DOT tomography

During past years the DOT was available to external users in the "Open DOT program" described in Rutten et al., A&A, 413, 1183, 2004.  At present, DOT availability depends on DOT partnerships (see DOT external usage). 

The table below defines the DOT tomography wavelengths.  The DOT standard product consists of co-spatial synchronous image sequences taken in these wavelengths with parallel cameras at 20-30 second speckle burst sampling cadence.

The field of view covers 91 x 72 arcsec with 0.071 arcsec/pixel sampling resolution for the blue wavelengths (Hitachi KPF100 cameras).  The newer Halpha and red continuum cameras (Redlake MegaPlus II ES4020 cameras) have 113 x 113 arcsec field of view at 0.110 arcsec/pixel.  In the speckle burst processing a few arcsec are lost at the field edges through tip-tilt correction; usually, more are lost when co-registering sequences from multiple cameras.

The guiding may or may not include solar rotation compensation, as desired.

These different diagnostics sample different heights in the solar atmosphere.  Precise heights of formation cannot be specified, because they depend on the actual time-dependent conditions.  Numerical modeling is needed to evaluate the response of each diagnostic to a particular type of solar structure or dynamical phenomenon.  However, generally the blue continuum samples the deep photosphere, the G band and red continua originate some tens of kilometers higher, the Ca II H line spans a few hundred kilometers from the outer wing to its core.  Halpha fibrils can be located at any height but generally lie one to a few thousand kilometers above the photosphere.  The Ba II 4554 line samples the upper photosphere.

Ba II 4554 is the major Ba II resonance line, similar to Ca II K in ionic structure but at lower opacity due to smaller abundance.  It has a boxy core due to hyperfine and isotope splitting.  The large atomic weight and the steep line flanks make it an excellent Doppler diagnostic.  It is a promising Zeeman and Hanle diagnostic, but polarimetry is not implemented at the DOT.

Both Ca II H and Ba II 4554 tend to have LTE opacities but scattering-dominated source functions.  Halpha has large departures from LTE in both its opacity and source function; Halpha fibrils can be optically thick or optically thin.  The far Halpha wings, especially the blue one, are excellent LTE diagnostics to locate and track intergranular magnetic elements unless these are shielded by Doppler-shifted fibrils.

DOT filters:
designation abbreviation wavelength (Å) filter FWHM (Å) type tuning
blue continuum bc 4320 6 interference fixed
red continuum rc 6550 2.4 interference tilt-shift to 6535 Å
red prominence pr 6563 2.3 interference tilt-shift to blue
G band (CH lines) gb 4305 10 interference fixed
Ca II H ca 3968 1.35 interference tilt-shift throughout violet wing
Halpha ha 6563 0.25 Lyot tunable to +/- 8 Å
Ba II 4554 ba 4554 0.08 Lyot tunable to +/- 2 Å
barium continuum bk 4505 5.4 interference fixed
Hbeta hb 4861 1.5 interference manual tilting

The wavelength setting of the tilt-shiftable and the two tunable Lyot filters can be changed between successive speckle bursts under program control.  The red continuum and red prominence (Halpha wide) filters are in a computer-controlled filterwheel, the choice is made from the control room.  Similarly, the barium continuum filter and the wide Hbeta filter are also in a filterwheel.  The Hbeta filter can only be tilted manually when the DOT is not observing. 

The continuum-near-Halpha and continuum-near-Ba II 4554 passbands serve for two-channel speckle restoration following Keller & von der Lühe (1992). In this technique, the wide-band wavefront estimation is used to restore the narrow-band frames.  When sequences of multiple wavelengths are chosen for the two Lyot filters (Halpha and Ba II 454), these can be taken in smaller subbursts (generally 100/N frames/burst, where N is the number of wavelengths, down to 20 fpb). This results in faster cadence and excellent rubber-sheet co-registration since the different subbursts are slaved to the single full-burst wide-band speckle reconstruction.  If the filters are kept fixed at one wavelength the cadence can be as fast as 2 seconds.  The disadvantage is lower image quality then for full 100 fpb burst reconstruction.  Because our cameras have low well depth while the exposures must be synchronous per pair, the photon flux for the wide-band cameras is reduced appropriately through the use of neutral density filters.  For an example movie see DOT speckle modes.

The prominence filter is a better choice for Keller & von der Lühe reconstruction at the limb (since Halpha shows fibrils crossing the limb, providig radial wave-front encoding).  It also serves to register the profile-integrated Halpha emission from an off-limb prominence. 

Tomographic speckle sequences can be taken whenever the DOT is manned and the seeing is at least reasonable, say Fried parameter above 6 cm.  At Fried parameter 10 cm (good but not perfect) the resolution already approaches the 0.2 arcsec diffraction limit.  This sometimes happens during multiple hours, also in afternoons.  Such high-resolution long-duration multi-camera sequences provide excellent science input in themselves, but are also highly valuable as context tomography for spectrometry and spectropolarimetry at other telescopes, and in combination with EUV imaging and spectroscopy from space.

The speckle-restored image sequences from the DOT are stored on and available from the DOT database server in Utrecht.

Rob Rutten 2014-07-22