I have inspected some DIS red biases that were obtained on Feb. 9. These biases display the increased level and "bimodality" of the effective read noise that has been mentioned previously. While there is no apparent dependence of bias level on two-dimensional position on the chip, there is a distinct pattern in the bias dependent on pixel number. (That is, numbering the pixels in a raster from left to right, and bottom to top.) Both Fourier analysis and phase dispersion minimization show a strong period at around 615 pixels. When binned in phase, the pattern resembles a sawtooth, or perhaps a sine curve with one or two low overtones. Such a fluctuation in bias level could account for the bimodality seen if the bias spends more time at the extremes of the imposed pattern. If the readout speed of DIS is around 40-thousand pixels per second, this seems to imply a frequency of around 60 Hz for the interference causing the new bias structure. All of these observations point to some sort of pickup noise as being the source of the new bias structure in the DIS red chip. Was DIS's ground changed during the telescope refurbishment? This should be examined in hopes of removing the bias structure from future observations. I have not looked into the effectiveness of Fourier filtering at getting rid of the bias pattern from existing data. -- Mark Hammergren, UW APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 29 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