1997 JCMT Pointing Report

A severe azimuth pointing effect was noticed in February, and was identified with a problem with a `hole' or pit in the central bearing race (analysis by Firmin Oliveira). An empirical (software) correction was installed in March and has been tested every 2-4 weeks by a tracking experiment. In general, the correction has proven valid and stable. Indeed, the lack of evolution of the problem may in itself be a further clue as to its nature. It is possible that the problem was present as early as July 1996. Engineering plans to replace the central bearing are in progress.

The welding of the track joints was completed on June 05. Since then, and particularly before and after a central bearing load adjustment in August, regular inclinometry measures of the track have kept that part of the pointing model updated. They do show, however, that changes to the track profile of order 4" can occur from one inclinometry run to another - possibly due to temperature changes. Regular inclinometry helps to maintain this part of the model.

SCUBA has been the primary instrument for obtaining pointing data in this period, and its sensitivity and speed have proven most useful. Pointing performance in this period is still fairly represented by rms errors in (azimuth, elevation) of (1.5",1.5").

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