A quick suggestion to distinguish between FITS writing artifacts and electronics problems: I suggest you take some DIS frames, storing the data in the IRAF HHH mode, and also in the FITS format. If imstat gives same behaviour for both then its likely in the electronics, otherwise likely a digital glitch. c. APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 55 in the apo35-dis archive. You can find APO the archive on http://www.astro.princeton.edu/APO/apo35-dis/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-dis@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO