Since Xiaohui and I were pretty much clouded out, we took a slew of dome flats. We attempted to battle the shutter timing problem and the rotation problem. To solve the shutter problem we put a 1% ND in front of the quartz lamps to lengthen the exposure time for a flat field. This worked pretty well. We used the ND for r,i, and z, and found the following exposure times gave an average number of counts near the half filled well value: r 30 sec i 25 sec z 35 sec No ND was used for the g, and u filters, adn the exposure times were 15 sec and 300 sec, respectively. (Note 300 gives nowhere near the half filled well value.) Now more important is the rotation problem. Specifically the illumination is uneven, and you need to rotate the DSC when taking flats to get an even flat, or so we were told. In fact, we took 3 images at each of 6 rotations (0,60,120,180,240,300), and median co-added them. The result-- an image that looks like you rotated the instrument six times. Thus our advice: DON'T USE ONLY DOME FLATS WITH THE DSC! even with rotation. Definitely get some twilight flats! If anyone wants to see one of our resulting flats, let me know. Gordon Richards Xiaohui Fan APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 16 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