Guiding system

The problem with the existing X-Y carriage guide-probe system is that the TCS occasionally looses count of the turns and the current method to reduce this error is to run the guide-probe slow, resulting in larger than necessary overheads during observing. A design for installing some simple electronics to take care of the guide probe X/Y turn counting has been made. This would eliminate the risk of loosing turns and thus loosing the guide probe correct position with 7 arcsec per turn of an axis. The speed of the guide probe can then be doubled and things like dithering used a lot in NOTCam observations can be speeded up. In addition some new amplifiers were identified that could replace the existing 25 year old devices. In parallel a spare amplifier will be build with the old design using similarly old spares.

In relation to the system possibly selecting a wrong guide star that happens to be in the field-of-view of the guide camera, it was noted that in principle the offset made by the system when detecting and centering the guide star should be of a similar size; the (variation in) offset being determined by the offset between the pointing model and the center of the guide camera, the RMS of the pointing model and the inaccuracies in the transformation of the guide probe X,Y position to RA & Dec. This implies that a sanity check can be made on the offset that is done when acquiring a guide star after a preset of the telescope, where a warning is given if an offset falls outside the expected range. As a first step for this the offsets that are made are being recorded. On the basis of the limited amount of data obtained up to now a relatively conservative upper limit of 50 arcsec has been set to any offset where a warning is given if the offset is larger.

A fairly fundamental part of current operations with the telescope is the use of the guide star server speeding up the process of acquiring a guide star as part of pointing the telescope. We had some problems with the hard disk which contains the catalogue used by the guide star server and we want to get a working backup disk with the catalogue such that the system can switch-over quickly (and automatically) in case of problems. In relation to this, the idea is to move the guide star server to its own dedicated computer. This can be an inexpensive basic machine where a second clone can be made that can serve as a complete back-up for the system.