Subject: using 3.5m to measure QSO colors

From: Michael Strauss

Submitted: Mon, 31 Jul 1995 10:20:41 -0400

Message number: 6 (previous: 5, next: 7 up: Index)

  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  This is message 6 in the apo35-general archive. You can find
APO  the archive in /u/strauss/apo/mailer/apo35-general on
APO  To join/leave the list, send mail to
APO  To post a message, mail it to