I'm now reducing some DSC data taken Apr 18/19, 1996. Looking at the values in the over-scan and pre-scan areas at the edges of the chip, I see a variation of about 50 counts between frames taken at "normal" rotator position, and those taken after rotation by 90 degrees. The difference occurs several times during an observing session, always in the same sense. And, of course, the bias varies when the DSC is taken on/off the telescope (by about the same amount, 50-60 counts). Michael Richmond APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 39 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