I agree with Michael Richmond that without a filter wheel, we really have substantial overheads. The proposal only makes sense if a filter wheel is indeed available. Tim McKay indicated at Fermilab that a filter wheel should be ready by 1 October, the beginning of the fourth quarter. With a filter wheel, the amount of time for slewing is reduced by a factor of five, because one observes an object in all five colors at once. Moreover, if one is clever about the order in which one does the observations, slew times will be << 1 minute; slew times at the APO for objects reasonably close in the sky can be as small as five seconds. If one isn't careful, slew times can indeed be *much* longer, of course, especially around the zenith. More thought needs to go into the exposure times, but a zeroth-order calculation goes like this: a rule-of-thumb is that the DSC at sidereal rate on the 3.5m goes roughly as deep as the 2.5m photometric camera (the differences being size of chip corresponding to effective exposure time, and size of pixels). The chip is 5 arc-minutes across, corresponding to an exposure time on the sky of 20 seconds at sidereal rate. Thus 120 seconds in stare mode is six times more exposure than we'll get on the 2.5m, which sounds like overkill to me. So let us assume 30 second exposures on object, + 10 seconds to move filter wheel, and 1 minute slew time between object. This is 260 sec per object. We will observe one standard star field every 10 QSO's, yielding a total of 110 objects, or 28,600 seconds, or eight hours of exposure. This is probably rather optimistic, but is probably within a factor of two of being realistic. I was planning to write the observing proposal in such a way that if the filter wheel is not available, that we will forfeit our time. -Michael Strauss APO APO APO APO APO Apache Point Observatory 3.5m APO APO APO APO APO This is message 6 in the apo35-general archive. You can find APO the archive in /u/strauss/apo/mailer/apo35-general 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-general@astro.princeton.edu APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO APO