Telescope drive

Again we had a case of some jumps of the telescope which affected the data. A more indepth investigation showed that occasionally relatively high tracking errors were seen in azimuth for specific positions of the telescope. Many things were tried, including cleaning the encoders of the azimuth motor, cleaning of the tachometers, checking the hydrostatic pads of the telescope which were slightly adjusted, changing the power amplifiers of the azimuth motors and replacing the brushes in both motors. No more problems were reported with significant excursions of the guide star or the presence of elongated star images which we believe was solved by changing the brushes, but the occasionally relatively high tracking errors are still present. The reported errors correspond to the difference in where the TCS thinks the telescope is and where it thinks it should be, and they typically do not get any larger than 0.1-0.2 arcsec. A detailed check of imaging data does not show any affect on the image quality, where in particular no difference is found in the data at the specific telescope positions were the increased noise is reported.

In the end it was found that the increased noise was directly related to the difference in temperature between the outside and the bottom of the telescope structure. The new cooling system was installed with thermal isolation around the tube where the cooling water goes through the center of the telescope fork where also the azimuth encoder is located. So, the encoder ends up being kept at the low temperature of the cooling water, very different from the outer temperature. By rising the cooling water temperature and later removing the isolation where the encoder is placed, the temperature difference was reduced and the jumps were reduced drastically. This probably could be improved further but we should wait for next summer with higher outside temperatures to be able to test this.