DOT technology

The DOT is a reflector with a parabolic mirror that sits out in the open at a height of 15 m (photograph). The mirror (presently Cervit, 45 cm diameter, focal length 200 cm) focuses the incoming beam onto a water-cooled 1.6 mm field stop that reflects most of the image out of the telescope and transmits only a 2.5 by 2.5 arcmin subfield to the re-imaging optics and cameras.

The 15 m high support tower, at a weight of 13 ton considerably lighter than the telescope itself, permits only lateral motion of the platform while inhibiting tilts.  Even in strong wind buffeting the telescope therefore maintains precise tracking.  The tower puts the telescope above much of the turbulent boundary layer caused by solar ground heating, especially when the strong trade wind blows up-slope from Northern directions in the best-seeing weather pattern.  The tower consists of open steel-tube triangles and is designed to withstand large ice loads and wind pressure.  The ladder and elevator cage may be filled by 30 tons of ice without harm!

The bad-weather canopy (photograph) opens clamshell-like and folds fully away to the sides.  It is made of heavy polyester fabric mounted on steel ribs and may be closed in winds up to 30 m/s (or opened, but that is less likely).  When closed it withstands the 70 m/s (Bf 12) or stronger winds that can hit Roque de los Muchachos in the harsh La Palma winter storms.  The coated fabric also resists ice deposition, a major problem at the Canary Island mountain summits where undercooled fogs often cause heavy upwind icicle growth.  The combination of massive ice loads with hurricane-strength storms is a major building risk at the La Palma and Tenerife observatories.  A scaled-up copy of this canopy for German GREGOR telescope has been constructed at DTO Delft under supervison of the DOT team and installed on the GREGOR platform by the DOT team.

The tower, platform, telescope, and canopy were mounted on the La Palma site during 1996-1997.  The optics were mounted in a full-size interferometer at Utrecht in order to construct a precise major-axis and focus defining laser system that guarantees high-precision alignment.  It was used to define the optical axis exactly when the mirror and secondary optics were installed.

The mirror (photograph) is mounted deformation-free with nine-point axial and three-point radial support in a parallactic telescope structure that is considerably overdimensioned as well as unbalanced in order to obtain extreme pointing stability at very low dissipation.  The latter amounts to only about 20 W (three orders of magnitude less than the heat production of the oil bearings in the nearby William Herschel Telescope). Brushless pairs of servo motors in push-pull preload configuration without backlash drive four-step gear trains (photograph) achieving 1:75,000 reduction with self-aligning gears.

The DOT top is constructed with very stiff supports capable of carrying large weight.  The resulting DOT aperture (shadow of the on-axis tube and support beams on the primary) is asymmetric; its unusual shape is accounted for in the speckle processing.  Here is an example of the DOT + speckle transfer function (amplitude modulation transfer function averaged over many specklegrams).

The DOT secondary optics consisted initially of on-axis re-imaging lenses, focus mechanism, and analog video camera.  All DOT movies from before April 2000 were made with this simple technology-demonstration system.  Even at 8-bit digitization (with a PC frame grabber), speckle reconstruction was found to be feasible and worthwhile.  Science-grade data followed with the installation of digital cameras.

Multi-channel observation was initiated by the installation of a second camera (the first one mounted besides the incoming beam) which observes continuum near the G-band and enables separation of granular and fluxtube motions through image subtraction, a technique that exploits the strict synchronicity of the DOT speckle imaging system (e.g. Nisenson, van Ballegooijen, de Wijn & Sütterlin, Ap. J. 587, 458, 2003).

Subsequently, an elaborate multi-wavelength system was designed using seven digital CCD cameras of which six are located, each with its own optimised re-imaging optics, in the DOT top besides the incoming beam.  Here is a schematic of the DOT top.  Beam splitters (including dichroic ones) divide the light between the G band (on-axis tube and camera), a continuum band near the G band, Ca II H, Halpha, a continuum band near Halpha, and Ba II 4554 with nearby continuum.  Detail is given in 2003hawaii-dot.pdf.

Interference filters are used for the continua, G band and Ca II H. The Halpha beam utilises a Zeiss Lyot filter from the former Ottawa River Solar Observatory which can be tuned rapidly through the line. The similar but even narrower-band tunable Lyot filter from Irkutsk provides Ba II 4554 Dopplergrams.  The narrow-band filters are mounted with telecentric re-imaging optics to produce bandpass homogeneity over the full field at the full resolution given by the primary-mirror diffraction limit at each wavelength.  The cameras run in synchronous speckle mode, each obtaining many-frame bursts at up to 12 frames/s rate.  The digital frames are transported per custom-made optical fiber links to the control room.  The telescope and camera operation is also remotely controlled through optical fibers.  The incoming speckle bursts are handled by a multi-computer network encompassing a control computer, image-storing computers, each with its own disks and connections to the DOT Speckle Processor.  For more detail see Rutten et al., A&A 413, 1183, 2004.

The continuum-near-Halpha and continuum-near-Ba II 4554 speckle registration serves for restoration following Keller & von der Lühe (1992). In this multi-channel technique, the wide-band wavefront estimation is used to restore the narrow-band frames.  An important advantage is that when the two Lyot filters for Halpha and Ba II 4554 are sequentially tuned to multiple wavelengths, smaller sub-bursts per wavelength suffice and so permit faster cadence, and also the different wavelenghth samples are perfectly co-registered through rubber-sheet slaving to the single wide-band channel speckle reconstruction.  However, independent full-burst reconstruction delivers higher quality.  A demonstration movie is presented and discussed under DOT speckle modes.

The DOT control room is located in the nearby Swedish telescope building, - where the DOT team enjoys generous hospitality - and adjacent to the Swedish 1-m Solar Telescope (SST) control room and image laboratory.  Their proximity obviously facilitates tandem operation of the two telescopes.

The on-site parallel DOT Speckle Processor delivers fast speckle processing.  The reduced data are disseminated via the DOT database in Utrecht.

Rob Rutten 2017-11-18