This has been known since the beginning, though it is perhaps not generally known; one cannot do astronomy, after all, with the instrument off the telescope. The noise is caused by the fact that the telescope ground is a few millivolts away from the ground at the coax-fiber interface box on the ground floor. The solution is to use fibers all the way, and Jon Brinkman is working on it. jim APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 26 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