Hi Tom,

Yes, we went from a very tail sun attitude, 171 pitch (cold), to a
forward sun attitude, 62 pitch (warm/hot). Also note, having both IRU's
on may be exaggerating the OBA flexing. I hope that the thermal profile
will return to normal after the IRU swap is completed and we have only 1
IRU on.

Rino

-----Original Message-----
From: Tom Aldcroft [mailto:aldcroft@head.cfa.cfa.harvard.edu]
Sent: Friday, June 27, 2003 4:21 PM
To: rgiordano@head.cfa.cfa.harvard.edu
Cc: dshropshire@head.cfa.cfa.harvard.edu; Dan Schwartz;
aspect_help@head.cfa.cfa.harvard.edu
Subject: Large telescope drift during science observation (obsid 3964)


Hi Rino,

Our aspect V&V folks brought to my attention a case where the apparentfid 
light positions drifted by about 4 arcsec during the course of a long HRC 
grating observation (obsid 3964 at 2003:175:07:20:16.34).  This is 
presumably due to thermal flexing of the optical bench.  Typically the
fid drift is less than about 2 arcsec.  

Was the thermal profile change from the previous attitude particularly bad
in this case?  Should we be expecting larger thermal flexure in general?

For ground aspect, this degree of drift is somewhat of a concern because
we begin to see larger fid light centroiding errors if their apparent
positions are drifting.

I've put a plot on the aspect web site which shows one fid light position
(red curve and red axis labels on right side) versus time (ksec).  The
black curve gives an indication of centroiding error, which in this is as
large as 0.1 arcsec.

http://asc.harvard.edu/mta/ASPECT/obs3964_drift/drift.gif

-Tom