We have noticed a large "effective" read-noise on the DIS red chip on two separate nights within the past two weeks. Upon inspection of image histograms of bias frames, we see that there appears to be a fluctuation of bias level which spreads out the pixel histogram. On one night, the histogram is distinctly bimodal, with each peak having a width of the previously determined readout noise of ~15 electrons, but with two peaks separated by ~20 DN, leading to an effective readout noise of ~28 electrons. The two peaks do NOT come from two distinct regions on the chip, odd vs. even rows/cols, or in any discernable pattern. On the second night, the histogram is broad and flatter topped, with a comparable effective rn of ~28 electrons, with a hint of bimodality, though not as clear. Has anyone noticed similar behavior recently? Can recent DIS users check their data for a similar effect? If this is a new problem, it needs to be addressed ASAP, because an effective readout noise of 28 electrons is really quite large! Jon Holtzman APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 27 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