Telescope drive

Several problems with the azimuth motion were encountered during the semester, some causing actual building crashes (caused by the telescope not moving) while others were seen as oscillations of the guide star image. These were caused by the brushes in the 2 azimuth motors having worn out which needed to be replaced on different occasions.

One of the main problem in diagnosing and solving these problem is that it is very hard to replace the brushes. A new system was made to `mount' the brushes which has been successfully used, but it still is difficult to get the brushes in to the motors and we are looking to see how this can be improved.

On the last occasion we had problems with the brushes it was discovered the observed azimuth oscillations were related to the actual position of the telescope and in fact occurred at 15 degree intervals in azimuth. This was noted as equal to the period of the motors been geared 24:1 to the telescope. It is not clear why we only got problems at certain azimuth angles, with an extent of $\sim$30 degrees of the azimuth motor rotations, though it was discovered than the previous problems also occurred at these positions. It is planned in the near future to open the motor and measure and examine the windings to see if anything obvious can be identified.

When trying to diagnose the problems with the azimuth motion the new alt/az amplifiers were re-installed. To fix the instability problems seen the last time they were used the gains have been reduced from 1.6 to 1.47 i.e. from just above the 1.5 previous amplifier gains to just below them. With NOTCam and ALFOSC mounted no problems were encountered but when SOFIN was on, oscillations sometimes occurred when powering on. After checking the drive it was found that there was an incorrect voltage level on the new enable signal going to the old PI (pre-amp) and pre-load boards. After this was changed to the right value no further problems were reported during the rest of the SOFIN run, or with any other subsequent instruments mounted.