Problem(s) Encountered:

The telescope focus slipped over time. For almost all of the night we were pointed at the same target, and the focus value changed from our best initial value of 6350 to either lower or higher values, with a change of about 20 units per half hour. We used the focus button on the hand paddle to correct it every so often. When we slewed back to zenith at the end of the night, the focus changed by 40 units.

Solutions:

The focus slip is normal behavior -- it always creeps a little when the telescope is moved.  The units are microns, so these are small changes.

Even if the focus number doesn''t move -- in which case the mirror assembly isn''t moving with respect to its mount -- the length of the truss varies quite a bit with temperature, so you need to refocus with any significant temperature change.  Typically this means chasing the focus for the first few hours of the night as the truss cools. [Thorstensen]