Guide star acquisition

Some modifications and improvements were made to the guide star acquisition. In the TCS a permanent offset can be adopted in the pointing (e.g., used when one wants targets to fall on a specific area of the detector) but this offset was not taken in to account when querying the guide star server. This resulted in the guide probe position not being corrected for the offset and depending on the size of the offset the guide star could fall outside of the field-of-view of the guide camera. The permanent offset is now incorporated in the query to the guide star server. A different way of applying (small) permanent pointing offsets is changing the position of the guide-star box in the field-of-view of the guide camera. Originally, the automatic guide star detection and centering system system has at start-up the guide-star box in the center of the field-of-view and at each new preset to a target the star box was recenter. This latter part of the procedure has been eliminated and any change in the position of the star box effectively provides an offset in the pointing.

One of the most stringent limitations on areas available for guide stars is that defined for beam-switching observations with NOTCam which require offsets larger than the field-of-view of the camera (4$\times$4 arcmin for the wide-field camera). To improve this, separate areas were defined for each direction where a bigger area can be used in each separate case. The different options still need to be verified in detail, while we are also looking to further optimize the areas available for guide star selection for different observing modes.

However, also in general it still is possible that no guide star is found. Possible ways to still find a guide star would be to change the target coordinates slightly, use a different field-rotation than the standard one, etc. However, the only way of checking this was by actually changing these values at the telescope. The online guide star search interface was modified to allow for user defined telescope position and instrument field as well as the current pointing. In this way availability of guide stars can be checked in advance, or quickly on the fly at the telescope.

To protect the guide camera very bright stars ($<$9 mag) are excluded from the search for guide stars, while for stars brighter than 11.7 mag a grey filter is put in the beam. It has been noted that for the faintest stars in this range the guide star can often only barely be seen and might not be detected by the system. Basically, the change from using the grey to not using it is to steep and a scheme will be implemented where an intermediately attenuating filter will be used for the faintest `bright' stars.

A different issue is that sometimes stars are selected that are near to stars that are considered too bright. When the automatic star detection and centering system is used this will lead to the wrong star to be selected and centered in the star box, with a corresponding off-set in the pointing. The system will be changed such that no guide star is selected near (closer that the size of the guide camera's field of view) to a star that is excluded because of its brightness.

Thomas Augusteijn 2010-02-09