The CCD news is almost all "bad" this time, and what's left is "ugly." The first piece of bad news is that our best large format CCD, Tek 2048 #4, suddenly died in December 1996 and attempts to revive it have not been successful. It does not see light, and all four amplifiers are inoperative. The CCD has been returned to SITe for a post-mortem. This CCD has mostly been used at the 4-m and 1.5-m telescopes for direct imaging and with the echelle spectrographs. We will continue to offer a Tek 2048 (either #3 or #5) for these applications. Tek 2048 #3 is mostly dedicated to direct imaging at the 0.9-m telescope, so it's likely that users on the larger telescopes will be scheduled with Tek 2048 #5. Although all four amplifiers of #5 function, one has higher than usual noise (~ 20e- rms); thus it is normally read out through two amplifiers. For direct imaging at the 4-m prime focus with broad band filters Poisson noise from the sky often dominates instead, and some users have preferred to read the CCD in quad mode to gain efficiency from halving the readout time. Otherwise, Tek 2048#5 is an excellent quality CCD, the only obvious cosmetic defect being a single adjacent pair of blocked columns. In particular, it does not have the myriad of low-level (few e-) traps that are an unfortunate characteristic of Tek 2048 #3, which although of little consequence for most direct imaging applications, degrades images with near-zero background. Therefore we will always try to schedule #5 for use with the echelle spectrographs.
Note that the BTC Mosaic Imager is also available for use at the 4-m prime focus, and is to be preferred for all programs except those for which the large area coverage is not important, or for programs requiring filters unavailable in 6 × 6 inch size.
The second CCD causing concern is the STIS 2048 CCD, which is dedicated to the Schmidt telescope. This CCD is front-illuminated, and has a coating of lumogen (Metachrome) that provides UV and blue sensitivity. For the past year this coating has been slowly delaminating, although no part as yet has become completely detached. Visually, on flat field exposures, the CCD appears to have several "cracks" and "bands" meandering over the surface. These are a few percent more sensitive than the majority of the CCD. These features are also wavelength dependent and are more prominent at shorter wavelengths. On closer examination under a microscope one can see a fine example of plate tectonics. The cracks are actually boundaries between two plates where material has piled up. It appears that as yet there are no regions on the CCD that are devoid of metachrome. Despite the bizarre appearance, standard flat-fielding techniques do a good job of producing flat object frames, and there are no discernible image profile variations when a star falls on a crack. However, the delamination is steadily progressing with time, and eventually it is likely that a large piece of the coating will separate from the CCD. If this happens, then one possibility is to remove the whole coating chemically. This is not without risk. Thus we have arranged that the identical CCD used on the Burrell Schmidt at KPNO will be sent to CTIO when that CCD is replaced by Case after it assumes responsibility for operations later this year.
Here is a brief list of our CCDs:
Tek 2048 #5: 4-m and 1.5-m direct imaging, echelles
Tek 2048 #3: 0.9-m direct imaging
STIS 2048: Schmidt direct imaging
Loral 3K: 4-m spectroscopy (R-C, Argus, Echelle)
Loral 1K: 1.5-m spectroscopy
Tek 1024 #2: Fabry-Perot, some direct imaging
All these CCDs are being operated by Arcon CCD controllers. See the CTIO WWW pages for more information on CCD characteristics and Arcon.
Alistair Walker