Report to the JCMT Board - April 2005

All-sky pointing
All-sky pointing has remained good throughout the period, despite the frequent unavailability of SCUBA, which is our FrontEnd of choice for checking or generating the pointing model. Stability of pointing is still well-described as being 1.5" rms in each coordinate during stable parts of the night.

A suspicion raised in March that there is a systematic (elevation-dependent) difference in the azimuth pointing between SCUBA (on the Nasmyth platform) and the heterodyne receivers (in the receiver cabin) will be pursued once we have SCUBA back. Note, however, that frequent, local pointing will mitigate this - indeed, any - systematic problem with the pointing model and ancilliary corrective algorithms, and so rigourous operational practices have maintained the advertised performance described above.

No faults were filed during the period concerning poor pointing.

Inclinometry
Measurements of the antenna track profile are made every week or so, and have shown only minor, gradual changes during the past 6 months: a model from September 2004 would propagate pointing errors of +2 arcsecs in some directions if used today.

Transit step and central bearing race defect
The 'transit step' phenomenon remains of negligible amplitude. The central bearing race defect had remained unchanged for 6 months when last measured in December 2004. A further check awaits the return of SCUBA; the faster rate of pointing data acquisition with SCUBA provides the necessary azimuthal resolution. Monitoring of both tracking problems requires the occasional 2-hour window of grade 3 observing time.

Temperature corrections
Algorithms that correct the pointing and focus for changes in the ambient or structural temperatures are checked each month and have required only occasional minor 'tweaking' during the reporting period.

Source Catalog
The brightnesses of blazars - used as SCUBA pointing sources - are updated each month, if they have been observed ! - and the catalog was supplemented at the end of 2004 by an additional 19 blazars with F(850um)>0.3Jy extracted from an observing program by I.Browne.
An algorithm has been drafted that will calculate the optimal integration time for a POINTING, given the source brightness, airmass and atmospheric opacity; the current default of n_integrations=2 (72 seconds) is not always useful. Implementation during observing has yet to be scheduled.

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