Tracking

In the past few months there were a few, but increasing number of reports about elongated stars in imaging data. This was investigated in detail and it is was found to be a problem with the corrections for the periodic errors in the drive system of the telescope. Specifically, the periodic error in altitude varies as a function of altitude which is corrected with the help of a look-up table. This table is constantly updated as a function of the autoguider corrections applied at a given position of the telescope. It turned out that the values in this look-up table showed some relatively large, sudden variations in altitude which effectively caused the telescope to make occasional jumps while tracking, which mostly effected data when the telescope was predominantly tracking in altitude. Switching off this look-up table has corrected this, and normal round star images are now obtained independent of the position of the telescope. However, it does mean that the periodic errors in the telescope drive are now not properly corrected which means that even for relatively short exposure times one needs to use the autoguider. It is also not really clear why the data in the look-up table contained such deviating values (which were apparently increasing in number with time) and we need to define a proper way to include these corrections again.