Date: Fri, 18 Sep 1998 06:13:40 -0600
From: long@apo.nmsu.edu (Dan Long)
Subject: 2.5m obslog 980917
                        THURSDAY SEPTEMBER 17, 1998
                     APACHE POINT OBSERVATORY SDSS 2.5m
                              OBSERVING  LOG
   Most of the first half of the night was spent tracking down an
intermittent communications problem with the imager.  It was eventually
traced to a recent PROM change.  The previous version of the PROM was
installed and the problem went away.
   We removed two of the brown weights from the back of the telescope to
allow the counterweights to have sufficient travel to balance the telescope
with the imager and its full load of nitrogen.
   There appears to be some imbalance back to front on the PSS such that
the altitude moves up more easily than down at moderately high elevations,
and is nearly balanced at low elevations.  This may have been the cause for
repeated failures of the altitude to move down when around 40 degrees.  It
would move a short distance then stop.  It was not hitting the windscreen
bumpers.
   The Neslab was not functioning properly.  With the set point at 18 C,
the temperature was running about 24 C.  The front panel showed no error,
and the cooling and refrigeration led were lit.  However the compressor was
only coming on for a few seconds every 5 minutes or so ( like something was
overheating or overloading and tripping a protection cct).  Power cycling
the unit did not change the condition.  At the end of the night when we
went to close up the Neslab _was_ working properly.  I don't know what
changed, but it bears watching.  The compressor may be nearing failure.
   We opened at about midnight.  The first thing we did was to set up on
Polaris.  As soon as we tried to move in azimuth, we found that while the
azimuth drives were enabled, the windscreen azimuth was not.  Swapping the
wires 83 and 84 on WB3 solved that problem, but still leaves the logic for
the stow position/ 15 degree stop such that altitude motion is not
permitted outside of the stow position.
  We lined up on Polaris and set the telescope position accordingly, using
the fiducials.  We then slewed to the equator at the meridian.  Here we
were greeted by quite poor images.  The primary was set to the position Pat
and I had found to work well with the engineering camera a few days ago.
The secondary was set to the tilts from Russell's list for ~57 deg
elevation.  In setmir terms that is
Primary      0  -35 31 1700 1900
Secondary 1350 -217  0    0  112
  After some experiments with tilt ( almost all error appeared to be
x-translation, so y-tilt was employed ), about +60 arcseconds of y-tilt
with all other parameters being zero, yielded good images.  Jim Gunn got a
number of -2.56mm x-translation with -46.7" arcseconds y-tilt to be the
translational equivalent of the tilt alone.  Unfortunately this was well
out of range of the primary's allowed motion.  The best we could do was
-1.24mm.  The rest was done with tilt.  the setmir numbers are listed
below.
Primary       0     0   30  -1240     0
Secondary  1200  -217    0      0   112
  We found that if the rotator fiducial is taken to be zero, with our
Polaris referenced position for Az, and El at the equator and meridian,
that the rotator position which best lines up for driftscanning is
89:50:38.
  Tonight there were several periods of widspread thin high clouds.  The
winds were light westerly, and humidity ranged from about 60% to 80%.
                                    Dan