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There were two faults causing 2 hours or more downtime during period 50.
The software routine to center a star on the fiber, and keep it on the fiber had a problem with one of the fibers. For an unknown reason the fiducial position for that fiber was very far off, outside the normal fiber of the fiber head. This caused the target to not be properly center, and let observers believe the target was in the fiber as no star was see outside the fiber. Beyond that the observers did not follow the instructions, the system also allowed unreasonable fiducial positions. The software has been upgrade to detect large deviations and warn the observers, pointing to the proper procedure to use. Also the instructions in the manual were updated to clarify this point.
During the night, with the telescope tracking on the sky, the telescope suddenly crashed giving an alarm about the rotator speed. It was soon established that the cause was a problem with the rotator motors. After a detailed investigations it was determined that the problem must be with one of the motors itself, not the electronics running the motors. However, to get to the motors, the whole rotator needs to be taken off which takes many hours, and it was decided to abandon for the night. The whole next day was used to dismount the rotator and dismantle the motor, where it was found that the motor brushes had failed as suspected. The brushes were replaced with a spare set, and the rotator reassembled, just in time for night time observations.
Thomas Augusteijn 2016-05-05