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Below are listed those faults for which 2 or more hours were lost.
The power supply unit of the azimuth drive of the telescope failed intermittently, often working for extended periods before failing again. This made it very hard to find the cause as several attempts at solving the problem actually resulted in the telescope working again for more than an hour before failing again. In the end, replacing the power supply unit solved the problem and it was noted that there was a poorly soldered diode in the unit which might have caused the intermittent failure. Also, as a result of the problems the hydraulic pads of the telescope had become displaced and had to be realigned.
Successive crashes occurred in which the telescope started to oscillate before failing. This also caused one of the hydraulic pads of the telescope to become displaced which had to be realigned. The main obstacle in solving the problem was that error messages given by the PLC control unit were not very specific. The PLC code was modified after which it was found that the error was due to one of the azimuth power amplifiers. The amplifier was replace, and we are currently looking for a spare.
The data acquisition system failed because for a still unknown reason the fuse of the computer rack was tripped. This brought down part of the data acquisition system. Several problems in bring the system back up caused the relatively large amount of downtime. We are planning to change the system such that each computer has its own fuse and one tripped fuse does not bring down the whole system.
Thomas Augusteijn 2006-02-02