MDM Trouble Report for 2006 Aug 31

Telescope: 2.4m
Observer(s): Thorstensen (Dartmouth)
Instrument: modspec + echelle

Problem(s) Encountered:
As I put the telescope to bed I noticed a very strange behavior
that appears to be reproducible and which I bet is related to
some of the other glitchy issues I've been seeing this run.

My usual procedure for stowing the telescope is to auto-slew to
a point a minute east of the zenith, let the telescope track in,
and turn off the tracking at the zenith, then turn off the drives.

Last night my last target (slit flat actually) was 1h w and
at +31 degrees.  I tried to execute this procedure but the 
telescope stopped setting about a minute west, and then started
crawling E at a very slow rate.  So I hit the xtcs stop,
used the paddle set/east buttons to manually move just E
of the meridian, tracked to HA=0, and turned off the tracking
and then the drives.  

But when I turned off the drives, the telescope jumped about
40 sec of time west!  I played with it a little, and it turns
out that if you manually set eastward for a while, then turn 
off the tracking and the drives, the telescope always jumps 
way to the west, 20 sec to almost a minute of time.  This 
isn't just the encoders, I see it in the tilt meters too, 
so it's a physical jump.  

I've used this telescope extensively for 21 years and I've 
never seen this before, so I believe it's new. Since
it's a weird behavior of the HA drive I suspect it's related 
to the other weird behaviors.  It's reproducible which means 
that it may be possible to diagnose (and maybe even fix).
Maybe it's a preload problem?

------------------------------

Submitted on 2006 Aug 31 [6:29:57]