This is to report on the noise problem noted by Charles Hoopes in his May 5 run with DIS. Charles noticed exceptionally high noise in both the red and blue chips of DIS. A histogram of intensities in individual bias frames shows a bi-modal distribution of intensities; two histograms with an empty region inbetween. In looking at this more carefully today, it appeared that the offset between the two histograms is 256, which suggests a problem in the A/D conversion, where a bit is dropped or added in the conversion. I am not sure why this problem would occur in both the red and blue chips (don't they have two different convertors?). Because the difference of 256 is so large, it has in fact been possible to correct bias frames and data by subtracting 256 from the pixels that are clearly too high. I should also note that regions of high intensity in the data (where the A/D conversion goes over to the next bit) appeared to be normal again. These pixels do not require a correction, and needed to be identified and fixed by visual inspection, after the automatic subtraction of 256 from high pixels. While we can probably salvage the one set of spectra we managed to get that night, this problem raises several questions: (1) Why was this problem apparently no longer present the following night? (2) Does it make sense that it showed up in both B and R chips at the same time? (3) Could the A/D convertor(s) be flaky, also causing the apparent increase in "readnoise" in the red chip, which seems to come and go. Note that an error in a different bit could easily cause some pixels to be high or low by any multiple of 2. For example, adding 32 to a bunch of pixels would go a long way to explaining a large increase in readnoise. The bi-modal noise histograms noted for the red chip by several users show a peak separation that may well be caused by one of these smaller offsets. I would like to get some feedback from DIS experts on this, and suggest that DIS users carefully check their data for possible A/D convertor errors. Would it be difficult to replace the A/D convertors? Rene Walterbos APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 52 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