I talked to Jim Gunn about the DIS efficiency. The 4-6% reported by Rene is definitely too low. Jim says that he measured it in May roughly and got something like 15% and felt that that number was already a factor of 1.5-2 lower than he expected. It was not clear to me from our discussion whether a changing efficiency was a plausible explanation. On technique, Jim felt it impossible to meaningfully measure throughput either with a slit or in direct mode through a filter. His technique, which he said was standard, is to use the dispersing element but no slit to make a short exposure on a bright standard. This includes the dispersing element efficiency and does not lose any light to a filter; it also does not lose light which does not get through the slit for whatever reason. On further reflection, it does not seem to me that it is quite fair, relevant might be a better word, to exclude the loses to the slit (image motion, wings of the psf, differential refraction, whatever) since one does use a slit for faint objects (the only time efficiency matters much) and suffers those loses presumably. Of course, most such loses cannot be cured by modifications of the spectrograph, an important point when thinking about the how/who/when/etc of the fix. Ed Turner APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 3 in the apo35-dis archive. You can find APO the archive in /u/strauss/apo/mailer/apo35-dis 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-dis@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO