Problem(s) Encountered:

The guider got stuck early in the night and would not move at all along the y-axis. It was also very far off position wise in the x-axis. Eric attempted to fix the guider remotely but was unsuccessful. I lost about an hour and half of observing time trying to fix the guider with Eric.

Using some guess and check work I was able to find guide stars that were along wherever the guider was stuck and was able to successfully guide for the remainder of the night. Guiding was not as good as previous nights but given the circumstances I was still able to observe.

Solutions:

This one was not fun.  Verified by a very ugly noise that something was wrong with y-stage of the MIS.  Pulled OSMOS and the bottom plate of the MIS.  Found that the end cap on the thread that the y-stage travels on had come off, dropping 2 of the 3 bolts used to hold it in place.  Amazingly, the tiny set bearing was still there, and thankfully so.  Subsequently opened up OSMOS and found the two screws that had fallen off inside the instrument.  Put everything back together in the MIS on the y-stage but found that the stage still seemed bound.  Ended up taking both end caps off, as well as the stage encoder and motor.  At this point we were verified that the two bearings (one on each end) were good, they were.  I started to fear something was amiss with the half-nut.  Working gingerly but firmly, Tony managed to finally get the stage to rotate freely again, verifying that the half-nut was still good.  But what caused the stage to so badly seize in the first place?  Looking more closely, the limit switch for the zero-position of the y-stage had failed.  Interestingly, this is the same limit switch to have failed on the 1.3m not so long ago.  The stage then drove itself until it could go no further.  At this point, the motor shook everything in there good enough to loosen the screws holding on the end cap.  Replaced the limit switch (I really need to buy a couple more spares!) and put the whole y-stage assembly back together.  Tested all and it works.  Buttoned up the MIS, re-installed OSMOS and I’m out of here, only 1.5 hours late.

Incidentally, while everything was pulled apart, I went ahead and replaced the y-stage drive belt.  Might as well.