Hi, This is update our knowledge of the CCD in the DSC. The chip is a 2048x2048 SITe UV anti-reflection coated device. In September, 1995 this chip was contaminated with water. The water spots are visible on a flat field image. Originally the spots had an amplitude of about 25%. After a cleaning of the device by SITe, the amplitude diminished to roughly 5%. For the most part, the water spots flat field out nicely. (Currently, the best flat fields are twilight flats.) The bias image has structure in it, probably due to clocking noise in the dewar. A typical bias image is available in the new dop documentation. I believe this will subtract out, given a good enough bias image. (Take many bias frames and median them together.) The chip has 4 working amplifiers: Amp Bias (Dn) read noise (DN) gain (e-/DN) 0 4865 3.5 * 1 5292 33.6 * 2 8499 3.9 * 3 8276 4.0 * The * means that we have not measured it yet. The gain used to be 2.5, but probably has changed. The most current values will be at $DOP_DIR/doc/www/dop.CCD.html on fnstar. We currently drift scan using amps 2 and 3, and stare through amp 2. If you really want to use amp 0 for staring, try the command fnstar-OAC> cc loadProgram stare_amp0 fnstar-OAC> goStare ... There remains small amounts of spurious charge in both the parallels and the serial registers. This is noticeable as ramps along both rows and cols. The amplitude has been much reduced by changes made at Fermilab by Connie Rockosi, and is now 10-15 DN in amps 2 and 3, and 8 DN in amp 0. These should subtract out with the bias frame, leaving a maximum noise term of 2.5 DN, assuming the gain is 2.5. (There may be spurious charge induced by the transfer gate, leaving another 1.5 DN of noise behind after subtraction). In the end, after subtraction, the DSC has a readnoise in the vicinity of 5 DN, or 13 e-, again assuming a gain of 2.5. A further fix while at Fermilab eliminated noise in the temperature sensor, which should produce a stable operating temperature. Before, the operating temperature in effect depended on the readout program and since spurious charge is worse at lower temperatures this created a situation where the amount of spurious charge could vary with time. Finally, we have been having problems with RF noise on images. This is the zig-zag or curvy pattern on images, distinct from the purely vertical banding of the bias structure. We think we have found the source of the RF noise (the camera electronics outside the dewar) and have taken steps to minimize it. In the near future Jon Brinkmann expects to rebuild the DSC support structure for Tim Mckay's new filter wheel box, and then we will mount the electronics in a place to eliminate the noise completely. Of course, then the RF noise will be dominated by other sources, examples of which we have already seen. James Annis APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 18 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