It's known that DSC has some vertical bias patterns. Right now, some overscan rows are added so that by taking the median of those overscan rows these patterns may be taken out. I have some older DSC images which suffered from these patterns, and there was no overscan row like now. I (suggested by Robert Lupton) tried to take out the median of every column of a flattened image. After taking out the medians, the bias patterns are basically gone. And since that bias pattern affected my sky determination of photometry quite much, esp. when doing aperture photometry for faint stars, after taking them out, the photometry is improved significantly. On the web site below, I put two figures, both the r' magnitude difference between same stars taken in different nights, one before substraction of bias pattern, the other after. The scatter of magnitude differences represents the photometric precision in both cases. You can see the improvement of the aperture photometry here. http://astro.princeton.edu/~fan/paper/qso96.html -- Xiaohui Fan Princeton University Observatory Peyton Hall Princeton, NJ 08544 email : fan@astro.princeton.edu tel : (609) 258-5153 (O) (609) 683-1315 (H) APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 37 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