The photographic plateholder has now been replaced by a CCD camera based on a JPL GRAF/Cassini 1024 x 1024 pixels CCD. The pixels size is 12 x 12 microns, corresponding to 1.65 "/pixel.
The field size is 27 x 27 arcminutes, which is quite sufficient for programmes that are presently undertaken at the telescope,but although we are using the smallest pixels available to us the spatial sampling is inadequate on nights with good seeing. With the CCD camera which offers quick determination of the telescope focus, we now find seeing values during the winter period down to 1.5 arc sec FWHM.
The low spatial sampling is improved by coadding multiple exposures with small offsets between exposures. Also a new reduction software, Teuber et.al. (1994) named "Rotate-and-Stare" has been developed in order to obtain effective subpixels resulution. The method is based on the assumption that the PSF is either circulary or elliptically symmetric.
The CCD detector is housed in an evacuated coldbox and is permanently cooled by a Peltier element. As the CCD camera is located inside the telescope tube it is considered essential, that excess heat dissipated from the camera to the surrounding is kept at an absolute minimum. This is achieved by circulating cold water with an antifreeze agent through a heat exchanger to remove the heat from the camera electronics as well as from the warm end of the Peltier cooler. Next the string of cooling water removes excess heat from the CCD on the basement of the building.
Besides the cryostat with the CCD, the camera contains a filter wheel and shutter and all associated electronics. Even the camera is compact enough that it does not cause any optical vignetting other than that of the original plateholder and spiders. The filter wheel is equipped with Johnson B and V and Gunn R,I and Z filters approximating the standard systems. A remaning sixth filter position is allocated for a Red focal extender to be used when better spatial sampling is required at the sarcrifice of field size.
The CCD controller is a prototype of a design "BroCam" which is currently beeing developed for large format CCD`s at telescopes at ESO, La Silla and the Nordic Optical Telescope, La Palma. All control signals and data are transmitted on two optical fibers providing complete galvanic isolation and noise free communication between the telescope and control room. Data acquisition is performed on 486 type PC equipped with an additional high speed image buffer. This PC is connected on Ethernet to a workstation running IRAF v2.10 software.
Sections of the CCD images in a compressed form may even be transmitted via modem to the observers home computer for evulation immediately after exposure thus enabling him to "look over the shoulder" of the observing assistant who is doing most of the observing task at the telescope.
The read-out noise is 19 electrons and the conversion factor 4.0 electrons/ADU. For direct imaging with the science CCD, the noise in exposures of more than 1 minute duration is dominated by photon shot noice of the sky background. Hence, long integrations can most efficiently be obtained as a number of 5 to 10 minute exposures.Near the meridian and at moderate zenith distances the tracking of the telescope is so good that autoguiding is not needed for such short exposures.
Figure 1.
The CCD camera with its shutter, filter wheel, and electronics
is located in a compact house at the position of the previus
plateholder.The camera is thermally neutral as the heat from
electronics and peltier is removed by circulating water.
Figure 2.
Show curves for the standard CCD filter set.
Here is a new test report on the CCD camera, in HTML format and as a compressed postscript file .
Brorfelde 10/10/1996
Karl Augustesen (kalle@astro.ku.dk)
Last updated May 25, 1999 by norup@astro.ku.dk