Dutch Open Telescope

Ø 45.00 cm
Responsible scientist
Oscoz Abad
Operation year

    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.

       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

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

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

       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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