MDM Trouble Report for 2012 Jul 03

Telescope: 1.3m
Observer(s): Gaidos (Hawaii)
Instrument: R4K

Problem(s) Encountered:
Not *really* trouble, but an observation which I was asked to record 
as a trouble report.  During a 2.5 hour defocused staring observation of 
a star with the R4K (DES z-band) the centroid of the defocused star image drifted by 13 pixels (4 ) in the y (N-S)
 direction despite the fact that the telescope was locked on a good guide star the entire
 time.  The change in airmass was from 1.75 to 1.1 during this time.
I assume this is due to flexure of the instrument in the image plane.
I verified that this was motion of the image center, not just the distribution
of light around the image.  The stellar image is near the center of the
R4K FOV.  

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Submitted on 2012 Jul 3 [12:22:26]