Problem(s) Encountered:

At the end of a 10-minute exposure the image looked like a bias frame -- noise and structure visible but no obvious cosmic rays.  I thought the shutter might have failed, but that didn''t explain the absence of CRs.

Solutions:

I powered down the Leach boards; the first image that way was similarly blank, but then it started working and I had no trouble with it the rest of the night. (Thorstensen)