Report to the JCMT Board; October 2005

All-sky pointing
All-sky pointing has remained good throughout the period, despite the eventual retirement of SCUBA in July. For the first time in some years the recovery of the pointing model following an adjustment to the load on the central bearing was accomplished using a receiver other than SCUBA. Stability (rms) of pointing - now only for the heterodyne receivers - is still well-described as being somewhat better than 1.5" in azimuth and about 1.5" in elevation; in stable weather, of course.

There was concern towards the end of the period about pointing excursions of about 10" that were tracked down to being due to the failure of the new SMU controller to effect complete SMU table updates. The concern was that science observations might suffer similarly. The problem was successfully solved by forcing the update in advance of the next (or pointing) observation, but a cleaner solution will be in place when the new SMU controller meets the new telescope control system following the '06 shutdown. The generally good pointing accuracy permits rapid detection of problems elsewhere in the system.

Inclinometry
Measurements of the antenna track profile are made every couple of weeks or so, and have shown only minor changes during the past 6 months. The track profile is a dynamic quantity depending as much upon the antenna load as upon the (unloaded) form of the track. The difference between the profile measured during clockwise motion of the antenna and that measured during counter-clockwise motion features in the construction of our hybrid CW/CCW models. These ensure that accurate pointing corrections are made for targets transitting both in the south (travelling CW) and north (CCW). The CW-CCW difference is measured less frequently - maybe twice per year - but results obtained in September confirm a remarkable stability over the years.

Transit step and central bearing race defect
Characterization of both these effects demands the highest possible resolution in azimuth and time, previously provided best by SCUBA in jiggle-map mode than by the heterodyne receivers in continuum-pointing mode. As a result the 'transit step' phenomenon remained unchecked during the reporting period, although there is no seredipitous evidence to suggest that its nature has changed from being "of negligible amplitude" as reported last time.

On the other hand, the central bearing race defect was checked in July using RxB and was found to be unchanged, with an amplitude at the horizon of 3".

Temperature corrections
Algorithms that correct the pointing and focus for changes in the ambient or structural temperatures are checked each month and have required no updates during the reporting period.