Report to the JCMT Board - September 2004

Inclinometry
Measurements of the antenna track profile are made every week or so, and have shown no substantial changes during the past 6 months. The use of hybrid (ClockWise & CounterCW) track models equalizes pointing quality in the north and south.

All-sky pointing
All-sky pointing has remained good throughout the period, in terms of both collimations and rms scatters. Values for the latter are 1.5" or better in each coordinate during stable parts of the night. The pointing model of 10 March 2004 has been used throughout to good effect; a routine update was always going to have to wait for the return to service of SCUBA, although there is still no obvious need. No faults were filed during the period concerning poor pointing; the coordinates of one source were found to be incorrect in May and were corrected, while the addition of new sources may result from recent blazar- and UCHII- science programs.

Transit step and central bearing race defect
The 'transit step' phenomenon (described last time) remains both a mystery and of negligible amplitude, while in May a defect in the central bearing race was detected, similar to that seen in 1997. Its amplitude is 6" at the horizon, decreasing as cos(elevation), but it, too, is being monitored regularly and corrected in software. There is no engineering plan to replace the bearing unless this software fix becomes unsatisfactory or the defect becomes gross. Monitoring of both tracking problems requires the occasional 2-hour window of grade 3 observing time.

Temperature corrections
The temperature of the antenna is used to correct both the elevation pointing and the Z-focus of the antenna. Following the upgrade of the enviromental task in June it was clear that the temperature corrections were not being applied properly, and some daywork was expended to track down the misassignment of temperature data channels. The opportunity was also taken to re-calibrate the temperature probes. The work was successfully concluded in August and elevation pointing quality was restored. It should be emphasized that the impact upon science was negligible throughout, since local, timely, relative pointing will always correct such systematics. However, the data taken in the new regime, and since the return to service of SCUBA, show a (small) difference between the thermal corrections needed for SCUBA and those for the heterodyne (cabin) receivers. This is still under investigation, but may need a new software parameter to resolve.

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