Dome Encoder Zeropoints

The dome encoders tend to come loose and are checked daily by Duty Technicians.

1. Dome (AZOFF). Should be monitored daily by setting dome to known azimuth (AP position on INT) and check against information display value. The easiest way to check for vignetting when on the sky is to observe a grossly out of focus bright star.
2. Shutters (TOPOFF and BOTOFF). As above, use out-of-focus star to check for and to define onset of vignetting and thus compare with telescope zenith distance (ZD). The dome shutter encoders seem to give sensible answers now. The shutters are usually positioned at their extreme positions, where the precise encoder values are not very relevant anyway.