. U.
2006ASPC..358..205S Altcode:
In order to expand the high-resolution, multi-wavelength imaging
capabilities of the Dutch Open Telescope (DOT), an additional
polarimetric channel based on a 80 mÅ tunable Lyot filter for Ba
II 4554 and Hβ has been designed and constructed. The large atomic
mass and the resulting steep line wings, make Ba II 4554 particularly
suitable for the creation of photospheric Dopplergrams and Stokes-V
magnetograms. The line also yields a significant degree of linear
(scattering) polarization for observations near the limb of the Sun,
which is modified by both horizontal and vertical weak-field topologies
through the Hanle effect and hyperfine-structure level crossing. The
polarimeter is based on liquid crystal variable retarders (LCVRs)
as polarization modulators in combination with the Lyot filter's
entrance polarizer. The tunability of the LCVRs is exploited to enable
specific wavelength calibration, selection of the reference frame of
linear polarization, and optimization of instrumental polarization
cross-talk, which for the DOT is constant in time. With the future
Ba II 4554 photospheric magnetograms, we expect to be able to discern
magnetic structures of about 150 km with field strengths down to 100 G,
and that Hanle-type observations can be performed at a resolution of
about 1 arcsec. The range of applicability of Hβ imaging polarimetry
has to be explored after installation.
---------------------------------------------------------
Title: SOLIS-VSM Solar Vector Magnetograms
Authors: Henney, C. J.; Keller, C. U.; Harvey, J. W.
2006ASPC..358...92H Altcode: 2006astro.ph.12584H
The Vector SpectroMagnetograph (VSM) instrument has recorded full-disk
photospheric vector magnetograms weekly since August 2003 as part
of the Synoptic Optical Long-term Investigations of the Sun (SOLIS)
project. After the full deployment of the VSM data processing system,
a typical observing day will include three Fe I 630.2 nm full-disk
photospheric vector magnetograms, one full-disk photospheric and
three Ca II 854.2 nm chromospheric longitudinal magnetograms, along
with three He I 1083 nm spectroheliograms. The photospheric vector
magnetograms will be available over the Internet in two stages: first,
as a quick-look product within minutes of data acquisition, and then
as a Milne-Eddington inversion product within a day of each observation.
---------------------------------------------------------
Title: High-Resolution Surface Observations and What They Can Tell
MHD Simulations
Authors: Keller, C. U.
2006ASPC..354....3K Altcode:
I review selected state-of-the art observations of the solar
photosphere at the highest possible spatial resolution with a special
emphasis on properties that can directly be compared with current
MHD simulations. Observations of the solar surface have made big
advances during the last decade thanks to new telescopes and new
data processing approaches. I will provide an overview of the latest
observations and discuss parameters such as the rms contrast and other
statistical parameters that can be directly related to state-of-the-art
MHD simulations.
---------------------------------------------------------
Title: Solar Carbon Monoxide, Thermal Profiling, and the Abundances
of C, O, and Their Isotopes
Authors: Ayres, Thomas R.; Plymate, Claude; Keller, Christoph U.
2006ApJS..165..618A Altcode: 2006astro.ph..6153A
A solar photospheric “thermal profiling” analysis is presented,
exploiting the infrared (2.3-4.6 μm) rovibrational bands of carbon
monoxide (CO) as observed with the McMath-Pierce Fourier transform
spectrometer (FTS) at Kitt Peak, and from above the Earth's atmosphere
by the Shuttle-borne ATMOS experiment. Visible continuum intensities
and center-limb behavior constrained the temperature profile of the deep
photosphere, while CO center-limb behavior defined the thermal structure
at higher altitudes. The oxygen abundance was self-consistently
determined from weak CO absorptions (for C/O≡0.5). Our analysis
was meant to complement recent studies based on three-dimensional
(3D) convection models, which, among other things, have revised the
historical solar oxygen (and carbon) abundance downward by a factor
of nearly 2, although in fact our conclusions do not support such a
revision. Based on various considerations, an ɛ<SUB>O</SUB>=700+/-100
ppm (parts per million relative to hydrogen) is recommended; the large
uncertainty reflects the model sensitivity of CO. New solar isotopic
ratios also are reported: <SUP>12</SUP>C/<SUP>13</SUP>C=80+/-1,
<SUP>16</SUP>O/<SUP>17</SUP>O=1700+/-220, and
<SUP>16</SUP>O/<SUP>18</SUP>O=440+/-6-all significantly lower than
terrestrial. CO synthesis experiments utilizing a stripped down
version of the 3D model-which has large temperature fluctuations in the
middle photosphere, possibly inconsistent with CO “movies” from the
Infrared Imaging Spectrometer (IRIS), and a steeper mean temperature
gradient than matches visible continuum center-limb measurements-point
to a lower oxygen abundance (~500 ppm) and isotopic ratios closer
to terrestrial. A low oxygen abundance from CO-and other molecules
like OH-thus hinges on the reality of the theoretically predicted
midphotospheric convective properties.
---------------------------------------------------------
Title: Design of a polarimeter for extrasolar planetary systems
characterization
Authors: Keller, Christoph U.
2006SPIE.6269E..0TK Altcode: 2006SPIE.6269E..26K
Unpolarized light from the central star that is reflected by exoplanets,
protoplanetary disks, and debris disks becomes partially polarized by
the reflection process. Imaging polarimetry is therefore the ideal
way to discriminate between the polarized light from circumstellar
environments and the unpolarized light from the nearby central star. A
sensitivity of 10 <SUP>-5</SUP> (fraction of polarized intensity to the
total intensity) must be achieved to detect exoplanets; 10 <SUP>-4</SUP>
is sufficient for disks. Based on extensive experience in precision
polarimetry of the Sun, the newly formed experimental astrophysics group
at Utrecht University, The Netherlands, will design, build, and use
a high-precision imaging polarimeter for use at the 4.2-meter William
Herschel Telescope. Since systematic errors typically limit conventional
imaging polarimeters to about 10 <SUP>-3</SUP>, laboratory setups
and theoretical models will be used to understand and then minimize
and/or calibrate systematic errors. Published catalogues of exoplanets
and stars that harbor disks will guide extensive observations with
this new polarimeter. The effort will focus on retrieving fundamental
properties of circumstellar environments that cannot be obtained with
other observational approaches.
---------------------------------------------------------
Title: Development of an IFU for diffraction-limited 3D
spectropolarimetry
Authors: Ren, Deqing; Keller, Christoph; Plymate, Claude
2006SPIE.6269E..5ZR Altcode: 2006SPIE.6269E.191R
Ground-based telescopes can achieve diffraction-limited images when
equipped with adaptive optics (AO). A major limitation of AO is the
small field of view, which is due to the limited isoplanatic patch
size. Nevertheless, conventional long-slit spectrographs cannot sample
the entire AO-corrected field of view in a single exposure. However,
equipped with a modern, large detector array, the Integral Field
Unit (IFU) technique will allow a 3-dimensional (3-D) data cube to
be recorded simultaneously over the entire AO corrected field of
view, with a conventional long-slit spectrographs. We are building a
state-of-the-art image slicer IFU for the National Solar Observatory's
(NSO) McMath-Pierce Solar Telescope (McMP). This will be the first time
that an advanced image slicer IFU is used for 3-D spectroscopy and
polarimetry at a solar telescope. The IFU consists of 25 slices that
will sample a 6.25" x 8" AO corrected field of view simultaneously,
and produces a 200" long slit for diffraction-limited 3-D spectroscopy
and polarimetry. This IFU 3-D technique will provide the most high
spatial, high temporal resolution with high throughput for solar
spectroscopy and polarimetry. This is critical for state-of-the-art
spectral diagnosis of solar velocity and magnetic fields. We discuss
the design, construction, and testing of this new IFU.
---------------------------------------------------------
Title: Mapping sodium distribution in the exosphere of Mercury with
tip-tilt image stabilization
Authors: Potter, A. E.; Plymate, C.; Keller, C.; Killen, R. M.;
Morgan, T. H.
2006AdSpR..38..599P Altcode:
Mapping the sodium emission about Mercury is a difficult observational
problem, since Mercury is seen either against the bright daytime sky,
or against a dark sky at very high air masses. The distribution of
sodium emission over the surface of Mercury is non-uniform, and changes
over time. These effects give clues to the processes that produce
the sodium and control its distribution, so that improved mapping
of sodium emissions over the Mercury surface will help clarify their
relative importance. We have adapted an image stabilizer utilizing a
piezoelectric driven tip-tilt correction mirror for daytime spectral
imaging of Mercury. The image stabilizer, which was originally developed
for solar observations at the McMath Pierce solar telescope, results
in a noticeable improvement in spatial resolution of our Mercury
sodium images. In this paper we give initial results from use of
the tip-tilt image stabilizer for observations of Mercury’s sodium
exosphere. Further systematic observations and improvements are planned
for the image stabilizer system, as well as experimental observations
with a low-order adaptive optics system incorporating a commercially
available 37-actuator deformable mirror.
---------------------------------------------------------
Title: C3Po --- The Ultimate Focal Plane Array for Imaging Polarimetry
from 200 nm to 20 μm
Authors: Keller, C. U.
2005ASPC..343...33K Altcode:
C^3Po is a novel array detector concept that is optimized for highly
sensitive and precise differential imaging. Chopping between two
or more independent image states (such as four linearly independent
polarization states) can be performed at speeds in the kHz domain to
provide virtually simultaneous images without the need to read out the
array at kHz frame rates. This allows the technology to be applied
to large arrays with precise, slow readouts. All independent image
planes are observed with the same physical pixel on the detector, which
renders normalized differences between image planes insensitive to the
gain of individual pixels. The detector concept has 100% geometrical
fill factor and a quantum efficiency approaching unity. The technology
can be applied to silicon to cover the 200--1100 nm wavelength range,
and to infrared-sensitive materials such as HgCdTe or InSb for the
1-20 μm wavelength range. The detector concept has a wide range of
potential applications outside of astronomy.
---------------------------------------------------------
Title: High precision polarimetry with the Advanced Technology
Solar Telescope
Authors: Socas-Navarro, Hector; Elmore, David F.; Keller, Christoph
U.; Seagraves, Paul H.; Streander, Kim V.; Card, Gregory L.; Warner,
Mark; Kuhn, Jeffrey R.; Mickey, Donald L.
2005SPIE.5901...52S Altcode:
This paper addresses the issue of calibrating the Advanced Technology
Solar Telescope for high-precision polarimetry, in particular of the
optical train above the Gregorian station (where suitable calibration
optics will be placed). Conventional techniques would not be adequate
for this telescope given its large aperture. Here we explore two
different methods that are currently being considered by the design
team. The first one is the "sub-aperture" method, which uses small
calibration optics above the primary mirror to calibrate a small
sub-aperture of the system. This calibration is then extended to the
full aperture by means of actual observations. The second method is
based on analyzing the polarization observed in a spectral line with
a peculiar Zeeman pattern, such as the FeII 614.9 nm line, which does
not produce any intrinsic linear polarization. Numerical simulations
are presented that show the robustness of both techniques and their
respective advantages and disadvantages are discussed.
---------------------------------------------------------
Title: Reduction of SOLIS/Vector Spectromagnetograph He I 1083
nm Observations
Authors: Jones, H. P.; Malanushenko, O. V.; Harvey, J. W.; Henney,
C. J.; Keller, C. U.
2005AGUSMSP51A..02J Altcode:
The SOLIS/Vector Spectromagnetograph (VSM) now continues the
three-decade record of full-disk He I 1083 nm spectroheliograms from
NSO/Kitt Peak (see poster by Henney et al.). Although the sensitivity
of the VSM interim cameras at 1083 nm is far greater than that of
previous NSO/KP instruments, the observations are compromised by
strong interference fringes produced within the detectors. We discuss
in some detail the extent to which this difficulty is overcome
in the simple algorithm used to produce the synoptic observations
and explore methods for removing remaining fringe effects for true
imaging spectroscopy. These techniques include Fourier and Wavelet
filtering, the flat-fielding algorithm used for earlier NASA/NSO
Spectromagnetograph (SPM) observations, and physical modeling of the
detector fringe pattern.
---------------------------------------------------------
Title: The Solar Oxygen Abundance, and the Rare Isotopes of C and O,
Derived from Infrared Spectra of Carbon Monoxide
Authors: Ayres, T. R.; Plymate, C.; Keller, C.; Kurucz, R. L.
2005AGUSMSP41B..09A Altcode:
A detailed abundance analysis is presented for solar oxygen based on
the ΔV=1 fundamental (4.6~μm) and ΔV=2 (2.3~μm) first-overtone
rovibrational bands of carbon monoxide observed above the Earth's
atmosphere at very high spectral resolution and high signal-to-noise by
the Shuttle-borne ATMOS Fourier transform spectrometer (FTS). Additional
observations to define the reference photospheric thermal structure
were taken of the CO fundamental bands in an atmospheric window at
2145~cm-1 (4.6~μm) using the 1~m FTS of the McMath-Pierce telescope
at Kitt Peak and a fast tip/tilt image stabilization system. The latter
allowed measurements at the extreme limb where the highly slanted rays
probe into the outer layers of the photosphere. High spatial resolution
"movies" of weak CO lines at disk center taken under excellent seeing
conditions with the Infrared Imaging Spectrometer (IRIS), also on
the McMath-Pierce telescope, further constrained thermal and velocity
fluctuations in the layers in which the abundance-sensitive CO lines
form. This work is meant to complement a series of recent studies
which have revised the previously recommended solar oxygen abundance
downward by nearly a factor of two; although in fact our conclusions
do not support such a revision. The oxygen abundance recovered in the
present work is 700±70~ppm (parts per million relative to hydrogen)
compared with the proposed downward revision to 460±60~ppm, and the
recommended value of 650±100~ppm of a decade ago. In our analysis,
a fixed C/O ratio of 0.5, derived in independent work, was assumed;
so the associated carbon abundance is 350~ppm. New accurate values
for the solar abundance ratios of the rare isotopes of C and O also
are reported: 12C/13C= 70, 16O/17O= 400, and 16O/18O= 2000. All three
ratios are lower than terrestrial or meteoritic values (indicating
higher isotopic abundances). We find no evidence in the ATMOS3 spectra
for measurable 14C16O lines.
---------------------------------------------------------
Title: Are You Being Served? - Solar Data Management at the National
Solar Observatory
Authors: Hill, F.; Suarez-Sola, I.; Harvey, J. W.; Henney, C. J.;
Keller, C.; Wampler, S.
2005AGUSMSH43B..04H Altcode:
Solar physics is a data driven field, where no progress can be made
without multi-wavelength and multi-cadence information about the
sun's behavior. The National Solar Observatory (NSO) has provided
data to the research community for over 30 years and thus has
considerable experience in the areas of data processing, archiving,
and distribution. This presentation will describe the development of
NSO's data services, outline what lessons have been learned along the
way, and attempt to forecast the future of data management at NSO.
---------------------------------------------------------
Title: First-Light Instrumentation for the Advanced Technology
Solar Telescope
Authors: Rimmele, T.; Balasubramaniam, K.; Berger, T.; Elmore, D.;
Gary, A.; Keller, C.; Kuhn, J.; Lin, H.; Mickey, D.; Pevtsov, A.;
Robinson, B.; Sigwarth, M.; Soccas-Navarro, H.
2005AGUSMSP34A..03R Altcode:
The 4m Advanced Technology Solar Telescope (ATST) is the next
generation ground based solar telescope. In this paper we provide
an overview of the ATST post-focus instrumentation. The majority of
ATST instrumentation is located in an instrument Coude lab facility,
where a rotating platform provides image de-rotation. A high order
adaptive optics system delivers a corrected beam to the Coude lab
facility. Alternatively, instruments can be mounted at the Nasmyth
focus. For example, instruments for observing the faint corona
preferably will be mounted at Nasmyth where maximum throughput
is achieved. In addition, the Nasmyth focus has minimum telescope
polarization and minimum stray light. We give an overview of the
initial set of first generation instruments: the Visible-Light
Broadband Imager (VLBI), the Visible Spectro-Polarimeter (ViSP),
the Near-IR Spectro-Polarimeter (NIRSP), which includes a coronal
module, and the Visible Tunable Filter. We also discuss the unique and
efficient approach to the ATST instrumentation, which builds on the use
of common components such as detector systems, polarimetry packages
and various opto-mechanical components. For example, the science
requirement for polarimetric sensitivity (10-5 relative to intensity)
and accuracy (5'10-4 relative to intensity) place strong constraints
on the polarization analysis and calibration units. Consequently,
these systems are provided at the facility level, rather than making
it part of the requirement for each instrument.
---------------------------------------------------------
Title: SOLIS-VSM Synoptic Products
Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U.
2005AGUSMSP51A..04H Altcode:
Since August 2003, the Vector Spectromagnetograph (VSM) instrument has
recorded full-disk photospheric and chromospheric magnetograms along
with He I 1083 nm spectroheliograms daily as part of the Synoptic
Optical Long-term Investigations of the Sun (SOLIS) project. In
addition, the VSM measured full-disk photospheric vector magnetograms
weekly. After the full deployment of the VSM data processing system,
a typical observing day will include three full-disk photospheric
vector magnetograms. Products from observations for the past year
and half are expected to become available during the year 2005, these
include Dopplergrams, equivalent width and line depth images. Besides
individual full-disk images, several derived products are also or
soon will be available via the SOLIS web site. These products include
integrated full-disk magnetogram and helium spectroheliogram time
series. Additionally, Carrington rotation and daily synoptic maps
are now available from the photospheric magnetograms. Daily coronal
hole estimate images using VSM data, along with the creation of solar
wind source maps will also be available. The VSM synoptic products
are available on the NSO-SOLIS web site cooperatively by NSF/NSO
and NASA/GSFC. The National Solar Observatory is operated by AURA,
Inc. under a cooperative agreement with the National Science Foundation.
---------------------------------------------------------
Title: Adaptive Optics Observations of the Sun
Authors: Keller, Christoph U.
2005sao..conf..119K Altcode:
Modern solar observations are performed close to the diffraction
limit of the existing solar telescopes since important, fundamental
processes on the Sun occur on scales of a tenth of an arc-second
and smaller. Spatial resolution is therefore paramount in solar
observations. Even though solar telescopes have relatively small
apertures as compared to modern night-time telescopes, their spatial
resolution is still limited by seeing, which is worse during the
day as compared to at night. Adaptive optics has thus become an
important addition to almost all large solar telescopes. While the
first solar adaptive optics system was among the first to be used at
astronomical telescopes, only in the past few years have such systems
become available for regular science use. Publications of solar science
obtained with adaptive optics are therefore still rare, but their number
is rapidly increasing. Here I discuss the existing solar adaptive optics
systems and summarize some of the science they have produced to date.
---------------------------------------------------------
Title: Helicity and the SOLIS Vector-Spectromagnetograph
Authors: Keller, Christoph U.; Harvey, John W.; Henney, Carl. J.;
Jones, Harrison P.
2005HiA....13..126K Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun)
is a suite of three innovative instruments that greatly improve
ground-based synoptic solar observations. The main instrument the
Vector Spectro-Magnetograph (VSM) is a compact high-throughput
vector-polarimeter that measures the magnetic field strength and
direction over the full solar disk within 15 minutes. Helicity can
be directly calculated from the full-disk vector field data provided
by the Stokes inversion of the observed polarized line profiles. We
present an anlysis of the difficulties and problems that one faces
when interpreting the the helicity derived from VSM data.
---------------------------------------------------------
Title: Radio Observations of the Quiet Sun
Authors: Keller, Christoph U.; Krucker, Säam
2004ASSL..314..287K Altcode:
While radio observations of the Sun have mostly focused on active
region phenomena, they also contribute unique data to our knowledge
of the quiet Sun, in particular through accurate measurements of the
temperature as a function of height in the atmosphere and through the
measurement of nonthermal emissions from chromospheric and coronal
heating events. Here we review observations of the quiet Sun using
radio telescopes and discuss current science problems that will be
addressed with future facilities such as the Frequency Agile Solar
Radiotelescope (FASR).
---------------------------------------------------------
Title: Instrumentation for the Advanced Technology Solar Telescope
Authors: Rimmele, Thomas R.; Hubbard, Robert P.; Balasubramaniam,
K. S.; Berger, Tom; Elmore, David; Gary, G. Allen; Jennings, Don;
Keller, Christoph; Kuhn, Jeff; Lin, Haosheng; Mickey, Don; Moretto,
Gilberto; Socas-Navarro, Hector; Stenflo, Jan O.; Wang, Haimin
2004SPIE.5492..944R Altcode:
The 4-m aperture Advanced Technology Solar Telescope (ATST) is the
next generation ground based solar telescope. In this paper we provide
an overview of the ATST post-focus instrumentation. The majority of
ATST instrumentation is located in an instrument Coude lab facility,
where a rotating platform provides image de-rotation. A high order
adaptive optics system delivers a corrected beam to the Coude lab
facility. Alternatively, instruments can be mounted at Nasmyth or
a small Gregorian area. For example, instruments for observing the
faint corona preferably will be mounted at Nasmyth focus where maximum
throughput is achieved. In addition, the Nasmyth focus has minimum
telescope polarization and minimum stray light. We describe the set of
first generation instruments, which include a Visible-Light Broadband
Imager (VLBI), Visible and Near-Infrared (NIR) Spectropolarimeters,
Visible and NIR Tunable Filters, a Thermal-Infrared Polarimeter &
Spectrometer and a UV-Polarimeter. We also discuss unique and efficient
approaches to the ATST instrumentation, which builds on the use of
common components such as detector systems, polarimetry packages and
various opto-mechanical components.
---------------------------------------------------------
Title: Solar and Space Weather Radiophysics - Current Status and
Future Developments
Authors: Gary, D. E.; Keller, C. U.
2004ASSL..314.....G Altcode: 2004sswr.book.....G
The book explores what can be learned about the Sun and interplanetary
space using present-day and future radio observations and
techniques. The emphasis is on interpretation of radio data with high
spatial and spectral resolution, motivated by the planned construction
of a new, powerful, solar-dedicated radio array called the Frequency
Agile Solar Radiotelescope (FASR). The book is unique in exploring a
broad frequency range, which corresponds to heights ranging from the
low solar atmosphere out to the Earth. The book contains a thorough
review of the entire field of solar and Space Weather radio research;
gives background information suitable for advanced undergraduates,
graduates, and researchers in solar and Space Weather research and
related fields; and looks at what new results may be expected in
the next two decades with FASR and other new instruments now under
development. The individual chapters are written by international
experts in each topic, and although each chapter may be read as a
stand-alone article, the ordering of the chapters and the topical
development makes the book readable from beginning to end, to provide
an excellent understanding of the field as a whole.
---------------------------------------------------------
Title: Evaluation of Seeing-Induced Cross Talk in Tip-Tilt-Corrected
Solar Polarimetry
Authors: Judge, Philip G.; Elmore, David F.; Lites, Bruce W.; Keller,
Christoph U.; Rimmele, Thomas
2004ApOpt..43.3817J Altcode:
We reanalyze the effects of atmosphere-induced image motions on the
measurement of solar polarized light using a formalism developed by
Lites. Our reanalysis is prompted by the advent of adaptive optics (AO)
systems that reduce image motion and higher-order aberrations, by the
availability of liquid crystals as modulation devices, and by the need
to understand how best to design polarimeters for future telescopes
such as the Advanced Technology Solar Telescope. In this first attempt
to understand the major issues, we analyze the influence of residual
image motion (tip-tilt) corrections of operational AO systems on the
cross talk between Stokes parameters and present results for several
polarization analysis schemes. Higher-order wave-front corrections are
left for future research. We also restrict our discussion to the solar
photosphere, which limits several important parameters of interest,
using some recent magnetoconvection simulations.
---------------------------------------------------------
Title: Improved Spatial Resolution of the Sodium Distribution
on Mercury
Authors: Potter, A. E.; Plymate, C.; Keller, C.; Killen, R. M.;
Morgan, T. H.
2004AAS...204.6407P Altcode: 2004BAAS...36R.780P
The distribution of sodium emission over the surface of Mercury
is non-uniform, and changes over time. These non-uniformities and
time-dependent changes give clues to the processes that produce the
sodium. However, observations of the distribution of sodium on Mercury
are hampered by the bad seeing that accompanies the need to observe
either in daytime, or at extremely large air masses in twilight. In
an effort to improve this situation, we have developed an image
stabilizer utilizing a piezoelectric driven tip-tilt correction mirror
for daytime spectral imaging of Mercury. The system is a modification
of one that was originally developed for solar observations at the
McMath-Pierce solar telescope. Use of image stabilization results in
a noticeable improvement in spatial resolution of our Mercury sodium
images. A series of sodium images taken over an eight-day period
show changes in which an emission peak in high southern latitudes
disappears, and is replaced by an emission peak at high northern
latitudes. Further systematic observations and improvements are planned
for the image stabilizer system, as well as experimental observations
with a low-order adaptive optics system incorporating a commercially
available 37-actuator deformable mirror. <P />This work was supported
by the NASA Planetary Astronomy Program.
---------------------------------------------------------
Title: High-accuracy Multiangle Spectropolarimetric Imaging Concept
for Aerosol Remote Sensing from Space
Authors: Diner, D. J.; Chipman, R. A.; Cairns, B.; Foo, L. D.; Keller,
C. U.; Macenka, S. A.; Bruce, C. F.
2004AGUSMGC51A..07D Altcode:
Satellite remote sensing has a key role in measuring the distribution,
radiative impact, and regional and global spatial context of
tropospheric aerosols. A synergistic combination of multispectral,
multiangle, and polarimetric approaches would improve the accuracies of
aerosol optical depth and particle property characterizations compared
to what is achievable using each method by itself. In this paper we
discuss the science benefits and technical feasibility of combining
key attributes of multiple aerosol remote sensing instruments into a
single instrument package. The features of the conceptual instrument
are: spectral coverage from the near-UV to the shortwave infrared;
global coverage within a few days; intensity and polarimetric imaging
simultaneously at multiple view angles; kilometer to sub-kilometer
spatial resolution; and measurement of the degree of linear polarization
in one visible and one shortwave-infrared spectral band, i.e., a
subset of the full spectral complement, with an uncertainty of 0.5%
or less. The polarimetric accuracy is the driving requirement of the
instrument design, and is stipulated in order to achieve uncertainty
goals in optical depth (0.01) and single scattering albedo (0.03) that
appear difficult to reach given the current state-of-the-art of the
calibration of intensity-only measurements. Bispectral polarimetry is
invoked to enable size-resolved retrievals of particle real refractive
index. After examining many approaches and technologies for imaging
polarimetry, we conclude that ultrafast time-multiplexing is the best
option for meeting the instrument performance requirements. The approach
is based upon innovative advances in high-precision imaging polarimetry
developed for ground-based solar astronomy. Rapid modulation of the
linear polarization Stokes components Q and U, coupled with synchronous
demodulation in a charge-caching focal plane, provides two essential
benefits: (1) the same detector is used to measure the relative
proportions of Q or U to the total intensity, thus circumventing
inaccuracies introduced by detector gain changes or uncertainties in
flight, and (2) rapid interlacing of the measurements at sub-pixel
scale insulates against false polarization signals as the spacecraft
flies over a spatially varying scene. Technology advances needed to
implement this approach are identified.
---------------------------------------------------------
Title: On the Origin of Solar Faculae
Authors: Keller, C. U.; Schüssler, M.; Vögler, A.; Zakharov, V.
2004ApJ...607L..59K Altcode:
Solar faculae appear as bright small features close to the solar
limb. Recent high-resolution images show these brightenings in
unprecedented detail. Our analysis of numerical MHD simulations
reproduces the observed small-scale features. The simulations reveal
that faculae originate from a thin layer within granules just below
largely transparent magnetic flux concentrations. This is basically
the “bright wall” model of Spruit. The dark, narrow lanes often
associated with faculae occur at the opposite side of the magnetic flux
concentration and are due to an extended layer with lower-than-average
temperature.
---------------------------------------------------------
Title: SOLIS-VSM Longitudinal Magnetogram Calibration
Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U.; Jones, H. P.
2004AAS...204.3702H Altcode: 2004BAAS...36..709H
The VSM (Vector SpectroMagnetograph) instrument started recording
daily full-disk magnetograms during August, 2003 at a temporary
site in Tucson, Arizona. Along with the other instruments that
constitute the SOLIS (Synoptic Optical Long-term Investigations of
the Sun) project, the VSM will be relocated to Kitt Peak in April,
2004. The VSM instrument provides a unique record of solar full-disk
vector magnetograms along with the high sensitivity photospheric and
chromospheric longitudinal magnetograms. In addition, the VSM takes
daily full-disk He I 1083 nm equivalent width observations. These
parameter-grams are constructed from individual scans in declination
of the projected solar image on the entrance slit with a relatively
large field angle of over a half of a degree in the spectrograph. The
calibration of the VSM longitudinal magnetic observations is reviewed,
along with a comparison between the VSM mean field, sun-as-a-star,
measurements with those by the Wilcox Solar Observatory. In addition,
the making of synoptic maps derived from the VSM magnetograms is
discussed, including the filling of data gaps in polar regions and
the use of accompanying weight maps. VSM magnetograms and synoptic
maps are available via the Internet at: http://solis.nso.edu/. This
research was supported in part by the Office of Naval Research Grant
N00014-91-J-1040. The National Solar Observatory is operated by AURA,
Inc. under a cooperative agreement with the National Science Foundation.
---------------------------------------------------------
Title: Early Results from SOLIS
Authors: Harvey, J.; Giampapa, M.; Henney, C.; Keller, C.; Jones, H.
2004AAS...204.3701H Altcode: 2004BAAS...36..708H
SOLIS (Synoptic Optical Long-Term Investigations of the Sun)
is a project that is replacing antiquated synoptic observing
equipment at the National Solar Observatory. SOLIS consists of
a suite of three instruments on an equatorial mount that will be
installed on Kitt Peak in April 2004. The major SOLIS instrument
is a vector spectromagnetograph (VSM) that maps magnetic fields
across the full solar disk using a slit spectrograph and one arc
sec pixels. Limited daily observations started at a temporary site
in August, 2003 and include line-of-sight component magnetograms in
the photosphere and chromosphere and, for the first time, full-disk
vector magnetograms. At a medium scan speed ( ∼ 10 minutes for the
full disk) noise is less than 1 Mx/cm<SUP>2</SUP>. This low noise,
combined with negligible instrumental polarization and well resolved
spectral line profiles, yields moderate resolution magnetograms of
unprecedented quality. Observations show magnetic flux nearly everywhere
in the photosphere from the disk center to the solar limb. Weak,
intranetwork fields are now routinely observed and show a tendency to
be of opposite polarity to the stronger surrounding fields. Diffuse
fields surround decaying active regions and appear to be distinct from
canopy fields. Vector magnetograms easily show the radial orientation
of network fields, and the diffuse component surrounding decaying active
regions. Near the disk center, the transverse magnetic fields of network
elements change on a time scale of minutes. Detailed quantitative
calibration of the observations is in progress. Good results have been
obtained from the other SOLIS instruments: a full-disk filter imager at
several narrow wavelengths and a double-pass grating spectrograph that
provides high-accuracy line spectra of integrated sunlight. SOLIS data
are freely available via the Internet and users are invited to submit
observing time requests for special observations. The National Solar
Observatory is operated by AURA, Inc. under a cooperative agreement
with the National Science Foundation. Additional support for the
development of SOLIS from NASA and ONR is gratefully acknowledged.
---------------------------------------------------------
Title: Innovative Information Technology for Space Weather Research
Authors: Wang, H.; Qu, M.; Shih, F.; Denker, C.; Gerbessiotis, A.;
Lofdahl, M.; Rees, D.; Keller, C.
2004AAS...204.5209W Altcode: 2004BAAS...36..755W
Solar activity is closely related to the near earth environment --
summarized descriptively as space weather. Changes in space weather
have adverse effect on many aspects of life and systems on earth and in
space. Real-time, high-quality data and data processing would be a key
element to forecast space weather promptly and accurately. Recently,
we obtained a funding from US National Science Foundation to apply
innovative information technology for space weather prediction. <P />(1)
We use the technologies of image processing and pattern recognition,
such as image morphology segmentation, Support Vector Machines (SVMs),
and neural networks to detect and characterize three important solar
activities in real-time: filament eruptions, flares, and emerging flux
regions (EFRs). Combining the real time detection with the recent
statistical study on the relationship among filament eruptions,
flares, EFRs, coronal mass ejections (CMEs), and geomagnetic storms,
we are establishing real time report of solar events and automatic
forecasting of earth directed CMEs and subsequent geomagnetic storms. <P
/>(2) We combine state-of-art parallel computing techniques with phase
diverse speckle imaging techniques, to yield near real-time diffraction
limited images with a cadence of approximately 10 sec. We utilize the
multiplicity of parallel paradigms to optimize the calculation of phase
diverse speckle imaging to improve calculation speed. With such data,
we can monitor flare producing active regions continuously and carry
out targeted studies of the evolution and flows in flare producing
active regions. <P />(3) We are developing Web based software tools to
post our processed data, events and forecasting in real time, and to
be integrated with current solar activity and space weather prediction
Web pages at BBSO. This will also be a part of Virtual Solar Observatory
(VSO) being developed by the solar physics community. <P />This research
is supported by NSF ITR program.
---------------------------------------------------------
Title: Evershed Flow of CO at Different Depths in a Sunspot Penumbra.
Authors: Clark, T. Alan; Plymate, C.; Bergman, M. W.; Keller, C. U.
2004AAS...204.3720C Altcode: 2004BAAS...36..712C
CO lines of different strengths near 4.7 microns have been used to
observe Evershed flow of molecular gas in the penumbra of a large
sunspot near to the solar limb. An adaptive optics system and IR
camera on the main spectrograph of the McMath-Pierce Solar Telescope
produced a sequence of diffraction-limited spectral-spatial images to
0.8 arc-second resolution as the telescope was scanned across this
sunspot. Dopplergrams constructed from this sequence for several
of these CO lines have been used to outline the characteristics of
this molecular flow as a function of depth within the penumbra. <P
/>Particularly noteworthy in these Dopplergrams is the change in
pattern of high-speed outflow, from a radial direction deep in the
penumbra from weak-line images, matching the structure in the continuum
penumbral image, to a spiral pattern more closely resembling the
appearance of penumbral structure at chromospheric heights. Typical
weak-line flow speeds were a few km/s, significantly lower than those
measured recently by Penn et al.,(2003) for weak CH lines at shorter
wavelengths, which are postulated to originate in dark fibrils deeper
in the penumbra. Strong CO-line images show lower flow speeds. Small
regions of narrow inverse Evershed flow channels appear in these latter
images near to the outer penumbral boundary. For all lines, the speed of
the Evershed flow reaches a maximum just before this boundary and all
flow appears to cease at or just beyond it. <P />Future Evershed flow
investigations will include observations of other potentially useful
spectral features within this spectral range such as weak OH lines,
in addition to several atomic FeI and SiI lines whose high Zeeman
sensitivity make them suitable for simultaneous penumbral magnetic
field mapping. <P />Penn, M.J, et al., Ap.J. 590, L119, 2003.
---------------------------------------------------------
Title: Measurement Scale of the SOLIS Vector Spectromagnetograph
Authors: Jones, H. P.; Harvey, J. W.; Henney, C. J.; Keller, C. U.;
Malanushenko, O. M.
2004AAS...204.3703J Altcode: 2004BAAS...36Q.709J
Longitudinal magnetograms obtained with the SOLIS Vector
Spectromagnetograph (VSM) during a cross-calibration period are
compared with similar data from the NASA/NSO Spectromagnetograph (SPM)
at the NSO/Kitt Peak Vacuum Telescope as well as with SOHO/MDI and GONG
magnetograms. The VSM began observations at the University of Arizona
agricultural test site and collaborative observations were obtained
with both the VSM and SPM from 2003 Aug 05 through 2003 Sep 21 when
the SPM was officially retired. The VSM replaces the SPM and continues
the 30-year NSO/Kitt Peak synoptic magnetogram record. Magnetograms
are compared by equating histograms and, for selected examples, by
pixel-by-pixel comparison of coregistered images. The VSM was not
corrected for polarization crosstalk and was operated without fast
guiding. Solar activity was at best moderate during this period. Over
the range of observed fields, the VSM magnetograms show greatly
improved sensitivity but are otherwise virtually identical with "raw"
SPM magnetograms. GONG magnetograms are also closely comparable with
the SPM while MDI flux values tend to be stronger by a factor of 1.2 -
1.4. Dependence of the results on seeing will be discussed. Partial
funding for this work was provided through Solar and Heliospheric
Research Supporting Research and Technology grants from NASA's Office
of Space Sciences.
---------------------------------------------------------
Title: Advanced Technology Solar Telescope: a progress report
Authors: Oschmann, Jim; Dalrymple, Nathan; Warner, Mark; Price, Ron;
Hill, Frank; Hubbard, Rob; Rimmele, Thomas R.; Keller, Christoph U.;
Keil, Stephen
2004SPIE.5171..160O Altcode:
The 4m ATST will be the most powerful solar telescope in the world,
providing a unique scientific tool to study the Sun and other
astronomical objects. The design and development phase for the Advance
Technology Solar Telescope (ATST) is progressing. The conceptual design
review (CoDR) for the ATST is scheduled for August 2003. We present a
brief description of the science requirements of ATST, and remind the
reader of some of the technical challenges of building a 4-m solar
telescope. We will discuss some of the design strategies that will
allow us to achieve the required performance specifications, present
conceptual designs for the ATST, and summarize the results of trades
we have made on our path to the CoDR. The thermal impacts to local,
self-induced seeing with respect to some of our system level trades
that have been completed will be discussed.
---------------------------------------------------------
Title: High-speed cameras and solar observing
Authors: Harvey, John; Keller, Christoph U.; Cole, Lonnie; Tucker,
Roy; Jaksha, David
2004SPIE.5171..258H Altcode:
The National Solar Observatory operates two facilities with
demanding needs for rapid image collection (i.e. > television
frame rates). The first is GONG, a global network of six identical
small telescopes devoted to nearly continuous observations of the
sun's surface vibrations in order to study its internal properties by
helioseismology. The second, SOLIS, is a suite of three instruments
that collects images and spectra of the sun needed to study the behavior
of solar activity on time scales of minutes to decades. Five different
types of cameras are installed in these instruments. High speed, high
sensitivity, large dynamic range, and good photometric performance
are key factors for cameras used to make measurements of subtle solar
signals that pass through the noisy terrestrial atmosphere. A camera
that combines all these characteristics is elusive. The combination
of high speed and good photometric performance, when observing small
intensity changes, is particularly hard to get in practice. High speed
in large format CCD and hybrid FPA cameras is achieved by dividing the
array into multiple channels that are read simultaneously. An unwanted
result of this technique is cross talk between signal channels. It
is of order 1 percent in the case of Silicon Mountain Design 1M60_20
cameras (1k x 1k, 60 fps) and Rockwell Scientific Company HyViSI-1024
cameras (1k x 256, 92 fps). Cross talk (and also successive-frame image
retention) are particularly hard to deal with since they may exhibit
non-linear characteristics that depend on illumination light level. We
describe these and other phenomena, attempts to mitigate the effects,
and results from solar observations.
---------------------------------------------------------
Title: Charge caching CMOS detector for polarimetry (C<SUP>3</SUP>Po)
Authors: Keller, Christoph U.
2004SPIE.5171..239K Altcode:
C<SUP>3</SUP>Po is a concept for a novel array detector concept that
is optimized for highly sensitive and precise differential imaging
such as needed for astrophysical polarimetry. Chopping between two
or more independent image states (such as four linearly independent
polarization states) can be performed at speeds in the kHz domain to
provide virtually simultaneous images without the need to read out the
array at kHz frame rates. This allows the technology to be applied
to large arrays with precise, slow readouts. All independent image
planes are observed with the same physical pixel on the detector, which
renders normalized differences between image planes insensitive to the
gain of individual pixels. The detector concept has 100% geometrical
fill factor and a quantum efficiency approaching unity. The technology
can be applied to silicon to cover the 200-1100 nm wavelength range,
and to infrared-sensitive materials such as HgCdTe or InSb for the
1-20 μm wavelength range. While the detector concept has a wide range
of potential applications outside of astronomy, we focus here on its
application to polarimetric observations of the Sun.
---------------------------------------------------------
Title: Mapping sodium distribution on the surface of Mercury
Authors: Potter, A.; Plymate, C.; Keller, C.; Killen, R.; Morgan, T.
2004cosp...35.1293P Altcode: 2004cosp.meet.1293P
The distribution of sodium emission over the surface of Mercury
is non-uniform, and changes over time. These non-uniformities and
time-dependent changes give clues to the processes that produce
sodium. Photon-stimulated desorption may be the major sodium source
process on Mercury, so the sodium density might be expected to maximize
at the subsolar point. Another source process may be sputtering by
solar particles that impact the surface at high latitudes in the polar
cusps, producing sodium emissions at these locations. Variations in
the geographic distribution of sodium-rich minerals might produce
localized emissions that do not change position with time. Mapping the
sodium emission is a difficult observational problem, since Mercury is
seen either against the bright daytime sky, or against a dark sky at
very high air masses. We have adapted an image stabilizer utilizing a
piezoelectric driven tip-tilt correction mirror for daytime spectral
imaging of Mercury. The image stabilizer was originally developed
for solar observations at the McMath-Pierce solar telescope. Use of
image stabilization results in a noticeable improvement in spatial
resolution of our Mercury sodium images. A series of sodium images
taken over an eight-day period show changes in which an emission peak
in high southern latitudes disappears, and is replaced by an emission
peak at high northern latitudes. Further systematic observations and
improvements are planned for the image stabilizer system, as well
as experimental observations with a low-order adaptive optics system
incorporating a commercially available 37-actuator deformable mirror.
---------------------------------------------------------
Title: First Results from SOLIS
Authors: Harvey, J.; Giampapa, M.; Henney, C.; Jones, H.; Keller, C.
2003AGUFMSH42B0545H Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a
project to replace antiquated synoptic observing equipment at the
National Solar Observatory. SOLIS includes a suite of three instruments
on an equatorial mount originally installed at a site in Tucson and will
be moved to Kitt Peak before the end of 2003. The major SOLIS instrument
is a vector spectromagnetograph that maps magnetic fields across the
full solar disk using a slit spectrograph and one arc sec pixels. Daily
observations include several line-of-sight component magnetograms in the
photosphere and chromosphere and, for the first time, full-disk vector
magnetograms. At a medium scan speed ( ∼10 minutes for the full disk)
noise is less than 1 Mx/cm<SUP>2</SUP>. This low noise, combined with
negligible instrumental polarization and well resolved spectral line
profiles, yields moderate resolution magnetograms of unprecedented
quality. Sample observations show magnetic flux nearly everywhere
in the photosphere from the disk center to the solar limb. The flux
is organized in large scale patterns that heretofore had been visible
only in strong flux elements or after substantial spatial smearing. Good
results have been obtained from the other SOLIS instruments: a full-disk
filter imager at several narrow wavelengths and a grating spectrograph
that provides high-accuracy line spectra of integrated sunlight. SOLIS
data are freely available via the Internet and users may submit
observing time requests for special observations. The National Solar
Observatory is operated by AURA, Inc. under a cooperative agreement
with the National Science Foundation. Additional support for the
development of SOLIS from NASA and ONR is gratefully acknowledged.
---------------------------------------------------------
Title: Linear Polarization Measurements of Chromospheric Emission
Lines
Authors: Sheeley, N. R., Jr.; Keller, C. U.
2003ApJ...594.1085S Altcode:
We have used the Zurich Imaging Stokes Polarimeter (ZIMPOL I)
with the McMath-Pierce 1.5 m main telescope on Kitt Peak to obtain
linear polarization measurements of the off-limb chromosphere with a
sensitivity better than 1×10<SUP>-5</SUP>. We found that the off-disk
observations require a combination of good seeing (to show the emission
lines) and a clean heliostat (to avoid contamination by scattered light
from the Sun's disk). When these conditions were met, we obtained the
following principal results:1. Sometimes self-reversed emission lines
of neutral and singly ionized metals showed linear polarization caused
by the transverse Zeeman effect or by instrumental cross talk from the
longitudinal Zeeman effect in chromospheric magnetic fields. Otherwise,
these lines tended to depolarize the scattered continuum radiation
by amounts that ranged up to 0.2%.2. Lines previously known to show
scattering polarization just inside the limb (such as the Na I λ5889 D2
and the He I λ5876 D3 lines) showed even more polarization above the
Sun's limb, with values approaching 0.7%.3. The O I triplet at λ7772,
λ7774, and λ7775 showed a range of polarizations. The λ7775 line,
whose maximum intrinsic polarizability, P<SUB>max</SUB>, is less than
1%, revealed mainly Zeeman contributions from chromospheric magnetic
fields. However, the more sensitive λ7772 (P<SUB>max</SUB>=19%) and
λ7774 (P<SUB>max</SUB>=29%) lines had relatively strong scattering
polarizations of approximately 0.3% in addition to their Zeeman
polarizations. At times of good seeing, the polarization spectra
resolve into fine structures that seem to be chromospheric spicules.
---------------------------------------------------------
Title: Calibration and Data Analysis for the SOLIS-VSM
Authors: Henney, C. J.; Harvey, J. W.; Keller, C. U.; Jones, H. P.;
SOLIS Team
2003SPD....34.0303H Altcode: 2003BAAS...35..808H
The Vector Spectromagnetograph (VSM) instrument of the Synoptic
Optical Long-term Investigations of the Sun (SOLIS) project will
provide a unique 25-year record of synoptic solar observations with
daily full-disk photospheric vector and high sensitivity longitudinal
magnetograms. In addition, the VSM will produce daily full-disk
chromospheric longitudinal magnetograms along with various He I 1083
nm parameter images. The VSM is being prepared for installation at a
temporary site in Tucson for a comparison period with observations from
the Kitt Peak Vacuum Telescope (KPVT). After the cross-calibration
period, the VSM will replace the KPVT spectromagnetograph at Kitt
Peak. We outline the various VSM data products, with emphasis on the
calibration and data reduction efforts. <P />The data transmission
and storage resources are such that the reduction of VSM data will be
performed at the observing site on Kitt Peak. Reduced data products
will be transmitted via a DS3 link from Kitt Peak to the National Solar
Observatory's digital archive in Tucson. During a typical observing day,
three full-disk photospheric vector magnetograms will be available over
the Internet in two stages: first, as a “quick-look” product within
10 minutes of data acquisition, and then as a full Milne-Eddington
(ME) inversion product within 12 hours of each observation. The
quick-look parameters will include estimates of the magnetic field
strength, azimuth and inclination based on Auer, Heasley, House (1977,
Solar Physics 55, p. 47). The high-precision vector products will be
determined with the High Altitude Observatory ME inversion technique
implemented by Skumanich and Lites (1987, ApJ, 322, p.473). The
flexible design of the VSM data handling system can incorporate
future improvements under consideration (e.g., principal component
analysis). <P />This research was supported in part by the Office of
Naval Research Grant N00014-91-J-1040.
---------------------------------------------------------
Title: Advanced Technology Solar Telescope - Approach to a Four-meter
Diffraction Limited Solar Telescope
Authors: Keil, S.; Rimmele, T.; Oschmann, J.; Warner, M.; Dalrymple,
N.; Hubbard, R.; Price, R.; Goodrich, B.; Keller, C.; ATST Team
2003SPD....34.2019K Altcode: 2003BAAS...35..847K
The Advanced Technology Solar Telescope (ATST) is intended to be
the next major step in ground based solar observatories. The ATST
will provide a laboratory for ultra high resolution, polarimetric
measurements of all layers of the solar atmosphere. Currently the
project is preparing a conceptual design to fulfill this mission,
including plans for the design, development, construction and operation
of this facility. Given the nearly three-fold increase in aperture
size over the largest existing solar facilities, our approach combines
techniques from the newest solar facilities with lessons from recent
nighttime telescope designs. This approach insures the ATST will meet
the scientific goals that include diffraction-limited performance in
the optical for high spatial resolution solar observations and very
low scattered light to advance coronal observation capabilities. The
current telescope design incorporates the latest active optics
techniques, fast focal ratios for the primary optics, an open design
for ventilation of locally produced seeing, an un-obscured off-axis
pupil and a very high order adaptive optics system built into the
telescope from the beginning. Examples of some of the current design
concepts for the telescope structure, optics, thermal management,
scattered light control, upgrade paths to multi-conjugate adaptive
optics, software and facilities to support future potential upgrades
and instrumentation are given along with some of the key challenges
that lie ahead. <P />The National Solar Observatory is sponsored and
supported by the National Science Foundation.
---------------------------------------------------------
Title: The SOLIS Vector-Spectromagnetograph (VSM)
Authors: Keller, C. U.; SOLIS Team
2003SPD....34.2023K Altcode: 2003BAAS...35..848K
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is
a suite of three innovative instruments that will greatly improve
ground-based synoptic solar observations. The main instrument, the
Vector Spectro-Magnetograph (VSM), is a compact, high-throughput
vector-polarimeter with an active secondary mirror, an actively
controlled grating spectrograph, and two high-speed cameras based on
silicon-on-CMOS-multiplexer hybrid focal plane arrays. It measures
the photospheric magnetic field strength and direction over the full
solar disk within 15 minutes using two iron lines at 630 nm. It can
also record the longitudinal magnetic flux in the chromosphere and
coronal proxies derived from the HeI 1083 nm spectral line. <P />The
VSM has just been assembled and is expected to see first light at
a temporary site in Tucson before the end of May. We will present
an overview of the instrument, describe its performance, and discuss
first results. <P />SOLIS has been supported by a specific grant from
NSF, NSO base funding, NASA/GSFC, and the ONR. The project has also
received significant technical assistance from HAO and the Solar and
Astrophysics Lab of Lockheed-Martin.
---------------------------------------------------------
Title: Site testing issues for the frequency agile solar
radiotelescope (FASR)
Authors: Gary, Dale E.; Keller, Christoph U.
2003SPIE.4853..523G Altcode:
The Frequency Agile Solar Radiotelescope (FASR) will be a broadband
synthesis-imaging array with 3-km or larger baselines, operating over
a broad frequency range of 0.1-30 GHz. The instrument demands a site
with low levels of Radio Frequency Interference (RFI) over this entire
band. The site also must be large enough to accommodate the expected
size of the array configuration and ideally would provide room to grow
with future upgrades. The site must have a benign environment in which
at least 100 separate elements will operate with little degradation
and weather-related downtime. Several sites in the U.S. are being
considered. We discuss what criteria are being used to assess the sites,
and give some initial results of testing some of the sites.
---------------------------------------------------------
Title: Solar polarimetry close to the diffraction limit
Authors: Keller, Christoph U.
2003SPIE.4843..100K Altcode:
Much progress has been made during the last years in obtaining
polarimetric observations of the Sun close to the diffraction
limit. Here I summarize the problems one encounters when observing close
to the diffraction limit of a large solar telescope, review techniques,
present examples of recent observations, and discuss the need for
further developments of instruments and image reconstruction techniques.
---------------------------------------------------------
Title: PEPSI spectro-polarimeter for the LBT
Authors: Strassmeier, Klaus G.; Hofmann, Axel; Woche, Manfred F.; Rice,
John B.; Keller, Christoph U.; Piskunov, N. E.; Pallavicini, Roberto
2003SPIE.4843..180S Altcode:
PEPSI (Postham Echelle Polarimetric and Spectroscopic Instrument) is
to use the unique feature of the LBT and its powerful double mirror
configuration to provide high and extremely high spectral resolution
full-Stokes four-vector spectra in the wavelength range 450-1100nm. For
the given aperture of 8.4m in single mirror mode and 11.8m in double
mirror mode, and at a spectral resolution of 40,000-300,000 as designed
for the fiber-fed Echelle spectrograph, a polarimetric accuracy between
10<SUP>-4</SUP> and 10<SUP>-2</SUP> can be reached for targets with
visual magnitudes of up to 17th magnitude. A polarimetric accuracy
better than 10<SUP>-4</SUP> can only be reached for either targets
brighter than approximately 10th magnitude together wiht a substantial
trade-off wiht the spectral resolution or with spectrum deconvolution
techniques. At 10<SUP>-2</SUP>, however, we will be able to observe
the brightest AGNs down to 17th magnitude.
---------------------------------------------------------
Title: SOLIS: an innovative suite of synoptic instruments
Authors: Keller, Christoph U.; Harvey, John W.; Giampapa, Mark S.
2003SPIE.4853..194K Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is
a suite of three innovative instruments under construction that
will greatly improve ground-based synoptic solar observations. The
Vector Spectromagnetograph (VSM) is a compact, high-throughput
vector-polarimeter with an active secondary mirror, an actively
controlled grating spectrograph, and two high-speed cameras with
silicon-on-CMOS-multiplexer hybrid focal plane arrays. It will measure
the magnetic field strength and direction over the full solar disk
within 15 minutes. The Full-Disk Patrol (FDP) takes full-disk solar
intensity and Doppler images in various spectral lines and in the
continuum at a high cadence through liquid-crystal tuned birefringent
filters. The Integrated Sunlight Spectrometer (ISS) uses a fiber-fed
spectrograph to measure minute changes of the Sun-as-a-star in
many spectral lines. A high degree of automation and remote control
provides fast user access to data and flexible interaction with the
data-collection process. SOLIS is currently in the final assembly
phase and will become operational early in 2003.
---------------------------------------------------------
Title: Design and development of the Advanced Technology Solar
Telescope (ATST)
Authors: Keil, Stephen L.; Rimmele, Thomas; Keller, Christoph U.;
Hill, Frank; Radick, Richard R.; Oschmann, Jacobus M.; Warner, Mark;
Dalrymple, Nathan E.; Briggs, John; Hegwer, Steven L.; Ren, Dauxing
2003SPIE.4853..240K Altcode:
High-resolution studies of the Sun's magnetic fields are needed for
a better understanding of solar magnetic fields and the fundamental
processes responsible for solar variability. The generation of magnetic
fields through dynamo processes, the amplification of fields through
the interaction with plasma flows, and the destruction of fields
are still poorly understood. There is still incomplete insight as
to what physical mechanisms are responsible for heating the corona,
what causes variations in the radiative output of the Sun, and what
mechanisms trigger flares and coronal mass ejections. Progress in
answering these critical questions requires study of the interaction
of the magnetic field and convection with a resolution sufficient to
observe scales fundamental to these processes. The 4m aperture Advanced
Technology Solar Telescope (ATST) will be a unique scientific tool,
with excellent angular resolution, a large wavelength range, and low
scattered light. With its integrated adaptive optics, the ATST will
achieve a spatial resolution nearly 10 times better than any existing
solar telescope. Building a large aperture telescope for viewing the
sun presents many challenges, some of the more difficult being: · Heat
control and rejection · Contamination and scattered light control ·
Control of telescope and instrument polarization · Site selection
This talk will present a short summary of the scientific questions
driving the ATST design, the design challenges faced by the ATST, and
the current status of the developing design and siting considerations
---------------------------------------------------------
Title: Low-cost solar adaptive optics in the infrared
Authors: Keller, Christoph U.; Plymate, Claude; Ammons, S. M.
2003SPIE.4853..351K Altcode:
We have developed a low-cost adaptive optics system for solar
observations in the infrared between 1 and 28 μm with the 1.5-m
McMath-Pierce solar telescope. The 37-actuator membrane mirror and a
fast tip-tilt mirror are controlled by a PC running Linux RedHat 7.1
that analyzes images from a 256 by 256 pixel, 1 kHz frame rate CCD
camera. The total hardware cost is less than $25,000, and the system
provides diffraction-limited performance under median seeing conditions
above 2.3 μm. The single Pentium III processor provides enough
computing power to analyze the 200 subapertures of the Shack-Hartmann
wavefront sensor in real time. We describe the hardware and software
implementations and show results from the first tests at the telescope.
---------------------------------------------------------
Title: Technical challenges of the Advanced Technology Solar Telescope
Authors: Rimmele, Thomas R.; Keil, Stephen L.; Keller, Christoph
U.; Hill, Frank; Briggs, John; Dalrymple, Nathan E.; Goodrich, Bret
D.; Hegwer, Steven L.; Hubbard, Rob; Oschmann, Jacobus M.; Radick,
Richard R.; Ren, Deqing; Wagner, Jeremy; Wampler, Stephen; Warner, Mark
2003SPIE.4837...94R Altcode:
The 4m Advance Technology Solar Telescope (ATST) will be the most
powerful solar telescope in the world, providing a unique scientific
tool to study the Sun and possibly other astronomical objects, such
as solar system planets. We briefly summarize the science drivers and
observational requirements of ATST. The main focus of this paper is on
the many technical challenges involved in designing a large aperture
solar telescope. The ATST project has entered the design and development
phase. Development of a 4-m solar telescope presents many technical
challenges. Most existing high-resolution solar telescopes are designed
as vacuum telescopes to avoid internal seeing caused by the solar heat
load. The large aperture drives the ATST to an open-air design, similar
to night-time telescope designs, and makes thermal control of optics
and telescope structure a paramount consideration. A heat stop must
reject most of the energy (13 kW) at prime focus without introducing
internal seeing. To achieve diffraction-limited observations at visible
and infrared wavelengths, ATST will have a high order (order 1000
DoF) adaptive optics system using solar granulation as the wavefront
sensing target. Coronal observations require occulting in prime focus,
a Lyot stop and contamination control of the primary. An initial set of
instruments will be designed as integral part of the telescope. First
telescope design and instrument concepts will be presented.
---------------------------------------------------------
Title: Design and development of the Advanced Technology Solar
Telescope
Authors: Keil, S.; Rimmele, T.; Keller, C.; ATST Team
2003AN....324..303K Altcode:
Led by the National Solar Observatory, plans have been made to design
and to develop the Advanced Technology Solar Telescope (ATST). The ATST
will be a 4-m general-purpose solar telescope equipped with adaptive
optics and versatile post-focus instrumentation. Its main aim will be
to achieve an angular resolution of 0.03 arcsec (20 km on the solar
surface). The project and the telescope design are briefly described.
---------------------------------------------------------
Title: Polarimetry with the ATST (Invited review)
Authors: Keller, C. U.
2003ASPC..286...31K Altcode: 2003ctmf.conf...31K
No abstract at ADS
---------------------------------------------------------
Title: Solar observing facilities
Authors: Fleck, B.; Keller, C. U.
2003dysu.book..403F Altcode:
An overview is given of current and planned ground-based solar
telescopes and instruments, balloon-borne and suborbital solar
telescopes, and solar and heliospheric space missions. These observing
facilities operate in all areas of solar physics, from the solar
interior to interplanetary space and from regimes of high energy to
observations requiring high resolution. The next generation of solar
telescopes and instruments promise us the ability to investigate solar
processes on their fundamental scales, whether sub-arc second or global
in nature.
---------------------------------------------------------
Title: Science Objectives and Technical Challenges of the Advanced
Technology Solar Telescope (Invited review)
Authors: Rimmele, T.; Keil, S. L.; Keller, C.; Hill, F.; Penn, M.;
Goodrich, B.; Hegwer, S.; Hubbard, R.; Oschmann, J.; Warner, M.;
Dalrymple, N.; Radick, R.; Atst Team
2003ASPC..286....3R Altcode: 2003ctmf.conf....3R
No abstract at ADS
---------------------------------------------------------
Title: Polarimetry with the Advanced Technology Solar Telescope
Authors: Keller, C. U.
2003ASPC..307...23K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The SOLIS Vector-Spectromagnetograph
Authors: Keller, C. U.; Harvey, J. W.; Solis Team
2003ASPC..307...13K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Helicity and the SOLIS Vector-Spectromagnetograph
Authors: Keller, Christoph U.; Harvey, John W.; Henney, Carl J.;
Jones, Harrison P.
2003IAUJD...3E..18K Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun)
is a suite of three innovative instruments that greatly improve
ground-based synoptic solar observations. The main instrument the
Vector Spectro-Magnetograph (VSM) is a compact high-throughput
vector-polarimeter that measures the magnetic field strength and
direction over the full solar disk within 15 minutes. Helicity can
be directly calculated from the full-disk vector field data provided
by the Stokes inversion of the observed polarized line profiles. We
present an anlysis of the difficulties and problems that one faces
when interpreting the the helicity derived from VSM data.
---------------------------------------------------------
Title: Data analysis for the SOLIS Vector Spectromagnetograph
Authors: Jones, Harrison P.; Harvey, John W.; Henney, Carl J.; Hill,
Frank; Keller, Christoph U.
2002ESASP.505...15J Altcode: 2002solm.conf...15J; 2002IAUCo.188...15J
The National Solar Observatory's SOLIS Vector Spectromagnetograph, which
will produce three or more full-disk maps of the Sun's photospheric
vector magnetic field every day for at least one solar magnetic cycle,
is in the final stages of assembly. Initial observations, including
cross-calibration with the current NASA/NSO spectromagnetograph (SPM),
will soon be carried out at a test site in Tucson. This paper discusses
data analysis techniques for reducing the raw data, calculation of
line-of-sight magnetograms, and both quick-look and high-precision
inference of vector fields from Stokes spectral profiles. Existing
SPM algorithms, suitably modified to accommodate the cameras, scanning
pattern, and polarization calibration optics for the VSM, will be used
to "clean" the raw data and to process line-of-sight magnetograms. A
recent version of the High Altitude Observatory Milne-Eddington (HAO-ME)
inversion code will be used for high-precision vector fields.
---------------------------------------------------------
Title: Spectroscopic Detection of the 3.934 Micron Line of Si IX in
the Solar Corona
Authors: Judge, P. G.; Tomczyk, S.; Livingston, W. C.; Keller, C. U.;
Penn, M. J.
2002ApJ...576L.157J Altcode:
We report the detection of the
2s<SUP>2</SUP>2p<SUP>2</SUP><SUP>3</SUP>P<SUB>1-->0</SUB> line of
Si IX using the McMath-Pierce telescope on Kitt Peak. Observations
were made of the solar disk and at various heights above the limb
between 2002 May 13 and 17, under nonideal sky conditions, using the
13.5 m vertical spectrograph and an InSb single-diode detector. We
report a new rest wavelength for the line and discuss its potential
use as a diagnostic of coronal magnetic fields using ground-based
telescopes. Our observations give λ<SUB>rest</SUB>=3.93434+/-0.00007
μm, consistent with earlier less accurate values, but it places the
blue wing of the line under a strong telluric N<SUB>2</SUB>O line. In
the active regions observed, the line's intensity is comparable to or
larger than predicted in earlier work for the quiet Sun.
---------------------------------------------------------
Title: The Advanced Technology Solar Telescope
Authors: Keller, C. U.; Rimmele, T. R.; Hill, F.; Keil, S. L.;
Oschmann, J. M.; ATST Team
2002AN....323..294K Altcode:
The Advanced Technology Solar Telescope is the largest solar optical
facility currently under development. The National Solar Observatory
and its partners have just started the design and development phase
with first light being planned for late this decade. The 4-m telescope
will provide an angular resolution down to 0.025 arcsec, a large photon
flux for precise magnetic and velocity field measurements, and access
to a broad set of diagnostics from 0.3 to 28 mu m. We summarize the
currently envisioned scientific capabilities of the telescope and its
suite of instruments along with a glimpse at some of the early concepts.
---------------------------------------------------------
Title: Spatial mapping of the Hanle and Zeeman effects on the Sun
Authors: Stenflo, J. O.; Gandorfer, A.; Holzreuter, R.; Gisler, D.;
Keller, C. U.; Bianda, M.
2002A&A...389..314S Altcode:
Spatial mapping of the Hanle and Zeeman effects on the Sun has been done
for the first time, through Stokes vector imaging with a narrow-band
(0.2 Å) universal filter. It is shown how the polarization signatures
of the Hanle and Zeeman effects can be cleanly distinguished from each
other by comparing the Stokes images recorded at different, specially
selected wavelengths within the Na I D<SUB>2</SUB>-D<SUB>1</SUB>
line system. Examples of the polarization signatures of sunspots,
faculae, the supergranulation network, and large-scale canopy fields
are shown. The most striking result of our observations is that the
scattering polarization has an extremely intermittent structure rather
than being a simple function of limb distance. These intermittent
scattering polarization signals are cospatial with the facular
and supergranulation network seen both in intensity and circular
polarization. The observed pattern can be explained in terms of magnetic
enhancement of the scattering polarization in the network and/or Hanle
depolarization of the scattering polarization outside the network. Since
however no magnetic fields are seen in circular polarization outside the
network, the relative absence of linear scattering polarization there
may be explained by Hanle depolarization only if the volume filling,
depolarizing magnetic field has mixed polarities on a subarcsec scale
that is not resolved.
---------------------------------------------------------
Title: Preliminary tests of a low-cost solar infrared adaptive
optics system
Authors: Ammons, S. M.; Keller, C. U.
2002AAS...200.5610A Altcode: 2002BAAS...34..736A
Images produced by the National Solar Observatory's McMath-Pierce
telescope on Kitt Peak, the largest solar telescope in the world,
have been at the mercy of atmospheric turbulence for decades. Work
is currently underway to install a low-cost adaptive optics system
with the goal of correction in the infrared for a total hardware cost
of \$25k. As a preliminary step, a slow AO system was constructed in
the lab to demonstrate the feasibility of the low-cost approach. The
design is a simple feedback loop that reads the wavefront shape with a
Hartmann wavefront sensor and makes corrections through a micromachined
membrane deformable mirror. A computer calculates the voltages to apply
to the 37-actuator mirror based on the wavefront information. The
system operates at 1 Hz and is able to correct a distorted laser
wavefront within several cycles. This test paves the way to deploy a
faster version of this system that runs at 500 Hz. Funded by NSF.
---------------------------------------------------------
Title: Stokes Inversion Techniques for the SOLIS-VSM
Authors: Henney, C. J.; Keller, C. U.; Jones, H. P.; SOLIS Team
2002AAS...200.5514H Altcode: 2002BAAS...34Q.734H
The Vector Spectromagnetograph (VSM) instrument of the Synoptic Optical
Long-term Investigations of the Sun (SOLIS) project will begin operation
in 2002 and provide a 25 year record of synoptic solar observations. The
50-cm aperture VSM will provide daily full-disk photospheric vector
and high sensitivity longitudinal magnetograms. In addition, the VSM
will produce daily full-disk chormospheric longitudinal magnetograms,
along with 1083 nm equivalent width images. Current data transmission
and storage resources are such that the reduction of VSM data will be
performed at the observing site on Kitt Peak. Reduced data products
will be transmitted via a DS3 link from Kitt Peak to the National
Solar Observatory's digital archive in Tucson. During a typical
observing day, three full-disk photospheric vector magnetograms will
be available over the web in two stages: first, as a “quick-look”
product within 10 minutes of data acquisition, and then as a full
Milne-Eddington (ME) inversion product within 24 hours of each
observation. The quick-look parameters will include estimates of
the magnetic field strength, azimuth and inclination based on Auer,
Heasley, House (1977, Solar Physics 55, p. 47). The high-precision
vector products will be determined with the High Altitude Observatory
ME inversion technique implemented by Skumanich and Lites (1987, ApJ,
322, p.473). The flexible design of the VSM data handling system can
incorporate future improvements under consideration (e.g., principal
component analysis). However, since the original Stokes profiles
observed will not be archived, a retrospective reduction of VSM data
will not be possible. Quick-look and full ME inversion results are
compared using simulated VSM Stokes profile data based on Kitt Peak
Vacuum Telescope magnetograms. This research was supported in part by
the Office of Naval Research Grant N00014-91-J-1040.
---------------------------------------------------------
Title: Observational Signatures of a Solar Small-Scale Global Dynamo
Authors: Keller, C. U.; Stein, R. F.; Nordlund, A.
2002AAS...200.8908K Altcode: 2002BAAS...34..792K
There is ample theoretical and observational evidence for the existence
of a dynamo operating in the solar convection zone that produces
small-scale, weak magnetic fields. The next generation of solar
telescopes such as the 4-m Advanced Technology Solar Telescope and
the 1.5-m GREGOR will be able to provide observational data on these
magnetic fields. In order to guide the development of instruments and
observational procedures to investigate these small-scale magnetic
fields, we have calculated polarized spectral line profiles from
numerical simulations of a small-scale global dynamo and analyzed
them as if they were actual observations of the Sun. The simulated
observations include realistic noise, spatial smearing from a partially
correcting AO system, and spectral smearing and scattered light from a
spectrograph. We identify the unique signatures of these magnetic fields
and relate them to the physical conditions in the numerical simulations.
---------------------------------------------------------
Title: The Advanced Technology Solar Telescope
Authors: Rimmele, T. R.; Keil, S. L.; Keller, C. U.; Hill, F.;
Oschmann, J. M.; Warner, M.; Dalrymple, N. E.; ATST Team
2002AAS...200.3408R Altcode: 2002BAAS...34..691R
The 4m aperture Advance Technology Solar Telescope (ATST) will be the
most powerful solar telescope in the world and a unique scientific tool
to study the Sun and other astronomical objects, such as planets. The
ATST will replace major existing national solar facilities at the end
of this decade. The ATST project has entered the design and development
phase. We present an overview of the ATST science drivers and discuss
preliminary design concepts and technical challenges. The ATST science
goals lead to the following general requirements for the ATST facility:
- Diffraction limited angular resolution in the visible and infrared
to study fundamental astrophysical processes with unprecedented
resolution enabling verification of model predictions. - A high photon
flux for accurate measurements of physical parameters throughout
the solar atmosphere, such as magnetic strength and direction,
temperature and velocity. - Access to a new diagnostics at relatively
unexplored infrared wavelength. - Low scattered light to enable
coronal observations. - Low instrumental polarization for accurate
measurements of magnetic fields. Development of a 4m solar telescope
presents many technical challenges. The large aperture drives the ATST
to an open-air design and makes thermal control of optics and telescope
structure a paramount consideration. To achieve diffraction-limited
observations at visible and infrared wavelength ATST will have a high
order solar adaptive optics system. Coronal observations require,
occulting in prime focus, a Lyot stop and contamination control of the
primary. An initial set of instruments will be designed as integral
part of the telescope. Preliminary telescope and instrument concepts
will be discussed.
---------------------------------------------------------
Title: Instrumentation for astrophysical spectropolarimetry
Authors: Keller, Christoph U.
2002apsp.conf..303K Altcode:
Astronomical spectropolarimetry is performed from the X-ray to
the radio regimes of the electromagnetic spectrum. The following
chapter deals with instruments and their components that are used
in the wavelength range from 300 nm to 20 μm. After introducing
the terminology and formalisms that are used in the context of
astronomical spectropolarimeters, I discuss the most widely used
optical components. These include crystal and sheet polarizers, fixed
monochromatic and achromatic retarders, and variable retarders such
as liquid crystals and photoelastic modulators. Since polarimetric
measurements are often limited by systematic errors rather than
statistical errors due to photon noise, I deal with these instrumentally
induced errors in detail. Among these errors, I discuss instrumental
polarization of various kinds and chromatic and angle of incidence
errors of optical components. I close with a few examples of successful,
modern night-time and solar spectropolarimeter.
---------------------------------------------------------
Title: The ionosphere of Titan
Authors: Cravens, T.; Vann, J.; Clark, J.; Yu, J.; Keller, C.
2002cosp...34E.399C Altcode: 2002cosp.meetE.399C
Titan has an atmosphere consisting mainly of molecular nitrogen and
methane. Solar extreme ultraviolet and x-ray radiation and energetic
electrons from SaturnSs magnetosphere interact with the upper atmosphere
producing an ionosphere. We will review our current understanding of
TitanSs ionosphere. We will emphasize recent models of the horizontal
and vertical ion density structure. The ionospheric chemistry is quite
complex and a large number of hydrocarbon ion species are present. The
ion chemistry operating in the ionosphere will be reviewed. The
ionosphere acts as an obstacle to the external magnetospheric flow
and this will also be briefly discussed.
---------------------------------------------------------
Title: 1000 years of climate change
Authors: Keller, C.
2002cosp...34E1791K Altcode: 2002cosp.meetE1791K
Solar activity has been observed to vary on decadal and centennial time
scales. Recent evidence (Bond, 2002) points to a major semi-periodic
variation of approximately 1,500 yrs. For this reason, and because high
resolution proxy records are limited to the past thousand years or so,
assessing the role of the sun's variability on climate change over
this time f ame has received much attention. A pressingr application
of these assessments is the attempt to separate the role of the sun
from that of various anthropogenic forcings in the past century and
a half. This separation is complicated by the possible existence
of natural variability other than solar, and by the fact that the
time-dependence of solar and anthropogenic forcings is very similar
over the past hundred years or so. It has been generally assumed that
solar forcing is direct, i.e. changes in sun's irradiance. However,
evidence has been put forth suggesting that there exist various
additional indirect forcings that could be as large as or even exceed
direct forcing (modulation of cosmic ray - induced cloudiness, UV-
induced stratospheric ozone change s, or oscillator -driven changes in
the Pacific Ocean). Were such forcings to be large, they could account
for nearly all 20th Century warming, relegating anthropogenic effects
to a minor role. Determination of climate change over the last thousand
years offers perhaps the best way to assess the magnitude of total
solar forcing, thus allowing its comparison with that of anthropogenic
sources. Perhaps the best proxy records for climate variation in the
past 1,000 yrs have been variations in temperat ure sensitive tree
rings (Briffa and Osborne, 2002). A paucity of such records in the
Southern Hemisphere has largely limited climate change determinations
to the subtropical NH. Two problems with tree rings are that the
rings respond to temperature differently with the age of the tree,
and record largely the warm, growing season only. It appears that
both these problems have been adequately solved although caution is
warranted. A promising adjunct to tree rings is actual measurement of
temperatures in boreholes. Inversion of such records gives low frequency
temperatures that are potentially more accurate than any proxy- derived
ones. All these records give a fairly consistent picture of at least
one major warming and cooling extreme (Medieval Warming Period (MWP) and
Little Ice Age (LIA). Many modeling efforts using direct solar forcing
have been done. These typically employ proxy data (sunspot number and
variations in Be-10 and C -14 calibrated by satellite observations) for
changes in solar forcing, and give the same general picture-- that of
a substantial warming 1,000 yrs ago (MWP) followed by cooling that was
particularly marked in the late 17th and early 19th centuries (LIA). The
resulting amplitude of temperature change between MWP and LIA agrees
well with paleo-temperature reconstructions and suggests that solar
forcing alone is inadequate to account for more than about half the
20th century warming (Lean et al 1995, Crowley and Lowry 2000). Since
these quantitatively reproduce climate variations in the past 1000
years, the role of indirect solar forcing is inferred to be small but
may be important (Lean and Rind 2001). Gerard Bond, Bernd Kromer,
Juerg Beer, Raimund Muscheler, Michael N. Evans, William Showers,
Sharon Hoffmann, Rusty Lotti-Bond, Irka Hajdas, and Georges Bonani,
(2001) Persistent Solar Influence on North Atlantic Climate During the
Holocene,Science 294: 2130-2136 Briffa and Osborne, (2002) Blowing Hot
and Cold, Science 295, 2227-2228. Lean, J., Beer, J., and Bradley, R.,
(1995) Reconstruction of solar irradiance since 1610: Implications
for climate change, Geophys. Res. Lett.., 22, 3195-3198. Crowley ,T.,
(2000) Causes of climate change over the past 1000 years, Science,289,
270- 277. Lean and Rind, (2001), Earth's Response to a Variable Sun,
Science, 292, 234-236.
---------------------------------------------------------
Title: Polarization Modulation for Solar Vector-Polarimeters using
Ferroelectric Liquid Crystals
Authors: Pifer, R.; Keller, C.
2001AGUSM..SP51B05P Altcode:
Polarization modulation for solar vector polarimetry can be performed
in various ways. Ferroelectric liquid crystals (FLC) have the advantage
of allowing modulation frequencies of up to several kHz, therefore
avoiding spurious signals due to seeing. A new modulation scheme using
two FLC modulators was designed for the SOLIS Vector-Spectromagnetograph
(VSM). Here we present experimental confirmation of this modulation
scheme from a laboratory setup using a fast CCD camera running at
300 frames per second. Furthermore, we also show measurements from a
circular polarization modulator that operates at 854 nm but uses one
FLC built for 630 nm. This provides a means to cover a large wavelength
range with a single, chromatic FLC modulator.
---------------------------------------------------------
Title: The Advanced Technology Solar Telescope -- Science Goals and
Instrument Description.
Authors: Rimmele, T. R.; Keil, S. L.; Keller, C. U.; Hill, F.
2001AGUSM..SH31D08R Altcode:
High-resolution studies of the Sun's magnetic fields are needed for
a better understanding of solar magnetic fields and the fundamental
processes responsible for solar variability. The generation of magnetic
fields through dynamo processes, the amplification of fields through
the interaction with plasma flows, and the destruction of fields are
still poorly understood. There is still incomplete insight as to what
physical mechanisms are responsible for heating the corona, what causes
variations in the radiative output of the Sun, and what mechanisms
trigger flares and coronal mass ejections. Progress in answering
these critical questions requires study of the interaction of the
magnetic field and convection with a resolution sufficient to observe
physical scales fundamental to these processes. The 4m aperture ATST
will be a unique scientific tool, with excellent angular resolution,
a large wavelength range, and low scattered light. With its integrated
adaptive optics, the ATST will achieve a spatial resolution nearly 10
times better than any existing solar telescope. The ATST will provide:
<P />Unprecedented angular resolution of 0.03 arcsec in the visible and
0.08 arcsec at 1.6 microns to enable us to clearly resolve and study the
fundamental astrophysical processes on their intrinsic scales and to
verify model predictions. A high photon flux for accurate and precise
measurements of physical parameters, such as magnetic field strength
and direction, temperature and velocity, on the short time scales
involved. Access to a broad set of diagnostics, from visible to thermal
infrared wavelengths. Low scattered light observations and coronagraphic
capabilities in the infrared, allowing measurements of coronal magnetic
fields. The ATST has been highly ranked by the latest Decadal Survey
of Astronomy and Astrophysics and the NAS/NRC study of ground-based
solar astronomy. A large part of the solar community will participate
in the design and development of the ATST. A strawman telescope design,
design challenges and instrument concepts will be discussed. Examples
of recent high resolution observations with adaptive optics, that
demonstrate the potential of this new technology will be shown.
---------------------------------------------------------
Title: Influence of magnetic fields on the coherence effects in the
Na I D<SUB>1</SUB> and D<SUB>2</SUB> lines
Authors: Stenflo, J. O.; Gandorfer, A.; Wenzler, T.; Keller, C. U.
2001A&A...367.1033S Altcode:
To clarify the physical nature of the enigmatic scattering
polarization in the Na i D<SUB>1</SUB> and D<SUB>2</SUB> line cores
we have explored their behavior with full Stokes vector polarimetry
in regions with varying degree of magnetic activity near the solar
limb. These observations represent the first time that ZIMPOL II,
the second generation of our CCD based imaging polarimeter systems,
has been used for a scientific program. With ZIMPOL II the four Stokes
images can be demodulated and recorded with a single CCD sensor such
that the resulting images of the fractional polarization Q/I, U/I,
and V/I are entirely free from spurious features due to seeing or
flat-field effects. The polarization in the cores of the lines, in
particular in D<SUB>2</SUB>, exhibits dramatic and unexpected spatial
variations in both Q/I and U/I, including polarization self-reversals
of the D<SUB>2</SUB> Q/I core peak. As the fluctuations in the Q,
U, and V parameters appear to be relatively uncorrelated, we have
parametrized the profiles and made scatter plots of the extracted
parameters. Comparison with synthetic scatter plots based on different
theoretical models suggests that the polarization signals in the cores
of the D<SUB>2</SUB> and D<SUB>1</SUB> lines have different physical
origins: While the D<SUB>1</SUB> core is likely to be governed by
ground-state atomic polarization, the D<SUB>2</SUB> core is dominated by
the alignment of the excited state and by effects of partial frequency
redistribution.
---------------------------------------------------------
Title: The SOLIS Vector-Spectromagnetograph (VSM)
Authors: Keller, C. U.; Solis Team
2001ASPC..236...16K Altcode: 2001aspt.conf...16K
No abstract at ADS
---------------------------------------------------------
Title: High Resolution Polarimetry and the Need for a Large-Aperture
Solar Telescope
Authors: Keller, C. U.
2001ASPC..236..389K Altcode: 2001aspt.conf..389K
No abstract at ADS
---------------------------------------------------------
Title: The Advanced Technology Solar Telescope
Authors: Keil, S. L.; Rimmele, T. R.; Keller, C. U.; Atst Team
2001ASPC..236..597K Altcode: 2001aspt.conf..597K
No abstract at ADS
---------------------------------------------------------
Title: Solar Chromospheric Magnetic Fields
Authors: Keller, C. U.
2001ASPC..248...61K Altcode: 2001mfah.conf...61K
No abstract at ADS
---------------------------------------------------------
Title: The Advanced Technology Solar Telescope: Science Goals and
Instrument Description
Authors: Rimmele, T.; Keller, C.; Keil, S.; Hill, F.; Atst Team
2001AGM....18S1006R Altcode:
High-resolution studies of the Sun's magnetic fields are needed for
a better understanding of solar magnetic fields and the fundamental
processes responsible for solar variability. For example, the
generation of magnetic fields through dynamo processes is still poorly
understood. There is still incomplete insight as to what physical
mechanisms are responsible for heating the corona, what causes
variations in the radiative output of the Sun. Progress in answering
these critical questions requires study of the interaction of the
magnetic field and convection with a resolution sufficient to observe
scales fundamental to these processes. The 4m aperture ATST will be
a unique scientific tool, which will provide unprecedented angular
resolution, high photon flux, access to a broad set of diagnostics,
from visible to thermal infrared wavelengths, and low scattered light
observations and coronagraphic capabilities in the infrared. Development
of a 4-m solar telescope presents several technical challenges. The
large heat flux makes thermal control of optics and telescope structure
a paramount consideration. To achieve diffraction-limited performance,
a powerful solar adaptive optics system is required. Low scattered
light is essential for observing the corona but also to accurately
measure the physical properties of small structures in, for example,
sunspots. Contamination control of the primary and secondary mirrors
must therefore be addressed. An initial set of instruments will be
designed as integral part of the telescope during the upcoming design
and development phase. A strawman telescope design and instrument
concepts will be discussed.
---------------------------------------------------------
Title: The Advanced Solar Telescope
Authors: Keil, S. L.; Rimmele, T. R.; Keller, C.; Hill, F.
2000AAS...197.1710K Altcode: 2000BAAS...32.1433K
The planned Advanced Technology Solar Telescope (ATST) will be a 4-m
aperture general-purpose solar telescope with integrated adaptive
optics and versatile post focus instrumentation. The ATST will achieve
an angular resolution of 0.03 arcsec (20 km on the solar surface)
in the visible, which is almost an order of magnitude better than
what is achieved with current solar telescopes. This will make it
possible to resolve the fundamental astrophysical hydrodynamic and
magnetohydrodynamic processes and structures in the solar atmosphere
such as the building blocks of solar magnetic fields that are believed
to be responsible for solar irradiance variations and the heating of the
outer solar atmosphere. The ATST will cover the wavelength range from
0.35 to 35 ?m and minimize scattered light. The initial set of post
focus instruments will exploit the unique capabilities of the ATST to
study magnetic fields at the highest spatial resolution in the visible
and near-infrared parts of the spectrum. The ATST was highly recommended
by the recent Decadal Study. A proposal for a four-year Design and
Development phase has just been submitted to the NSF. Construction is
expected to start in FY2005. The National Solar Observatory is operated
by the Associated Universities for Research in Astronomy and is funded
by the National Science Foundation under a cooperative agreement.
---------------------------------------------------------
Title: Results from a revisit to the K<SUB>2V</SUB> bright points
Authors: Sivaraman, K. R.; Gupta, S. S.; Livingston, W. C.; Damé,
L.; Kalkofen, W.; Keller, C. U.; Smartt, R.; Hasan, S. S.
2000A&A...363..279S Altcode:
We have used pairs of temporally simultaneous CaII K-line
spectroheliograms and magnetic area scans to search for spatial
correlation between the CaII K<SUB>2V</SUB> bright points in the
interior of the network and corresponding magnetic elements. We find
that about 60% of the K<SUB>2V</SUB> bright points spatially coincide
with magnetic elements of flux density > 4 Mx cm<SUP>-2</SUP>. About
25% of the K<SUB>2V</SUB> bright points with equally enhanced emission
lie over bipole elements where the fields are > 4 Mx cm<SUP>-2</SUP>
for both polarity elements which merge and presumably cancel and
result in low fields. The rest, 15%, of the bright points coincide
with areas of fields < 4 Mx cm<SUP>-2</SUP> which is the noise
level set by us for the magnetic scans. When magnetic elements of
opposite polarity merge and form bipoles, the associated K<SUB>2V</SUB>
bright points show excess emission. Although such excess emission is a
magnetic-field-driven phenomenon, the measured value of the field at the
site of the bipole is typically low, and these cases would therefore be
excluded in the count of coincidences of excess emission with excess
magnetic fields. In our opinion, these cases of excess emission at
the sites of the bipoles, as well as at the sites of fields >
4 Mx cm<SUP>-2</SUP>, are both instances of magnetic-field-related
emissions. If the former are not taken into account as coincidences,
the correlation will drop down and this might be interpreted as not an
obvious correlation. Our present results, taking into account the low
fields of merging bipoles, establish the association of K<SUB>2V</SUB>
bright points with magnetic elements.
---------------------------------------------------------
Title: New Initiatives for Synoptic Observations
Authors: Keller, C. U.
2000JApA...21..127K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: New Results from the Flare Genesis Experiment
Authors: Rust, D. M.; Bernasconi, P. N.; Eaton, H. A.; Keller, C.;
Murphy, G. A.; Schmieder, B.
2000SPD....31.0302R Altcode: 2000BAAS...32..834R
From January 10 to 27, 2000, the Flare Genesis solar telescope
observed the Sun while suspended from a balloon in the stratosphere
above Antarctica. The goal of the mission was to acquire long time
series of high-resolution images and vector magnetograms of the
solar photosphere and chromosphere. Images were obtained in the
magnetically sensitive Ca I line at 6122 Angstroms and at H-alpha
(6563 Angstroms). The FGE data were obtained in the context of Max
Millennium Observing Campaign #004, the objective of which was to study
the “Genesis of Solar Flares and Active Filaments/Sigmoids." Flare
Genesis obtained about 26,000 usable images on the 8 targeted active
regions. A preliminary examination reveals a good sequence on an
emerging flux region and data on the M1 flare on January 22, as well
as a number of sequences on active filaments. We will present the
results of our first analysis efforts. Flare Genesis was supported
by NASA grants NAG5-4955, NAG5-5139, and NAG5-8331 and by NSF grant
OPP-9615073. The Air Force Office of Scientific Research and the
Ballistic Missile Defense Organization supported early development of
the Flare Genesis Experiment.
---------------------------------------------------------
Title: Evolution of small-scale magnetic fields from combined adaptive
optics and phase-diverse speckle imaging
Authors: Keller, C. U.; Rimmele, T. R.; Paxman, R. G.; Seldin, J. H.;
Carrara, D.; Gleichman, K.
2000SPD....31.0301K Altcode: 2000BAAS...32..833K
We have obtained movies of the photospheric magnetic field at a
sustained resolution of 0.2 arcsec by combining the adaptive optics
system at the Dunn Solar Telescope with the Zurich Imaging Polarimeter I
(ZIMPOL) and processing the data with Phase-Diverse Speckle Imaging and
speckle deconvolution. The adaptive optics was correcting the low-order
aberrations with an update rate of about 1.5 kHz and fed a narrow-band
channel through the Universal Birefringent Filter in the wing of the CaI
610.3 nm line and two white-light channels that were used to obtain one
in-focus and one out-of-focus image for the phase-diversity processing,
which removes the remaining aberrations. All three channels were
equipped with ZIMPOL I cameras running simultaneously at 5 frames
per second. The narrow-band intensity and magnetogram images were
reconstructed using speckle deconvolution. This combined attack to
obtain the best magnetogram movies of the solar surface was very
successful and led to spectacular time sequences with a consistent
spatial resolution of better than 0.2 arcsec. We will present the
first scientific results on the evolution of the small-scale magnetic
fields in an active region. This work was supported by the National
Science Foundation.
---------------------------------------------------------
Title: Center-to-limb variation of the enigmatic Na bt I D_1 and
D_2 polarization profiles
Authors: Stenflo, J. O.; Gandorfer, A.; Keller, C. U.
2000A&A...355..781S Altcode:
The remarkable polarization structure of the Na i D_1 and D_2 lines
that is due to coherent scattering has remained an enigma, since it has
not yet been possible to find an explanation that is consistent with
both current understanding of quantum mechanics and the astrophysical
properties of the Sun's atmosphere. To guide future theoretical efforts
we have here explored the detailed center-to-limb variation of the
linearly polarized profiles in non-magnetic regions. In particular we
find that the unexplained narrow polarization peaks in the Doppler
cores of the two lines become even more pronounced with respect to
the relative profile shape as we move away from the limb towards the
center of the solar disk.
---------------------------------------------------------
Title: Anomalous polarization effects due to coherent scattering on
the Sun
Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A.
2000A&A...355..789S Altcode:
The richly structured linearly polarized spectrum that is produced
by coherent scattering in the Sun's atmosphere contains a number of
spectral features for which no explanation has been found within the
standard scattering theory. According to this quantum-mechanical
framework, the intrinsic polarizability of a given line should be
determined by the total angular momentum quantum numbers of the
atomic levels involved in the scattering transition (which may be
resonant or fluorescent). Well defined polarization peaks have been
observed in many lines, which according to these theoretical concepts
should be intrinsically unpolarizable. A possible explanation for
these anomalous spectral structures could be that the initial ground
state of the scattering transition becomes polarized by an optical
pumping process. However, such an explanation is contradicted by
other observations, since it seems to require that much of the solar
atmosphere must be filled with extremely weak magnetic fields (<~
10 mG). We have searched through the whole visible solar spectrum
for lines with the quantum numbers that should normally make them
unpolarizable, and have carried out a systematic observing program
for the most prominent of these lines. Here we report on the observed
properties of the polarized line profiles of these lines and explain
in what respect their behaviors are anomalous and cannot be understood
within current conceptual frameworks.
---------------------------------------------------------
Title: 5,000 by 5,000 Spatial by 15,000 Spectral Resolution Elements:
First Astronomical Observations with a Novel 3-D Detector
Authors: Keller, C. U.
2000ASPC..195..495K Altcode: 2000iutd.conf..495K
No abstract at ADS
---------------------------------------------------------
Title: Deconvolution of narrowband solar images using aberrations
estimated from phase-diverse imagery
Authors: Seldin, John H.; Paxman, Richard G.; Carrara, David A.;
Keller, Christoph U.; Rimmele, Thomas R.
1999SPIE.3815..155S Altcode:
Phase-Diverse Speckle (PDS) is a short-exposure data- collection and
processing technique that blends phase- diversity and speckle-imaging
concepts. PDS has been successfully used for solar astronomy to achieve
near diffraction-limited resolution in ground-based imaging of solar
granulation. Variants of PDS that involve narrow-band, spectroscopic,
and polarimetric data provide more information observations. We
present results from processing data collected with the 76-cm Richard
B. Dunn Solar Telescope (DST) on Sacramento Peak, NM. Three-channel
data sets consisting of a pair of phase-diverse images of the solar
continuum and a narrow-band image were collected over spans of 15 - 20
minutes. Point-spread functions that are estimated from the PDS data are
used in a multi-frame deconvolution algorithm to correct the narrow-band
imagery. The data were processed into a number of time series. A rare,
short-lived continuum bright point with a peak intensity at a factor
of 2.1 above the mean intensity in the continuum was observed in one
such sequence. The field of view spans multiple isoplanatic patches,
and strategies for processing these large fields were developed. We will
discuss these methods along with other techniques that were explored
for accelerating the processing. Finally, we show the first PDS
reconstruction of adaptive-optics (AO) compensated solar granulation
taken at the DST. As expected, we find that these data are less
aberrated and, thus, the use of AO in future experiments is planned.
---------------------------------------------------------
Title: Infrared lines as probes of solar magnetic
features. XV. Evershed flow in cool, weak penumbral fields
Authors: Rüedi, I.; Solanki, S. K.; Keller, C. U.
1999A&A...348L..37R Altcode:
Observations of Ti I lines at 2.2 mu m show that the Evershed flow takes
place in cool, almost horizontal channels with a low magnetic field
strength (~ 500-900 G) that does not appear to change significantly
across the penumbra. This property might allow an outward directed
siphon flow to exist along such cool flux tubes.
---------------------------------------------------------
Title: Evolution of Protoplanetary Accretion Disks including Chemistry
and Transport Processes
Authors: Duschl, W. J.; Gail, H. -P.; Keller, C.; Tscharnuter, W. M.
1999AGAb...15...22D Altcode: 1999AGM....15..B14D
According to our current understanding, planetary systems form in
protostellar accretion disks. At late stages of the star formation
process, conditions become favourable for the dust component in
the disk to agglomerate first into km-sized planetesimals and by a
complicated hierarchy of further growth process into planets. The
simulation of the structure and evolution of protoplanetary accretion
disks are a necessary prerequisite for understanding the formation
of planetary systems. An important topic in this context is the study
of the chemistry of gas and dust, the mineralogical and petrological
properties of the dust, and of radial and vertical transport processes
in the disc, since this determines the temporal evolution and radial
distribution of the composition of the disk material, from which the
planetary system bodies are formed. The construction of hydrodynamic
evolution models including chemical and transport processes is
part of a project within the SFB 359 "Reactive Flows, Diffusion, and
Transport" at Heidelberg University. The present state of this project
and the future plans are briefly discussed. As a special result, we
discuss the chemistry and structure of a stationary disk model, which
combines self-consistently chemical equilibrium calculations for the
gas phase and the mineral assemblage in the disk with the complete
set of equations for the radial and vertical disk structure in the
1+1 dimensional approximation. Convection and radiative transfer is
included in the model calculation. We briefly discuss the mineral
composition predicted by this model with (i) results of earlier
condensation calculations, and (ii) the mineral assemblage observed
for primitive meteorites.
---------------------------------------------------------
Title: Optimum Apodization for Speckle Imaging of Extended Sources
Authors: Keller, C.
1999ASPC..183..342K Altcode: 1999hrsp.conf..342K
No abstract at ADS
---------------------------------------------------------
Title: The Advanced Solar Telescope: I. Science Goals
Authors: Keller, C.
1999ASPC..183..169K Altcode: 1999hrsp.conf..169K
No abstract at ADS
---------------------------------------------------------
Title: Phase-Diversity Data Sets and Processing Strategies
Authors: Paxman, R.; Seldin, J.; Keller, C.
1999ASPC..183..311P Altcode: 1999hrsp.conf..311P
No abstract at ADS
---------------------------------------------------------
Title: Scattering polarization in the chromosphere
Authors: Keller, C. U.; Sheeley, N. R., Jr.
1999ASSL..243...17K Altcode: 1999sopo.conf...17K
No abstract at ADS
---------------------------------------------------------
Title: Infrared lines as probes of solar magnetic features. XIV. TI
i and the cool components of sunspots
Authors: Rueedi, I.; Solanki, S. K.; Keller, C. U.; Frutiger, C.
1998A&A...338.1089R Altcode:
The first systematic observations of sunspot umbrae using the lines
of the Ti Ii multiplet at 2.2 mu m are presented. Their diagnostic
capabilities are investigated, developed and used to investigate the
magnetic and velocity structure of a sunspot. These lines are most
sensitive to cool plasma. In addition, they are extremely Zeeman
sensitive. We find that a sunspot is composed of two distinct cool
magnetic components. One of them is fairly vertical, has a large
magnetic field strength and is associated with the central (umbral)
part of the sunspot. The other component is strongest near the outer
boundary of the spot (penumbra), is much more inclined, has a very
low magnetic field strength and shows the signature of the Evershed
effect. In contrast to the smooth transition of field strength from
the darkest part of the umbra to the outer penumbral boundary usually
visible in observations carried out in other spectral lines, the Ti
Ii lines exhibit a sharp transition between the two magnetic components.
---------------------------------------------------------
Title: SOLIS: a modern facility for synoptic solar observations
Authors: Keller, Christoph U.
1998SPIE.3352..732K Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite
of instruments that will modernize and greatly improve synoptic solar
observations carried out by the National Solar Observatory. It will
provide fundamental data necessary to understand the solar activity
cycle, sudden energy releases in the solar atmosphere, and solar
spectral irradiance changes. State-of-the-art instrumentation and data
collection techniques will be employed to enhance both the quality
and quantity of data. A high degree of automation and remote control
will provide faster user access to data and flexible interaction
with the data- collection process. The instruments include a vector
spectromagnetograph that will measure the magnetic field strength and
direction over the full solar disk in 15 minutes, a full disk patrol
delivering digital images in various spectral lines at a high cadence,
and a Sun-as-a- star precision spectrometer to measure changes in many
spectral lines.
---------------------------------------------------------
Title: SOLIS: A Modern Facility for Synoptic Solar Observations
Authors: Keller, C. U.
1998STIN...9946076K Altcode:
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is a suite
of instruments that will modernize and greatly improve synoptic solar
observations carried out by the National Solar Observatory. It will
provide fundamental data necessary to understand the solar activity
cycle, sudden energy releases in the solar atmosphere, and solar
spectral irradiance changes. State-of-the-art instrumentation and data
collection techniques will be employed to enhance both the quality
and quantity of data. A high degree of automation and remote control
will provide faster user access to data and flexible interaction
with the data-collection process. The instruments include a vector
spectromagnetograph that will measure the magnetic field strength and
direction over the full solar disk in 15 minutes, a full disk patrol
delivering digital images in various spectral lines at a high cadence,
and a Sun-as-a-star precision spectrometer to measure changes in many
spectral lines.
---------------------------------------------------------
Title: Complex magnetic fields in an active region
Authors: Bernasconi, P. N.; Keller, C. U.; Solanki, S. K.; Stenflo,
J. O.
1998A&A...329..704B Altcode:
High-resolution observations of the full Stokes vector in Fe\sc i
spectral lines around 5250 Angstroms obtained at the Swedish Vacuum
Solar Telescope on La Palma with the ZIMPOL I Stokes polarimeter in a
complex active region reveal the presence of anomalously shaped Stokes
profiles indicating the coexistence of at least two magnetic components
within the same spatial resolution element. These Stokes profiles have
been analyzed with an inversion code based on a 3-component atmospheric
model with two magnetic and one field-free component. The fits to
the observations in a magnetic region that resembles a small penumbra
reveal the presence of a horizontal magnetic field component with an
average field strength of /line{B}=840 G, a mean filling factor of
/lineα=0.12, and an average temperature /line{T}=5400 K at log {tau_
{5000}}=-1.5 embedded in the main “penumbral” magnetic field that
has /line{B}=1500 G, /lineα=0.56, and /line{T}=4900 K. The horizontal
component exhibits a mean outflow of 2.7 km s(-1) which is mainly due
to the Evershed flow. In a region where there are strong downflows up
to 7 km s(-1) , we infer the possible presence of a shock front whose
height changes along the slit. The height variation can be explained by
a change of the gas pressure at the base of the photosphere below the
shock front as proposed by Thomas & Montesinos (1991). Small plages
with field strengths below 900 G have been observed in the vicinity
of some pores. Finally, we present a puzzling field structure at the
boundary between two adjacent pores. Ambiguous results suggest that
although the inversion code is able to successfully invert even very
complex Stokes profiles, we are far from a complete description of the
field structure in complex magnetic regions. We warn that magnetograms
and fits to data involving only a single magnetic component may hide
the true complexity of the magnetic structure in at least some parts
of active regions.
---------------------------------------------------------
Title: SOLIS --- A Modern Facility for Synoptic Solar Observations
Authors: Keller, C. U.; Nso Staff
1998ASPC..154..636K Altcode: 1998csss...10..636K
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is
a proposed suite of instruments that will modernize and greatly
improve synoptic solar observations carried out by the National Solar
Observatory. The primary scientific goal is to provide fundamental data
necessary to understand the solar activity cycle, sudden energy releases
in the solar atmosphere, and solar spectral irradiance changes. An
operational goal is to produce real-time and near real-time data for
forecasting space weather, and to augment the scientific yield from
space missions such as SOHO and TRACE, and ground-based projects
including RISE and GONG. State-of-the-art instrumentation and data
collection techniques will be employed to enhance both the quality
and quantity of data. A high degree of automation and remote control
will provide faster user access to data and flexible interaction
with the data-collection process. The instruments include a vector
spectromagnetograph that will measure the magnetic field strength and
direction over the full solar disk in 15 minutes, a full disk patrol
delivering digital images in various spectral lines at a high cadence,
a coronal emission line imager and photometer that will provide
photometric and velocity images in at least five spectral lines,
and a Sun-as-a-star precision spectrometer to measure changes in many
spectral lines.
---------------------------------------------------------
Title: SOLIS Instrumentation Aspects
Authors: Keller, C. U.; NSO Staff
1998ASPC..140..539K Altcode: 1998ssp..conf..539K
No abstract at ADS
---------------------------------------------------------
Title: Population number inversion in disk-impinging-clumps
Authors: Fiebig, D.; Els, S. G.; Keller, C.
1998AGAb...14...22F Altcode: 1998AGM....14..B27F
The model of accretion disk-impinging-clumps is numerically investigated
with respect to the occurrence of a population number inversion between
the J_{K_{-1} {K_{+1}} = 6<SUB>16</SUB>, 5<SUB>23</SUB> ortho-{H_2O}
rotational levels. Former studies (Fiebig 1997) have shown that the
collision of small, dense clumps onto the protostellar accretion disk
around the deeply embedded YSO in L 1287 can account for the appearence
of the associated position-velocity structure of identified 22 GHz
{H_2O} masers. Recently another source was found which indicates a very
similar structure (Torrelles et al. 1998). Since the origin of the
22 GHz {H_2O} maser emission is assigned to shock heated disk-clump
collisions, a 2-D radiative transfer code was developed to calculate
the population numbers of water molecules in the interaction region
of a single collision event. Starting from the results obtained by
a 2-D SPH-Code (Keller & Fiebig \ this conference) the spatial
distribution of gas and dust temperature, density, and velocity
were adopted as boundary conditions. Since dust was found to be the
dominant cooling agent (Els & Fiebig \ this conference), the
radiative transfer between water molecules could be decoupled from
the heating/cooling processes incorporated into the SPH-Code. The 2-D
radiative transfer code makes use of a modified short characteristics
method, considers Rayleigh scattering, and is usual ly run for a number
of 60 rotational levels covering corresponding levels temperatures up
to 2600 K. The computational results clearly show the occurrence of
compact regions of population number inversion to be identified with
observable 22 GHz {H_2O} maser emission.
---------------------------------------------------------
Title: Differential Hanle effect and the spatial variation of
turbulent magnetic fields on the Sun
Authors: Stenflo, J. O.; Keller, C. U.; Gandorfer, A.
1998A&A...329..319S Altcode:
While diagnostic techniques based on the ordinary Zeeman effect
(e.g. magnetograms) are almost “blind” to a turbulent magnetic field
with mixed magnetic polarities within the spatial resolution element,
the Hanle effect is sensitive to this domain of solar magnetism. We
present observational evidence that the turbulent magnetic field that
fills the 99\ts%\ of the volume between the kG flux tubes in quiet
solar regions does not have a unique field-strength distribution,
but the rms turbulent field strength can vary by an order of magnitude
from one solar location to the next. The varying Hanle depolarization
in combinations of spectral lines with different sensitivities to the
Hanle effect is conspicuously evident from direct visual inspection
of the spectra. To quantify these variations we have extracted the
polarization amplitudes for a selection of spectral lines observed in 8
different solar regions with different turbulent field strengths, and
then applied an inversion technique to find the field strengths and
calibrate the selected lines. The inversion gives stable solutions
for the turbulent field strengths, in the range 4--40\ts G, but
the field-strength scale is presently very uncertain. The inversion
exercise has helped to expose a number of problem areas which need to be
attended to before the differential Hanle effect can become a standard,
reliable diagnostic tool. One major problem is the extraction of the
line polarization when the contributions from the line and continuum
are of the same order of magnitude, which is the usual case. For
exploratory purposes we have applied a heuristic, statistical approach
to deal with this problem here.
---------------------------------------------------------
Title: The USAF Improved Solar Observing Optical Network (ISOON)
and its Impact on Solar Synoptic Data Bases
Authors: Neidig, D.; Wiborg, P.; Confer, M.; Haas, B.; Dunn, R.;
Balasubramaniam, K. S.; Gullixson, C.; Craig, D.; Kaufman, M.; Hull,
W.; McGraw, R.; Henry, T.; Rentschler, R.; Keller, C.; Jones, H.;
Coulter, R.; Gregory, S.; Schimming, R.; Smaga, B.
1998ASPC..140..519N Altcode: 1998ssp..conf..519N
No abstract at ADS
---------------------------------------------------------
Title: The Procyon campaign: Observations from Kitt Peak
Authors: Pilachowski, C. A.; Barden, S.; Hill, F.; Harvey, J. W.;
Keller, C. U.; Giampapa, M. S.
1998IAUS..185..319P Altcode:
Time series spectra of the F5IV star Procyon (alpha CMi) were obtained
at the Kitt Peak National Observatory during a 35-night observing run
in January-February 1997. The observations were obtained as part of an
international collaboration to detect and study acoustic oscillations
in solar-type stars. Spectra covered the wavelength range from 4000
to 5300 AA , with a resolving power of approximately 3500 (1.3 AA
resolution). The sampling rate was one observation per minute, and
the typical S/N ratio per observation is in excess of 1000. At the
time of writing, we have obtained over 10,000 spectra. The spectra
will be analyzed to identify any periodic signals due to acoustic
oscillations in Procyon. In addition to measuring the equivalent widths
of the three Balmer lines (Kjeldsen et al. 1995) covered by our spectra
(Hβ, Hγ, and Hdelta) we will also examine the spectra for variations
in the average metal line strength. Preliminary power spectra will
be presented.
---------------------------------------------------------
Title: Asteroseismology from equivalent widths: a test of the sun
Authors: Keller, C. U.; Harvey, J. W.; Barden, S. C.; Giampapa, M. S.;
Hill, F.; Pilachowski, C. A.
1998IAUS..185..375K Altcode:
Kjeldsen et al. (1995) reported a probable detection of solar-like,
low-amplitude, p-mode oscillations of eta Bootes using equivalent
width measurements from low-resolution spectra of the hydrogen Balmer
lines. We tested the usefulness of this technique using observations of
the Sun in integrated light. Despite the very high signal-to-noise ratio
of the data stretching over six continuous days, no solar oscillation
signal was found so far in the equivalent width of Hβ. Spatially
resolved observations of the Hβ equivalent width at solar disk center
reveal that the oscillation signal is suppressed in the wings of Hβ
as compared to the continuum. Extrapolation of the oscillation signal
seen in the spatially resolved data suggests an amplitude of about
1ppm for integrated light measurements, which is about a factor of
5 lower than expected from simple theoretical arguments. We explore
other methods to deduce an oscillation signal from all spectral lines
simultaneously. cont has: Deng, L. et al.; auths fixed below
---------------------------------------------------------
Title: Center-to-limb variation of the second solar spectrum.
Authors: Stenflo, J. O.; Bianda, M.; Keller, C. U.; Solanki, S. K.
1997A&A...322..985S Altcode:
The linear polarization that is caused by scattering processes in the
solar atmosphere has been refered to as the "second solar spectrum",
since it is structurally as rich as the ordinary intensity spectrum
but quite different in appearance and information contents. One
of the most used and theoretically best understood lines in the
second solar spectrum is the SrI 4607Å line, which has served as a
diagnostic tool for determinations of spatially unresolved, turbulent
magnetic fields via the Hanle effect. Here we present the detailed
center-to-limb variation of the scattering polarization in this line
for a number of new data sets obtained both with an electrooptical
modulation system (ZIMPOL) and a non-modulating beam splitter system
(at IRSOL, Locarno), to provide improved observational constraints for
theoretical modelling. The amplitude and width of the polarization
profile, the amount of continuum polarization, as well as the depth
and width of the intensity profile have been evaluated and carefully
corrected for spectral broadening and stray light. While there is
generally good agreement between the five data sets, some systematic
differences are shown to be of solar rather than instrumental origin,
most likely due to spatially varying Hanle depolarization across the
solar disk. A number of other spectral lines have been observed with
the ZIMPOL system at two different limb distances (μ=0.1 and 0.2) to
allow us to compare the steepness of the center-to-limb variation of
their polarization amplitudes. The steepest variation is exhibited by
the continuum polarization, which declines by approximately a factor of
6 when going the 15 arcsec distance from μ=0.1 to μ=0.2. The spectral
lines with the steepest center-to-limb variation are molecular lines,
the CaII infrared triplet, and Hα. In contrast the SrI 4607 and BaII
4554Å lines have only moderately steeper center-to-limb variations
than that of an ideal, purely dipole-scattering atmosphere, for
which the polarization ratio between μ=0.1 and μ=0.2 is 1.38. These
center-to-limb variations may be used to constrain temperature-density
models of the upper photosphere and chromosphere.
---------------------------------------------------------
Title: ISOON: The Improved Solar Observing Optical Network
Authors: Neidig, D.; Confer, M.; Wiborg, P.; Dunn, R.; Balasubramaniam,
K. S.; Frederick, R.; Kutzman, R.; Soli, R.; Keller, C.; Gullixson,
C.; Alios, Inter
1997SPD....28.0224N Altcode: 1997BAAS...29..897N
Efforts are under way to replace the existing SOON system, which was
designed in the 1970s, with a new system (ISOON) based on a fully
tunable narrow-band filter and CCD detector. ISOON would feature
autonomous, rapid-cadence solar imaging and remote operation at four
sites, and would transmit solar images in near real time to central
facilities at Falcon AFB and Boulder CO, for use in space weather
forecasting. The ISOON technical approach is to retain the front
end of the existing SOON telescope, but replace the optical bench,
birefringent filter, and spectrograph with a dual Fabry-Perot filter
system and secondary optics contained in a single pod. ISOON data
products will include full-disk H-alpha, continuum, and line-of-sight
magnetograms on 1-arcsecond pixels. High- resolution images (limited
field, 0.3-arcsecond pixels) would be available via a future upgrade
in the secondary optics. ISOON will also be capable of acquiring
vector magnetic field images via a software upgrade to be added at a
future time.
---------------------------------------------------------
Title: The second solar spectrum. A new window for diagnostics of
the Sun.
Authors: Stenflo, J. O.; Keller, C. U.
1997A&A...321..927S Altcode:
The Sun's radiation becomes linearly polarized by coherent scattering
processes in the solar atmosphere. With a novel polarimetry system
that achieves a precision of 10^-5^ in the degree of polarization, the
previously largely unexplored territory of scattering physics on the Sun
is now fully accessible. The observations reveal a polarized spectrum
that looks very different as compared with the ordinary, unpolarized
solar spectrum but has an astounding wealth of spectral structures. It
is therefore refered to as the "second solar spectrum". In the present
paper we show how the second solar spectrum is governed by different
physical processes, which provide new diagnostic opportunities and tools
that are complementary to those of the ordinary intensity spectrum. We
illustrate the effects of quantum interferences and hyperfine structure,
isotope abundances, partial frequency redistribution, molecular
contributions, and magnetic canopies. Also shown are polarization
features, for which the underlying physics has not yet been identified.
---------------------------------------------------------
Title: Chromospheric Events in the Quiet Network
Authors: Keller, C.; Bastian, T.; Benz, A.; Krucker, S.
1997SPD....28.1304K Altcode: 1997BAAS...29..917K
Time sequences of a quiet network region close to disk center have
been simultaneously recorded with the VLA, various instruments on SOHO,
and the solar telescopes on Kitt Peak. The analysis of the Hα spectra
obtained at the McMath-Pierce telescope revealed down-flows with
apparent velocities of more than 2.5 km/s associated with magnetic
field structures in the quiet network. During such events, the Hα
spectra show a pronounced asymmetry. The photospheric magnetic field was
determined from rapid scans in three iron lines with the Zurich Imaging
Stokes Polarimeter. Up- and down-flow velocity excursions outside of
magnetic field regions are compatible with chromospheric waves. We
describe the properties of these events as seen in the observations
of the visible part of the spectrum and their signatures at radio
and UV wavelengths. The final goal of this study is the construction
of a time-dependent 3-D picture of the quiet solar atmosphere and the
understanding of the dynamical coupling of photospheric magnetic fields
with the chromosphere and the corona.
---------------------------------------------------------
Title: Observations of the quiet Sun's magnetic field.
Authors: Grossmann-Doerth, U.; Keller, C. U.; Schuessler, M.
1996A&A...315..610G Altcode:
The profiles of Stokes I and V of FeI 5247A, CrI 5247A, FeI 5250.2A and
Fe I 5250.6A have been measured in the quiet photosphere with ZIMPOL,
a new polarimeter, with very low noise level. The spatial resolution
element of about 1Mm was essentially determined by seeing. In about
15% of the observed area, the signal-to-noise ratio was sufficiently
large to determine the strength of the magnetic field and to derive
various properties of the Stokes V profiles (asymmetries, zero-crossing
shift and thermal line ratio). Our results show that, at least in this
fraction of area, the magnetic field in the quiet Sun is dominated by
strong flux concentrations which must be rather small. The data on the
Stokes V properties provide observational constraints for theoretical
models of flux concentrations.
---------------------------------------------------------
Title: Concept for a miniature solar magnetograph
Authors: Keller, Christoph U.; Harvey, John W.
1996SPIE.2804...14K Altcode:
We present a novel concept for a solar magnetograph that uses a
photo-refractive crystal to reflect and focus the light from the wings
of many spectral lines onto a camera. The crystal acts simultaneously as
multiple, narrow-band filters and as an off-axis telescope. Polarization
measurements are performed close to the final focus. Since this approach
uses the light from many spectral lines simultaneously, the required
telescope aperture is substantially reduced and exposure times can be
so short that accurate tracking is not necessary. Such a concept is
particularly attractive for NASA's Minimum Solar Mission where very
compact, light-weight instruments are required.
---------------------------------------------------------
Title: Time series restoration from ground-based solar observations
Authors: Seldin, John H.; Paxman, Richard G.; Keller, Christoph U.
1996SPIE.2804..166S Altcode:
Many processes of interest in the solar atmosphere have spatial scales
of much less than one second of arc. If the processes are related to
magnetic fields, the relevant scales are even smaller. Noticeable
evolutions of solar features occur on time-scales of less than a
minute if a spatial resolution of better than one second of arc is
reached. It is, therefore, of great interest to recover time-series
imagery with near diffraction-limited spatial resolution and good
temporal resolution on a consistent basis and over extended periods of
time using ground-based techniques. Phase diversity is a post-collection
technique for restoring fine-resolution detail when imaging in the
presence of phase aberrations such as atmospheric turbulence. Incident
energy is split into two channels: one is collected at the conventional
focal plane, the other is intentionally defocussed a known amount
and collected by a second detector array. Phase-diverse speckle is an
extension of phase diversity whereby a time sequence of short-exposure
image pairs is collected. The maximum-likelihood estimate of a common
object and a set of phase aberrations is performed jointly using
all images. A phase-diverse speckle set of images of a plage region
was collected over a span of 13.5 minutes using the 76-cm Vacuum
Tower Telescope at the National Solar Observatory on Sacramento
Peak. A phase- diverse pair of broad-band images at 6563 angstroms
was collected along a third, narrow-band image in the wing of H-
(alpha) . A set of restorations was made into a movie depicting the
highly dynamic photosphere at scales below 0.3 arcsec. We conclude that
the combination of fine spatial and temporal resolution achieved with
phase-diverse speckle opens a new window to the study of the dynamics
of the solar atmosphere from ground-based observatories.
---------------------------------------------------------
Title: Flare Genesis Experiment
Authors: Murphy, Graham A.; Rust, David M.; Strohbehn, Kim; Eaton,
Harry A.; Keil, Stephen L.; Keller, Christoph U.; Wiborg, P. H.
1996SPIE.2804..141M Altcode:
In January 1996, the Flare Genesis Experiment was carried for 19
days by a 29.4 M cu. ft helium-filled balloon in the stratosphere
above Antarctica, during which over 14000 images of the Sun were
recorded. Long-duration ballooning provides a relatively inexpensive
means to observe the Sun under near-space conditions and to develop
instrumentation and techniques that will be used on future solar space
missions. The purpose of the flight was to improve understanding of
the mechanisms involved in many different types of solar activity,
particularly flares and solar filament eruptions. Achieving this goal
demanded the development of a platform for an 80-cm F/1.5 optical
telescope that would be stable to 10 arcseconds. In addition, we
developed an image motion compensation system capable of holding the
Sun's image to better than the system's 0.2 arcsecond diffraction
limit. Other key elements on board included a lithium-niobate
Fabry-Perot etalon filter to provide a tunable 0.016-nm bandpass over
a wide wavelength range, a fast 1534 X 1024-pixel Kodak CCD camera,
and 180 GBytes of on-board storage. There was also a system for
sending commands and receiving telemetry and a high-speed downlink
for sending images during periods when the payload was in line of
sight of the ground station. On- board computers provided a command
and control system capable of near-autonomous operation. During most
of the flight, contact with the payload was sporadic, so operation
was primarily under autonomous control.
---------------------------------------------------------
Title: Evaluation of Phase-Diversity Techniques for Solar-Image
Restoration
Authors: Paxman, Richard G.; Seldin, John H.; Loefdahl, Mats G.;
Scharmer, Goeran B.; Keller, Christoph U.
1996ApJ...466.1087P Altcode:
Phase-diversity techniques provide a novel observational method for
overcoming the effects of turbulence and instrument-induced aberrations
in ground-based astronomy. Two implementations of phase-diversity
techniques that differ with regard to noise model, estimator,
optimization algorithm, method of regularization, and treatment of edge
effects are described. Reconstructions of solar granulation derived
by applying these two implementations to common data sets are shown to
yield nearly identical images. For both implementations, reconstructions
from phase-diverse speckle data (involving multiple realizations of
turbulence) are shown to be superior to those derived from conventional
phase-diversity data (involving a single realization). Phase-diverse
speckle reconstructions are shown to achieve near diffraction-limited
resolution and are validated by internal and external consistency tests,
including a comparison with a reconstruction using a well-accepted
speckle-imaging method.
---------------------------------------------------------
Title: Imaging Coronal Emission Lines under High Sky-Background
Conditions
Authors: Keller, C. U.; Smartt, R. N.
1996SoPh..166..311K Altcode:
We have obtained images in solar coronal emission lines under high
sky-background conditions by making precise differential measurements
between the coronal emission line and the near-by continuum, which is
primarily due to scattered light from the solar disk. Chopping between
the two wavelengths was performed at 100 kHz to avoid artifacts from
fast-flying dust particles and other aerosols, and also from seeing
effects. The differential signal was detected with a novel CCD camera
that demodulates signals up to 100 kHz. These preliminary observations
show coronal emission at the 0.2% level of the scattered-light
background and pave the way to efficient and precise imaging of coronal
emission features under less than ideal `coronal-sky' conditions.
---------------------------------------------------------
Title: The Flare Genesis Experiment
Authors: Rust, D. M.; Murphy, G. A.; Strohbehn, K.; Keil, S. L.;
Keller, C. U.
1996AAS...188.6705R Altcode: 1996BAAS...28Q.934R
The goal of the Flare Genesis Experiment is to make solar observations
at the highest practicable resolution in order to improve understanding
of the mechanisms involved in many different types of solar activity,
particularly flares and solar filament eruptions. Achieving this goal
demanded the development of a balloon-borne platform for an 80-cm F/1.5
optical telescope that could maintain 10 arcsec pointing stability. The
first flight of the Flare Genesis Experiment took place in January
1996. In the stratosphere, 37 km above Antarctica, for more than 19
days, the Flare Genesis telescope pointed at the Sun with the planned
stability. While the primary science objective, to measure the vector
magnetic fields using two liquid crystal polarization modulators, was
not achieved on this flight, 18,000 continuum images were obtained. They
demonstrate that the major engineering challenges for such a flight
were overcome. In addition, we developed an image motion compensation
system capable of limiting the motion of the Sun's image on the focal
plane to less than the system's 0.2 arcsec diffraction limit. Other key
elements on board included a lithium-niobate Fabry-Perot etalon filter
to provide a tunable 0.016-nm bandpass over a wide wavelength range,
a 1538 x 1024- pixel CCD camera and 100 GBytes of on-board storage. We
will describe the payload design and how the instruments performed. We
will discuss how the constraints of long duration Antarctic ballooning
guided the final design and impacted the results. Two more flights
are planned before the next solar maximum. Such long-duration balloon
flights provide a relatively inexpensive means to observe the Sun at
the highest resolution and to develop instrumentation and techniques
for future space missions.
---------------------------------------------------------
Title: Asteroseismology via equivalent widths - tests on Procyon,
Eta Bootis, and Alpha Trianguli.
Authors: Harvey, J. W.; Pilachowski, C.; Barden, S.; Giampapa, M.;
Keller, C. U.; Hill, F.
1996BAAS...28..917H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Second Solar Spectrum
Authors: Keller, C. U.; Stenflo, J. O.
1996AAS...188.5704K Altcode: 1996BAAS...28..912K
The Sun's radiation becomes linearly polarized by coherent scattering
processes in the solar atmosphere. With a novel imaging polarimeter
(Zurich Imaging Stokes Polarimeter I) that achieves a precision of
10(-5) in the degree of polarization, the previously largely unexplored
territory of scattering physics on the Sun is now fully accessible. The
observations reveal a polarized spectrum that looks very different from
the ordinary, unpolarized solar spectrum but has an astounding wealth
of spectral structures from atoms as well as molecules. This second
solar spectrum will be used as a new source of information to learn
more about atomic physics, abundances and isotope ratios, radiative
transfer physics, solar magnetic fields, and the thermodynamics of
the solar atmosphere.
---------------------------------------------------------
Title: Asteroseismology via Equivalent Widths -- Tests on Procyon,
Eta Bootes, and Alpha Triangulum
Authors: Harvey, J.; Pilachowski, C.; Barden, S.; Giampapa, M.;
Keller, C.; Hill, F.
1996AAS...188.5903H Altcode: 1996BAAS...28S.917H
Recently, Kjeldsen et al. reported a probable detection of solar-like
low-amplitude p-mode oscillations of Eta Bootes using equivalent
width measurements from low-resolution spectra of the H Balmer
lines. This technique has the potential to provide stellar oscillation
measurements good enough to allow the asteroseismic inference of stellar
structure. Here we report on the preliminary analysis of data from three
observing runs with the Kitt Peak Coude Feed and 2.1-m telescope in
November 1995 (Alpha Triangulum), February 1996 (Procyon), and March
1996 (Eta Bootes). These runs are being used to develop observing
and data reduction techniques, such as a synchronized timing system
to maintain evenly spaced temporal samples, a continuous unshuttered
CCD readout to increase the duty cycle of the observations, and a
simulation of the probability of a detection as a function of observing
run length. We observed the region around the H beta, gamma, and delta
lines with a spectral dispersion of about 0.4 Angstroms per pixel,
extracted equivalent widths, and performed time series analysis. The
temporal spectrum of Alpha Triangulum contains a significant peak near
the theoretical prediction, however, we do not yet know the origin of
this peak.
---------------------------------------------------------
Title: SOLIS - A Modern Facility for Synoptic Solar Observations
Authors: Harvey, J.; Keller, C.; November, L.; NSO Staff
1996AAS...188.6703H Altcode: 1996BAAS...28..934H
SOLIS (Synoptic Optical Long-term Investigations of the Sun) is
a proposed suite of instruments that will modernize and greatly
improve synoptic solar observations carried out by the National
Solar Observatory on behalf the solar and solar-terrestrial physics
communities. The primary scientific goal is to provide fundamental data
necessary to understand the solar activity cycle, sudden energy releases
in the solar atmosphere, and solar spectral irradiance changes. An
operational goal is to produce real-time and near real-time data
for forecasting space weather, and to augment the scientific yield
from space mission such as SOHO and TRACE, and ground-based projects
including RISE and GONG. State-of-the-art instrumentation and data
collection techniques will be employed to enhance both the quality
and quantity of data. A high degree of automation and remote control
will provide faster user access to data and flexible interaction
with the data-collection process. The instruments include a vector
spectromagnetograph that will measure the magnetic field strength and
direction over the full solar disk in 15 minutes, a full disk patrol
delivering digital images in various spectral lines at a high cadence,
a coronal emission line imager and photometer that will provide
photometric and velocity images in at least five spectral lines,
and a Sun-as-a-star precision spectrometer to measure changes in
many spectral lines. The choice of sites for the instruments depends
on potential partnerships with other observatories and the level of
funding that can be obtained. The goal is to place the instruments at
sites with large amounts of sunshine and coronal observing conditions
as appropriate. The SOLIS proposal is currently under review by the
National Science Foundation.
---------------------------------------------------------
Title: NIM-2 -- A Near Infrared Imaging Vector Magnetograph
Authors: Rabin, D.; Keller, C.; Jaksha, D.
1996AAS...188.6706R Altcode: 1996BAAS...28R.934R
NIM-1 is a spectrograph-based Stokes polarimeter for measuring the
strength and orientation of magnetic fields in the solar photosphere
using two Zeeman-sensitive Fe I lines (g = 3 and g_geff = 1.53) near
1565 nm. NIM-2, now under construction, also uses these spectral
lines but is based on a high-resolution Fabry-Perot etalon. NIM-2
will eliminate the image scanning and consequent spatial distortions
of NIM-1 and will be compact and light enough to serve as a prototype
for balloon or space instruments. The Queensgate etalon will provide a
spectral resolving power of 10(5) over a 1-nm free spectral range. The
initial detector will be the 256(2) InSb array shared with NIM-1, but
NIM-2 is designed to accommodate a 512(2) or 1024(2) “Aladdin” InSb
array. The data system, also shared with NIM-1, is being upgraded to
handle the faster switching speed ( ~ 8 ms) of improved liquid-crystal
variable retarders. NOAO is operated for the NSF by the Association of
Universities for Research in Astronomy. Near-infrared magnetometry at
NSO is supported by the the NASA Space Physics Division through the
SR&T program in solar physics.
---------------------------------------------------------
Title: Measurement of the full Stokes vector of He I 10830 Å
Authors: Rüedi, I.; Keller, C. U.; Solanki, S. K.
1996SoPh..164..265R Altcode:
First observations of the full Stokes vector in the upper chromosphere
are presented. The He I 10830 Å line, which has been shown to give
reliable measurements of the line-of-sight component of the magnetic
field vector, has been used for this purpose. It is shown that the
difference between the appearance of chromospheric and photospheric
magnetic structures observed close to the solar limb is largely
due to the difference in height to which they refer and projection
effects. The observations do suggest, however, that the magnetic field
above sunspot penumbrae is somewhat more vertical in the chromosphere
than in the photosphere.
---------------------------------------------------------
Title: Recent Progress in Imaging Polarimetry
Authors: Keller, C. U.
1996SoPh..164..243K Altcode:
Recent instrumental developments in imaging polarimetry allow array
detectors to reach a polarimetric sensitivity of 1 × 10<SUP>−4</SUP>
of the intensity. New instrumental effects appear at these levels of
sensitivity and generate spurious polarization signals with amplitudes
of up to 5 × 10<SUP>−4</SUP>. Here I discuss these effects and
present methods to avoid them. Polarized spectra with an rms noise of
6 × 10<SUP>−6</SUP> may then be obtained. Furthermore a method is
brought to the reader's attention that allows polarization measurements
at the 1 × 10<SUP>−4</SUP> level with regular array detectors,
e.g. in the near-infrared.
---------------------------------------------------------
Title: Balloon-Borne Polarimetry
Authors: Rust, D. M.; Murphy, G.; Strohbehn, K.; Keller, C. U.
1996SoPh..164..403R Altcode:
For about two weeks in 1995, the balloon-borne Flare Genesis
Experiment will continuously observe the Sun well above the turbulent,
image-blurring layers of the Earth's atmosphere. The polarization-free
80 cm telescope will supply images to a liquid-crystal based vector
magnetograph, which will measure magnetic features at a resolution
of 0.2 arcsec. An electrically tunable lithium-niobate Fabry-Perot
provides a spectral resolution of about 0.015 nm. In a follow-up
series of Antarctic balloon flights, the Flare Genesis Experiment
(FGE) will provide unprecedented details about sunspots, flares,
magnetic elements, filaments, and the quiet solar atmosphere.
---------------------------------------------------------
Title: Direct measurements of flux tube inclinations in solar plages.
Authors: Bernasconi, P. N.; Keller, C. U.; Povel, H. P.; Stenflo, J. O.
1995A&A...302..533B Altcode:
Observations of the full Stokes vector in three spectral lines
indicate that flux tubes in solar plages have an average inclination
in the photosphere of 14^o^ with respect to the local vertical. Most
flux tubes are inclined in the eastwards direction, i.e., opposite
to the solar rotation. We have recorded the Stokes vector of the
FeI 5247.1A, FeI 5250.2A, and FeI 5250.7A lines in nine different
plages with the polarization-free 20cm Zeiss coronagraph at the Arosa
Astrophysical Observatory of ETH Zuerich. The telescope has been
modified for solar disk observations. The chosen spectral lines are
particularly sensitive to magnetic field strength and temperature. To
determine the field strength and geometry of the flux tubes in the
observed plages we use an inversion code that numerically solves the
radiative transfer equations and derives the emergent Stokes profiles
for one-dimensional model atmospheres consisting of a flux tube and
its surrounding non-magnetic atmosphere. Our results confirm earlier
indirect estimates of the inclination of the magnetic fields in plages.
---------------------------------------------------------
Title: Handling of huge multispectral image data volumes from a
spectral hole burning device (SHBD)
Authors: Graff, Werner; Rosselet, Armel C.; Wild, Urs P.; Gschwind,
Rudolf; Keller, Christoph U.
1995SPIE.2480..445G Altcode:
We use chlorin-doped polymer films at low temperatures as the
primary imaging detector. Based on the principles of persistent
spectral hole burning, this system is capable of storing spatial and
spectral information simultaneously in one exposure with extremely high
resolution. The sun as an extended light source has been imaged onto
the film. The information recorded amounts to tens of GBytes. This data
volume is read out by scanning the frequency of a tunable dye laser and
reading the images with a digital CCD camera. For acquisition, archival,
processing, and visualization, we use MUSIC (MUlti processor System with
Intelligent Communication), a single instruction multiple data parallel
processor system equipped with the necessary I/O facilities. The huge
amount of data requires the developemnt of sophisticated algorithms to
efficiently calibrate the data and to extract useful and new information
for solar physics.
---------------------------------------------------------
Title: Speckle spectrography of extended objects.
Authors: Keller, C. U.; Johannesson, A.
1995A&AS..110..565K Altcode:
We present a method to obtain diffraction limited spectrograms of
extended sources from a series of ground-based slit-spectrograms. The
method is a combination of speckle deconvolution (Keller & von
der Luehe 1992) and a rapid spectrograph scanning scheme (Johannesson
et al. 1992). The slit of a spectrograph is scanned over the solar
surface while simultaneous slit-jaw images and spectrograms are
recorded. The exposure time is short with respect to seeing-induced
variations. A Knox-Thompson speckle reconstruction scheme is applied
to the slit jaw images. From the individual slit-jaw images and their
speckle reconstruction the instantaneous point spread function can
be determined for any location. The spectrograms are reconstructed
by inverting a matrix that describes the action of the point-spread
function on the spectrograms. The final product is a three-dimensional
cube formed by the two spatial coordinates and the wavelength. In
the spatial domain the data is nearly diffraction limited while the
spectral resolution is not affected by the reconstruction. The method
is applied to observations of the quiet solar granulation.
---------------------------------------------------------
Title: Weak Magnetic Fields in the Network
Authors: Keller, C. U.; Harvey, J. W.
1995SPD....26..206K Altcode: 1995BAAS...27..952K
No abstract at ADS
---------------------------------------------------------
Title: The spectral hole-burning device: a 3-dimensional photon
detector.
Authors: Keller, C. U.; Gschwind, R.; Renn, A.; Rosselet, A.; Wild,
U. P.
1995A&AS..109..383K Altcode:
We present the principles of a new device that combines a
high-resolution spectrometer (0.001nm spectral resolution, 10nm
bandwidth) with maximum efficiency, a large detector (pixel size
1μm by 1μm, overall size 2cm by 2cm), and a very high density data
storage device (up to 400TB) in a single optical device. This device
is based on persistent spectral hole-burning in a dye-doped polymer at
temperatures of a few K. It is called a Spectral Hole-Burning Device
(SHBD) in analogy to Charge Coupled Devices (CCD). A SHBD can be
thought of as a color film with extremely high spectral resolution:
it records the intensity in the visible or the near-infrared according
to the two-dimensional position and the wavelength. Therefore it is a
3-dimensional photon detector. We show first results from laboratory
measurements that prove the feasibility of a SHBD.
---------------------------------------------------------
Title: Visible and near-infrared polarimetry with LEST.
Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
1995LFTR...59.....K Altcode:
This document describes the LEST vector polarimeters for the visible
and the near-infrared part of the solar spectrum. After some general
remarks on precise polarimetry with large telescopes, the authors
present the specifications based on scientific reasons and some
general design considerations. The proposed instrument design for the
visible is based on the ZIMPOL II concept. They present two different
concepts for vector polarimetry in the near infrared. One is based
on a beam-splitter system combined with liquid crystal modulators,
while the other is based on the same modulator package as used in the
visible and optical demodulation in the final focus.
---------------------------------------------------------
Title: Infrared Capabilities of the Large Earth-Based Solar Telescope
(LEST)
Authors: Keller, C. U.
1995itsa.conf..215K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Lest Detector and Data Acquisition System - Part One -
Specifications
Authors: Collados, M.; Keller, C. U.; Steiner, P.
1995lest.rept....1C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Visible and Near Infrared Polarimetry with lest
Authors: Keller, C. U.; Bernasconi, P. N.; Egger, U.; Powel, H. P.;
Steiner, P.; Stenflo, J. O.
1995lest.rept....1K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: LEST detector and data acquisition systems.
Authors: Collados, M.; Keller, C. U.; Steiner, P.
1995LFTR...61.....C Altcode:
Contents: 1. Specifications (M. Collados, C. U. Keller,
P. Steiner). 2. Design considerations for the data acquisition system
(P. Steiner).
---------------------------------------------------------
Title: First light for an astronomical 3-D photon detector.
Authors: Keller, C. U.; Graff, W.; Rosselet, A.; Gschwind, R.; Wild,
U. P.
1994A&A...289L..41K Altcode:
Spectroscopic studies of extended astronomical sources at optical
wavelengths are hampered by the two-dimensional nature of current
photon detectors. Most photons of interest are not utilized because a
spectrometer must scan in the spatial or spectral domain. We present
the first solar spectrum recorded with a three-dimensional, highly
wavelength-sensitive photon detector based on a dye-doped polymer film
at 1.6K. The achieved spectral resolution is comparable to the best
solar spectra obtained with a Fourier transform spectrometer. Our
experiment paves the way for a photon detector that improves the
efficiency of spectroscopy of extended sources by orders of magnitudes.
---------------------------------------------------------
Title: Two-dimensional polarimeter with a charge-coupled-device
image sensor and a piezoelastic modulator
Authors: Povel, H. P.; Keller, C. U.; Yadigaroglu, I. -A.
1994ApOpt..33.4254P Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the strength of solar intra-network fields.
Authors: Keller, C. U.; Deubner, F. -L.; Egger, U.; Fleck, B.; Povel,
H. P.
1994A&A...286..626K Altcode:
The combination of the German Vacuum Tower Telescope and the prototype
of ZIMPOL I (Zuerich Imaging Stokes Polarimeter I), a novel, very
sensitive imaging polarimeter, has resulted in the first spectra
of solar intra-network (IN) fields in circular polarization. The
sensitivity in terms of flux density is 0.7 Mx/cm^2^. While magnetic
fields in solar spots, pores, plages, and in the network have
predominantly kG field strengths, the magnetic field strength of
the IN flux is a controversial subject due to the absence of direct
measurements. We first summarize the current ideas on IN fields and
examine previous arguments for their field strength. Our measurements
of the magnetic line ratio formed between the amplitudes of the Stokes
V profiles of Fe I 5247.1 A and Fe I 5250.2 A are consistent with a
field strength well below 1 kG. Since the sensitivity of the magnetic
line ratio becomes low for small field strengths, we can only set an
upper limit on the field strength of IN fields of 500 G at the level
of line formation with a probability of 68% and an upper limit of 1
kG with a probability of 95%. We emphasize that these are the first
observations of a magnetic line ratio of unity near disk center.
---------------------------------------------------------
Title: Application of multiframe iterative blind deconvolution for
diverse astronomical imaging
Authors: Christou, Julian C.; Hege, E. Keith; Jefferies, Stuart M.;
Keller, Christoph U.
1994SPIE.2200..433C Altcode: 1994aisi.conf..433C
We present applications of a recently developed iterative blind
deconvolution algorithm to both simulated and real data. The
applications demonstrate the algorithm's performance for a wide range
of astronomical imaging. We demonstrate the effectiveness of using
multiple observations of the same object convolved with different
point spread functions. We also show the extension of the algorithm
to phase retrieval when the object Fourier amplitude is available.
---------------------------------------------------------
Title: Speckle techniques for spectroscopic observations
Authors: Keller, C. U.
1994ASIC..433...43K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Direct Measurements of Fluxtube Inclinations in Plages
Authors: Bernasconi, Pietro N.; Keller, Christoph U.; Stenflo, Jan Olof
1994ASPC...68..131B Altcode: 1994sare.conf..131B
No abstract at ADS
---------------------------------------------------------
Title: High spatial resolution observations of solar magnetic fields
Authors: Keller, C. U.
1994smf..conf..325K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Some aspects of polarimetry with LEST
Authors: Keller, C. U.
1994ASIC..433...37K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Narrow-Band Speckle Imaging
Authors: Keller, C.; von der Lühe, O.
1993rtpf.conf..129K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Small-Scale Structure in Active Regions (Invited)
Authors: Keller, C.
1993ASPC...46....3K Altcode: 1993IAUCo.141....3K; 1993mvfs.conf....3K
No abstract at ADS
---------------------------------------------------------
Title: Resolution of magnetic flux tubes on the Sun
Authors: Keller, C. U.
1992Natur.359..307K Altcode:
MAGNETIC flux at the surface of the Sun is predominantly concentrated
in discrete areas with kilogauss field strengths<SUP>1</SUP>. Except
for sunspots, these areas are too small to have been resolved
by conventional observations. These magnetic flux tubes are an
essential part of the physics of the activity and heating of the
outer atmosphere of the Sun and other late-type stars<SUP>2</SUP>, but
although their average properties have been studied in considerable
detail<SUP>3,4</SUP>, direct observations of them have been lacking
because of turbulence in the Earth's atmosphere, which limits
resolution to ~400 km. Using a newly developed technique of speckle
inter-ferometry<SUP>5</SUP>, we have obtained simultaneous direct
observations of the white-light and magnetic field signature of flux
tubes. Individual flux tubes are seen, with resolved diameters of
~200 km and continuum brightness contrast of at least +30%. Magnetic
features larger than 300 km in size tend, however, to be darker than
their surroundings.
---------------------------------------------------------
Title: Solar speckle polarimetry
Authors: Keller, C. U.; von der Luehe, O.
1992A&A...261..321K Altcode:
The combination of a polarimeter with real-time frame selection
and differential speckle imaging results in diffraction-limited
magnetograms that provide new insight into the morphology of solar
small-scale magnetic fields. The method to record diffraction limited
narrow-band filtergrams of solar features is based on two cameras
taking simultaneous short exposure images through a broad-band and
a narrow-band filter, respectively. Speckle imaging reconstructs the
image in the broad-band channel. This reconstruction determines the
instantaneous optical transfer function (OTF) for each individual
broad-band exposure. Each simultaneously recorded image in the
narrow-band channel is then corrected for the instantaneous OTE To
recover all spatial frequencies in the narrow-band channel the so
corrected single images are averaged. We have applied the method
to polarimetric observations of a solar active region by tuning
the narrow-band filter to the wing of a Zeeman sensitive spectral
line. The most active part of the region shows no more normal granules
but features with a diameter of 0.3 to 0.5 arcsec. The smallest
magnetic fields are concentrated in regions with sizes at or below
the diffraction limit of the telescope.
---------------------------------------------------------
Title: Application of Differential Speckle Imaging to Solar
Polarimetry
Authors: Keller, C. U.; von der Luhe, O.
1992ESOC...39..453K Altcode: 1992hrii.conf..453K
No abstract at ADS
---------------------------------------------------------
Title: High Spatial Resolution Magnetograms of Solar Active Regions
Authors: Keller, C. U.; Stenflo, J. O.; von der Luhe, O.
1992A&A...254..355K Altcode:
Using the Universal Birefringent Filter at the Sacramento Peak Vacuum
Tower Telescope we have obtained simultaneous observations of left and
right circular polarization in various solar magnetic features with a
resulting spatial resolution of 0".7 in the magnetograms. We describe
the data reduction in some detail and discuss the various instrumental
effects. In particular we show that seeing can create features in
magnetograms. A penumbra near disk center shows small-scale features in
the magnetogram which are associated with the bright filaments. Bright
features in the umbra of a small spot exhibit considerable polarization
signals. In a pore region opposite polarities are found within a few
seconds of arc.
---------------------------------------------------------
Title: Zurich Imaging Stokes Polarimeter Zimpol-I - Design Review
Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
1992lest.rept....1K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: GAP: yet another image processing system for solar
observations.
Authors: Keller, C. U.
1992lest.rept....3K Altcode:
GAP is a versatile, interactive image processing system for analyzing
solar observations, in particular extended time sequences, and for
preparing publication quality figures. It consists of an interpreter
that is based on a language with a control flow similar to PASCAL
and C. The interpreter may be accessed from a command line editor and
from user-supplied functions, procedures, and command scripts. GAP is
easily expandable via external FORTRAN programs that are linked to
the GAP interface routines. The current version of GAP runs on VAX,
DECstation, Sun, and Apollo computers. Versions for MS-DOS and OS/2
are in preparation.
---------------------------------------------------------
Title: High resolution observation of solar magnetic fields
Authors: Keller, Christoph Ulrich
1992PhDT.......203K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Zürich Imaging Stokes Polarimeter - ZIMPOL I. Design review.
Authors: Keller, C. U.; Aebersold, F.; Egger, U.; Povel, H. P.;
Steiner, P.; Stenflo, J. O.
1992LFTR...53.....K Altcode:
This document describes the design of ZIMPOL I, the first Zürich
Imaging Stokes Polarimeter. This solar vector polarimeter will mainly
be used for observations of the solar magnetic field at high spatial
and/or spectral resolution. A brief overview of ZIMPOL I is given
in the preface. The scientific requirements are then specified in
detail. They lead to an instrument concept which consists of several
parts: the optical system, the camera system, the real-time image
processing system, and the graphical user interface. Data reduction
and analysis of observations recorded with this polarimeter are also
dealt with in detail. Prototypes of the modulator package and the CCD
camera have been tested at various observatories. Results from these
tests are presented and discussed.
---------------------------------------------------------
Title: Demodulation of all four Stokes parameters with a single CCD -
ZIMPOL II. Conceptual design.
Authors: Stenflo, J. O.; Keller, C. U.; Povel, H. P.
1992LFTR...54.....S Altcode:
It is shown how it is possible to simultaneously record images of
all four Stokes parameters with a single CCD detector chip when
fast (50 kHz), piezoelastic modulation of the polarization state
is used. As the four image planes use the identical pixels of the
CCD, all gain-table or flat-field effects vanish when forming the
fractional polarization images. For each group of four pixel rows,
one row collects the photons, while the other three are used for fast
buffer storage. There are no light losses caused by masking of the pixel
rows used for buffer storage, sinced a microlens array collects all the
photons and directs them to the unmasked pixel rows. The efficiency of
the system for simultaneous recording of all four Stokes parameters
is six times greater than that of ZIMPOL I, the first generation of
the Zürich Imaging Stokes Polarimeter, since no beam splitter with
three separate CCD cameras is needed and no significant light losses
occur at the masked pixel rows. The theoretically possible efficiency
limit is thereby practically reached. The system is planned to be
developed as ZIMPOL II, the second generation of the Zürich Imaging
Stokes Polarimeter.
---------------------------------------------------------
Title: Round table discussion.
Authors: Darvann, T. A.; Keller, C. U.
1992lest.rept..131D Altcode:
The discussion session was divided into two parts: 1) Image processing
software requirements for the future LEST, and 2) strategies at the
present time to increase the collaboration and communication on image
processing software within the solar community, and for the successful
achievement of point 1.
---------------------------------------------------------
Title: Multicolor Continuum Analysis of the Solar Granulation in
Quiet and Active Regions
Authors: Keller, C. U.; Koutchmy, S.
1991ApJ...379..751K Altcode:
A set of narrow-band filtergrams of the solar photosphere recorded in
three widely separated true continuum windows and in Mg I b1 with a
spatial resolution of about 0.5 arcsec is analyzed. The influence of
small-scale magnetic fields on the granulation is studied by comparing
various statistical parameters in selected active and quiet regions,
and the temperature stratification of faculae at the level of continuum
formation is investigated. In active regions there exist more points
with enhanced continuum intensity as compared with quiet regions, and
there is more power at small spatial scales in active regions, whereas
more power exists at large scales in quiet regions. Quiet regions
near and far away from a large sunspot do not show any significant
difference. It is concluded that changes in the granular pattern near
sunspots are caused by the small-scale magnetic fields often found near
sunspots. Faculae, identified by their brightness in the Mg I b1 wing,
show an enhanced continuum intensity and a reduced ratio between the
blue and the red continuum, which is consistent with current flux
tube models.
---------------------------------------------------------
Title: Fine-Scale Magnetic Field in a Sunspot Penumbra and Adjacent
Photosphere
Authors: Frank, Z. A.; Scharmer, G. B.; Keller, C.; Lundstedt, H.
1991BAAS...23.1052F Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Two-dimensional polarimeter with CCD image sensors and
piezo-elastic modulators.
Authors: Povel, H. P.; Keller, C. U.; Stenflo, J. O.
1991sopo.work..100P Altcode:
A new type of polarimeter for application in solar physics is described,
which combines fast polarization modulation and high spatial resolution
using piezoelastic modulators and charge coupled device (CCD) image
sensors. The problem of incompatibility between the slow read-out of
CCD sensors and fast modulation has been solved. First two-dimensional
Stokes Q and V images, free from gain-table noise, and with an rms
noise level of about 10<SUP>-3</SUP> have been obtained.
---------------------------------------------------------
Title: High spatial resolution polarimetry using filtergrams.
Authors: Keller, C. U.
1991sopo.work..124K Altcode:
Using tunable filters at the Sacramento Peak Vacuum Tower
Telescope Stokes V images of various solar magnetic features have
been observed. Simultaneous recording of left and right circular
polarization leads to high spatial resolution magnetograms. Various
instrumental effects of these observations are discussed in detail.
---------------------------------------------------------
Title: Inversion of Stokes V profiles: The structure of solar magnetic
fluxtubes and its dependence on the filling factor.
Authors: Keller, C. U.; Solanki, S. K.; Stenflo, J. O.; Zayer, I.
1991sopo.work..387K Altcode:
The authors present results from an inversion procedure that derive the
temperature stratification, the turbulent velocity, and the magnetic
field strength of the photospheric layers of magnetic fluxtubes from
observed Stokes V spectra near disk center. In a first step the
inversion has been applied to 10 Fe I and Fe II Stokes V profiles
of a plage and a network region to obtain reliable models of the
fluxtubes. In a second step the dependence of the fluxtube structure
on the filling factor has been studied with spectra of 3 Fe I lines
from 23 different regions based on the models derived in the first step.
---------------------------------------------------------
Title: Dependence of the properties of solar magnetic flux tubes on
filling factor. II - Results of an inversion approach
Authors: Zayer, I.; Stenflo, J. O.; Keller, C. U.; Solanki, S. K.
1990A&A...239..356Z Altcode:
The dependence of the properties of solar magnetic elements on the
magnetic filling factor is studied using Stokes V spectra of three
lines observed near the center of the solar disk. The inversion
technique developed by Keller et al. (1990) is applied to three
neighboring spectral lines, and the average temperature difference,
the magnetic field strength, and the nonstationary velocity in the
relevant line-forming layers are quantitatively determined. Quantitative
evidence is provided for the dependence of the temperature within
flux tubes on the amount of magnetic flux. The flux tubes are found
to become cooler and their field strengths, at a given optical depth,
to become larger as the filling factor increases. The presence of
kilogauss field strengths within flux tubes is reconfirmed.
---------------------------------------------------------
Title: Solar magnetic field strength determinations from high spatial
resolution filtergrams
Authors: Keller, C. U.; Stenflo, J. O.; Solanki, S. K.; Tarbell,
T. D.; Title, A. M.
1990A&A...236..250K Altcode:
Circularly polarized images with high spatial resolution (better than 1
arcsec) of a solar active region, obtained with a tunable filter in the
wings of Fe I 5247.1 A and Fe I 5250.2 A, have been analyzed in terms of
the magnetic line ratio technique introduced by Stenflo (1973). Whenever
a measurable amount of polarization is present, the distribution of
the observed magnetic-line ratio is compatible with a unique value,
which is randomly blurred by noise due to the photon statistics,
the CCD camera, and atmospheric distortions. There is no need for a
distribution of field strengths to explain the observed distribution of
the magnetic line ratio. Consequently, the observations are compatible
with a unique magnetic field strength in solar small-scale magnetic
elements of about 1000 G at the level of line formation. For a thin
flux tube, this corresponds to a field strength of approximately 2000
G at the level of continuum formation, which is in excellent agreement
with previous field strength determinations from low spatial resolution
spectra (4-10 arcsec).
---------------------------------------------------------
Title: Structure of solar magnetic fluxtubes from the inversion of
Stokes spectra at disk center
Authors: Keller, C. U.; Steiner, O.; Stenflo, J. O.; Solanki, S. K.
1990A&A...233..583K Altcode:
The paper presents an inversion procedure that derives the temperature
stratification, the turbulent velocity, and the magnetic field strength
of the photospheric layers of small-scale magnetic fields from observed
Stokes V spectra and the continuum intensity. The inversion is based
on the determination of a small number of model flux parameters by a
nonlinear least squares fitting algorithm. The minimization of the sum
of the squared differences between observed and synthetic observables
makes it possible to determine the temperature stratification and the
magnetic field strength.
---------------------------------------------------------
Title: Empirical Photospheric Fluxtube Models from Inversion of
Stokes V Data
Authors: Keller, C. U.
1990IAUS..138..121K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Restoration of distorted images as a variational problem:
a dynamic programming approach
Authors: Keller, C. U.
1989hsrs.conf..208K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: High resolution photographic Stokes polarimetry of small
scale magnetic flux (poster)
Authors: Keller, C. U.; Koutchmy, S.
1989hsrs.conf..443K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Properties of solar magnetic fluxtubes from only two spectral
lines
Authors: Solanki, S. K.; Keller, C.; Stenflo, J. O.
1987A&A...188..183S Altcode:
A method for the determination of the magnetic field strength,
velocity, and temperatures inside solar flux tubes, in addition to
their inclinations and filling factors, is presented which requires
only the Stokes V and Q profiles of the Fe I 5250.2 A and Fe I 5247.1
A spectral lines. Application of the procedure to spectra of the two
lines obtained at various distances from the solar limb shows that
considerable velocity broadening is required at all positions on the
disk in order to reproduce the polarimeter data. The center to limb
variation of the 5250/5247 Stokes V and Q line ratios is found to
contain little information on the height variation of the magnetic
field in the context of one-dimensional models.
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Title: Comet Rudnicki (1966e)
Authors: Nielsen, A. V.; Milet, B.; Pereyra, Z. M.; Keller; Rodriguez,
J. J.; Mintz, B.
1967IAUC.1987....2N Altcode:
Dr. Axel V. Nielsen, Ole Romer Observatory, points out that at its
descending node Comet Rudnicki passes very near the orbit of the
earth. Its heliocentric distance would then be 1.02 AU. He suggests
the possibility of observing meteors associated with the comet on about
1967 June 7. Further precise positions have been reported as follows:
1966 UT R.A. (1950) Decl. Mag. Observer Nov. 16.81487 1 13 14.71 -
6 54 55.1 Milet 16.82318 1 13 12.28 - 6 55 08.3 " Dec. 1.03499 0 06
33.99 -11 24 24.3 9.0 Pereyra 1.04892 0 06 29.72 -11 24 35.6 9.0 "
4.06458 23 50 37.02 -12 12 58.6 Mintz 4.08542 23 50 30.51 -12 13 14.7
" 5.01250 23 45 31.56 -12 27 10.9 " Milet (Nice Observatory). Pereyra
(Cordoba Observatory). Measurer: Keller. Computer: Rodriguez. Mintz
(U.S. Naval Observatory, Washington). Measured on ADAMM.