I used the 3.5-m plus DSC for three hours each on Aug 26/27 and Sep 03/04 1995. My plan was to scan at 0.2 sidereal (exposure time 96 seconds) and look at distant Abell clusters in the SDSS r' band. Images taken on the first run suffered from huge halos around bright sources. Bright stars resembled automobile headlights viewed at a distance through a thick fog. I tried to measure the radial profile of some stars, and here's what I found: radius (pix) radius (arcsec) fraction of light(*) 6 1.7 14% 10 2.8 31 20 5.5 61 50 13.8 60 100 28 79 150 41 98 200 56 100 (by definition) The last column is "fraction of all the light which falls within a circle of 200 pixels, contained in a smaller circle of N pixels". The seeing was about 8 pixels FWHM = 2 arcsec. Some clouds sneaked in and out near dawn. On Sep 03/04, I again had three hours. Once again, I used the r' band and a drift rate of 0.2 sidereal. This time, the seeing was better (about 1.5-1.6 arcsec), and stellar images did NOT appear to suffer from the "fog halo" effect. I have not yet reduced the data, but preliminary inspection doesn't show it. The big problem on this run was that the telescope drives threatened to act up. Tim McKay's run on Sep 02/03 suffered from the following problem. When he issued an "offset" command (just before starting a drift scan), the drive would fail. Here's his comment, forwarded to me by Karen Gloria: >Couple of notes that may be useful tomorrow. First the halting of the >axis. This happened every time I executed a telescope offset using the >"tcc offset inst/pabs" command with a DC offset. So to drift across my >object I had to move to where I wanted to start the drift and use "tcc >offset inst/pabs 0.0,0.0,-0.00041666667,0.0", i.e. only the 0 DC >offset allowed it to work. Don't know why this is a problem, it never >has been before. So, as Tim suggested, I just figured out where I should point the telescope to place it just to the west of my objects, and then scanned past them, as normal, with tcc offset inst/pabs 0.0,0.0,-0.00083333,0.0 And this worked out fine. The weather was very nice on this run, the sky beautifully clear. I look forward to examining the results. Michael Richmond APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 3 in the apo35-dsc archive. You can find APO the archive in /u/strauss/apo/mailer/apo35-dsc on astro.princeton.edu 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