Observatorio del Roque de los Muchachos

Dutch Open Telescope

DOT
DOT
Diámetro
Ø 45.00 cm
Científico responsable
Alejandro
Oscoz Abad
Año de operación
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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