Guiding system

Two upgrades have been implemented in the guiding system. When guiding during cloudy weather (especially if there is some Moon light) it sometimes happens the system guides on bright features in the sky which are not real stars, and it can drift away from the proper position without noting there is no a real star. If the system detects that there is a longer-term drift in the position of the estimated guide star position it now will provide a warning. The box in which the guide star is placed was always at the center of the camera, which caused this specific part of the camera to become less sensitive due to the star image ``burning in''. This is an issue as the contrast between the guide star and the background is reduced, which can be a problem when the sky background is high (during twilight). To avoid this area a new feature is in use where at a preset the star box is placed on a circle with about 5 arcsec radius around the center. This circle has 100 positions, updated one step at every new observing run. The guide probe is counter moved so the pointing remains the same as if the star box is at center. Also the background box is now placed at an optimal position to give the best possible contrast.

As reported earlier, the TCS occasionally looses count of the number of turns the guide-probe motors make and to minimise this problem the speed of the probe is limited. To not overload the TCS with this task, originally a hardware solution had been designed to do the turn counting. In the end it was decided to replace the encoders themselves with encoders that have the same mechanical properties as the existing ones, but that incorporate the multi-turn counting facility. These new encoders have been ordered. A modification to the TCS will be needed to read and control the extra turn information, but it will be possible to operate the new encoders in exactly the same way the existing ones are used now.

With the new encoders, guide probe speed can be considerably increased and the TCS system load be decreased by lowering the execution speed for the adapter program. Now it runs at highest frequency just for the guide probe turn counting.

Thomas Augusteijn 2013-12-02