In our current set-up for the guiding camera, the box in which the guide star is place is always at the center of the camera. This has 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 star and the background is reduced, which can be a problem when the sky background is high (during twilight). To avoid this the idea is to change the position of the star box regularly to avoid affecting specific areas on the camera too much. The plan is to change (on a daily basis) the default position of the ``star box'' in a random way over an area close to the center of the camera. By displacing the camera probe which holds the camera by the same amount in the opposite direction the star box will, to high precision, point to the same area on the sky so this should not affect the resulting pointing. By making the positioning of the star box and the correction in the position of the guide probe an integrated part of the TCS, the effect of changing the star box position will be complete transparent to outside users and none of the observing system software or observing scripts would need to be changed.
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 a hardware solution has been designed to do the turn counting. A printed circuit board has been produced and assembled and is currently been tested.
Thomas Augusteijn 2013-05-10