Looking quickly at the biases I took last Sunday night (February 2nd) I find that an imhistogram of the red chip bias frames does seem to be bimodal with a separation between the peaks of ~20-25 pixels. Looking at older data I find no such second peak (data taken November 11, 1996). It is hard to say with a quick look like this whether the widths of the two peaks are the "expected" 15 DN (they may be slightly narrower), but the overall width of the distribution is of order 30 DN essentially doubling the read noise. I have not been able to detect any pattern either yet (I haven't really had time to look that much). I hadn't noticed this before as I was really looking for something in my blue spectra, and hadn't really done more than glance at the latest red data. Jim Lauroesch APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 28 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