Report to the JCMT Board - April 1999

Central bearing defect
The empirical correction for the central bearing problem remains in effect - but hopefully not for much longer, with the replacement of the central bearing scheduled for May 1999. Monitoring of the defect has continued, and some variations have been seen and accomodated. Errors have not been allowed to exceed a couple of arcseconds at the horizon - the effect in other parts of the sky varying as cosine(elevation).

Inclinometry 'spikes'
The variability reported last time had become worrisome, and prompted an intensive effort to understand its cause. This culminated, around the end of the year, in the theory that the front wheel loadings are sufficiently small, and the thermal effects sufficiently large, that under certain thermal conditions, and particularly at track joints - where the geometry of the antenna and track produce maximum leverage - the loads on the front wheels reduce to zero. The wheels then do not follow the track profile. Differences between inclinometry runs taken under different thermal conditions then show spikes of several arcseconds amplitude at the track joints. This was our primary diagnostic. Of more concern to the observer, however, was that the antenna azimuth pointing would be then incorrectly adjusted. Errors on the sky again vary with cosine(elevation).

The chosen solution, to load the front wheels with 3 additional tons, is being implemented.

Pointing accuracy summary
For reasons unknown, the inclinometry 'spikes' abruptly disappeared on or about 14 Nov 1998. Dedicated pointing runs on 15 Dec 1998 and 20 Jan 1999 confirmed that the pointing accuracy is currently best described by rms errors in each coordinate of (1.5",1.5"). There are additional possible azimuth errors due to the central bearing problem and, prior to 14 Nov 1998, to the 'spikes' problem, as described above.

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