I recently had a couple of short slots on the APO 3.5-m and tried to use the DSC to look at a very faint, distant SN. I noticed a few items while reducing the data that might interest other DSC users. 1. There is a change in the bias level when the camera is attached to the telescope. Before my time on the 3.5-m, I took a series of 300-sec dark exposures; then the camera was hooked up to the telescope, and I took a series of 300-sec exposures; and afterwards, when the DSC had been removed from the telescope, I took another series of 300-sec darks. The bias levels (in the pre-/over-scan areas of the chip) varied roughly as follows: before telescope: 7196 DN on telescope: 7148-7128 DN (decreasing with time) after telescope: 7200 DN I do not know if the pattern of the bias changed, since I did not take any bias or dark frames when the camera was attached to the telescope. Perhaps, next time, I shall (even though I'll lose time to observe my targets). 2. The 300-sec dark exposures reveal non-uniform levels across the chip; there was a general pattern with amplitude about 30 DN from lowest to highest like so: ------------------------- | low med high | | | | | | | | med highest | | | | | | high high | ------------------------- I suspect that some of this is due to charge generation in the chip ("spurious charge"), and some is due to scattered light; the highest area has a roughly circular shape, which leads me to suspect a light leak. I compared dark frames taken before my observing time to those afterwards; there was a difference of about 4-5 DN, which showed the same pattern as above. This change in properties over the course of only 2 hours again seems consistent with a light leak, if the camera was positioned slightly differently in its stowed position. The later darks had LOWER counts than the earlier ones, and the moon was LOWER in the sky at the later time ... 3. There is a pair of bad columns not far from the center of the chip (cols 695-696), so one should place the target slightly off-center, on the side opposite to these columns. 4. I made small offsets between each of several exposures of a field in order to be able to create a clean image by taking median of the series. I discovered that offsets a) should be more than 10 arcsec in size (maybe 20 arcsec) b) should be chosen carefully so that no exposures allow the same bad column to line up with itself This means that one may have to plan the offsets between exposures carefully, in advance; I was certainly unable to adapt to the situation at 4 AM. I hope that these hints might help other users of the DSC. Michael Richmond APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 29 in the apo35-dsc archive. You can find APO the archive on http://astro.princeton.edu:82/apo35-dsc/INDEX.html APO To join/leave the list, send mail to apo35-request@astro.princeton.edu APO To post a message, mail it to apo35-dsc@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO