Secondary top unit

Since the aluminisation last year we have had problems with the secondary displacement mechanism which compensates for the bending of the telescope with altitude. Extensive work was done on the secondary unit, mostly to restore it to the state it was in before aluminisation. It was found that friction between metal parts had caused the positioning problems. The mechanism was properly re-positioned to the original specifications. There is still some residual friction, and the position errors in the mechanism are still higher than before, but it is now at a level that should not effect the image quality.

Unfortunately during this work the encoder wires for part of the tilt mechanism used to align the secondary mirror were damaged. A software fix has been implemented to get around the lack of servo control of this mechanism which provides a satisfactory solution. The next time the secondary will be dismounted the encoder wiring can be repaired.