MDM Trouble Report for 2009 Apr 8

Telescope: 2.4m
Observer(s): Atlee (OSU)
Instrument: TIFKAM

Problem(s) Encountered:
When using a Prospero script to dither the telescope in a
prescribed pattern for sky subtraction, I discovered that the
'NORTH', 'SOUTH', etc. commands do not yield reproducible
position offsets.  This problem was not associated with any
error message, and it was not until I realized that a bright star
in a field I was imaging was creeping steadily out of frame.

I tinkered with my script for quite some time, including simply
performing a series of position offsets without taking any images
and found that performing cancelling motions (e.g. north 10, 
south 10) did not return the guide star to its original position.
I eventually figured out a workaround for this by manually moving
the guide star back to its starting position at the end of each
dither sequence and setting the RA/Dec encoders to the correct
values before beginning the next dither sequence.  Failing to
apply this correction led to such bizarre behaviors as calling
'west 10' and watching the telescope move east by ~90 .

My workaround does not yield identical dither positions for
each sequence.  One can easily see a bright star in one field
appear at different places for the same place in the dithering
sequence in sequential dithers.

In addition to this difficulty, I saw a very strange
malfunction.  The telescope suddenly started tracking faster
than it should have, and neither the auto-guider nor the paddle
were able to correct the problem.  In fact, the paddle stopped
responding altogether.  I tried turning the tracking off and
then turning it back on again, but that did not solve the
problem.  I ultimately resolved it by returning the telescope
to the zenith before returning to my object, at which time the
problem miraculously went away.

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Submitted on 2009 Apr 9 [7:06:32]