On the positive side our new Tek 2048 CCD (SITe 2048 #6) continues to live up to its early promise as a really excellent device. It will primarily be used for Direct imaging and Echelle spectroscopy at the 4-m and 1.5-m telescopes. Although during its initial use at the telescope the full well capacity was rather low for one of the four amplifiers, this has been rectified by further adjustment in the laboratory. Full well is now greater than 256K e- on the two upper amplifiers and greater than 180K e- on the lower pair, very similar to the performance of our other two SITe CCDs. The only remaining fly in the ointment is that although, after prolonged periods in the dark the dark current of this detector is a very low 0.7e-/hour, even quite short periods of exposure to room lights results in a catastrophic level of dark current (8e-/hour after a 1 minute exposure and 65e-/hour after only 10 minutes). This excess dark takes many days to decay away. This is of particular concern for Echelle users especially as it is very difficult to install the CCD on the spectrograph without exposing the detector to some light. Nonetheless, experience during a recent echelle run shows that with careful handling during the installation the excess dark current can be kept to a level where it does not appreciably contribute to the noise budget, even in long exposures. When we have encountered this phenomenon in the past it has been due to fluorescence of the dewar window; however, in this case the dewar window is known not to suffer from this problem.
On the bad side the freshly deposited lumogen coating on the spare STIS 2048 CCD obtained from KPNO (see NOAO Newsletter No. 51 p. 27) fractured due to thermal stress and began to de-laminate after only a few carefully controlled thermal cycles during laboratory characterization. This CCD also proved not to be very good after more extensive laboratory testing. Inspection of our original STIS CCD re-coated at the same time revealed that its coating was also fractured after only a single cooling cycle. Consequently we still have no CCD that can be dedicated to use on the Schmidt. Hope is in sight in that KPNO has agreed to loan us the STIS CCD previously used on the Burrell Schmidt, although this device was used with only a single amplifier so the properties of the others are not yet known. In the meantime we have been able to dedicate SITe 2048 #5 to the Schmidt for the remainder of the second semester 1997 and hope to be able to do so during first semester 1998. The higher QE of this thinned CCD is an advantage for many programs, but its larger pixels (2.3"/pixel compared to 2.0 for the STIS) is a problem for others. This situation does leave us dangerously short of any backup in the event of the failure of another of our CCDs and makes it very difficult to schedule time for preventive maintenance or other laboratory work on our CCD systems.
As someone commented recently, "It's like a bad off-Broadway play. There seems to be a different cast every night." Here is the cast at the time of writing: SITe 2048 #6 for 4-m and 1.5-m direct imaging and echelle spectroscopy; Tek 2048 #3 for 0.9-m direct imaging SITe; 2048 #5 for Schmidt direct imaging; Loral 3K for 4m spectroscopy (R-C, Argus, Echelle) Loral 1K for 1.5-m spectroscopy (Cass. Spec) Tek 1024 #2 for Fabry Perot, some direct imaging. See the CTIO WWW pages for more information on the characteristics of each CCD.
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 covered is not important, or which require filters not available in 6 x 6 inch size.
Speaking of the BTC, users of this instrument in particular will be pleased to know that a Sun Ultra 2-200 workstation with 54Gb of disk space has now been installed as the data reduction computer at the 4-m telescope. With both the BTC and Arcon we expect that the resulting data will be written directly to disk on this machine, although the acquisition system will run on a different machine. The BTC has also been hooked up to Save the Bits, so data from all instruments in use at CTIO are now being backed up for an archive.
Steve Heathcote, Roger Smith
sheathcote@noao.edu,
rsmith@noao.edu