Problem(s) Encountered:

When we were testing spectrograph flexure at high hour angle (~5 hours) we heard a noise and noticed that the rotator angle had changed to ~84 deg. We brought the telescope back to Zenith and reset the rotator angle to zero.

Solution:

The rotator is a friction-drive system.  Due to the extremely heavy nature of the instrument, the spring plate did not maintain enough friction to keep the rotator from turning.  This has only happened once previously, with OSMOS—another heavy instrument.  Spring plate was slightly tightened.