Report to the JCMT Board - October 1999

The period was dominated by two events in May : the replacement of the defective central bearing, and the front loading of the antenna.

The c.b. defect caused a deflection in azimuth of as much as 10 arcseconds (* cos(elevation)) every 16.33 degrees, and was corrected using a software 'lookup table'. The replacement process is described elsewhere, but antenna pointing models no longer need or use this 'empirical correction'.

The combination of the amplitude of the track irregularities, the thermal properties of the antenna, particularly the differential temperature between the parts of the antenna frame above and below the observing floor, and the general balance of the antenna, occasionally produced conditions where the front wheels were totally unloaded. They would not then follow the track model, and consecutive inclinometry runs taken under different differential thermal conditions would appear quite different. The corrective action chosen was to load the front wheels with 3t of weights. The burnout of one carousel motor in July has restricted the frequency of inclinometry data acquisition to once per month or less, but inclinometry data have been very repeatable in this period.

Pointing accuracy summary
Nightly pointing data come mainly from SCUBA (55% of 2000 measures made in this period), with RxA generating 21%, RxB 17% and RxW 6%.
General pointing performance is still well represented by rms errors in each coordinate of (1.5",1.5").

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