Roque de los Muchachos Observatory

Dutch Open Telescope

DOT
DOT
Diameter
Ø 45.00 cm
Responsible scientist
Alejandro
Oscoz Abad
Operation year
1997

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.

Optics
   Prime mirror (Cervit): aperture 45 cm, focal length 200 cm
   Quality: rms 0.011 micron = wavelength/50
   Prime-focus field stop (hole in water-cooled mirror): 1.6 mm
   Field of view: 150 arcsec

Telescope
   Mount: parallactic, brushless push-pull pairs of servo motors
   Gears: 1:75000 reduction in four floating-gear steps
   Weight: 16 tons

Tower
   Design: permits only parallel platform motion, no tilts
   Construction: open steel framework
   Height: 15 meter
   Weight: 13 tons

Canopy
   Design: fold-away clam-like shell
   Construction: teflon-like-coated PVC fabric on heavy steel ribs
   Diameter: 7 meter

Image acquisition system
   Blue cameras: Hitachi KP-F100 (1296 x 1030 px, square 6.7 micron px, well depth 16000, 10 bits, 12 frames/s, bad non-linearity)
      Field of view: 92 x 73 arcsec, 0.071 arcsec/px
   Red cameras: Redlake MegaPlus II ES4020 (2K x 2K px, square 14.84 micron pixels
      Field of view: 113 x 113 arcsec, 0.110 arcsec/px
   Fiber links: dual way, custom made (IGF)

 

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