Report to the JCMT Board - October 1997

An empirical (software) correction for the central bearing problem, itemized in the previous report, has been in place since March 1997. Every 2-4 weeks since then a specific test of the algorithm has been made by tracking a bright source through one of the azimuths particularly affected. 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.

The date of first occurrence of the problem, however, is still unknown. One concientious observer has submitted evidence that it may have been present as early as July 1996. Limitations on the acquisition of pointing data in that period were addressed in the last report, although it is clear that problems that affect small parts of the azimuth-elevation plane might escape detection by our usual `all-sky' pointing checks. The aforementioned tracking experiments will become regular features of the Pointing project henceforth.

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 1-2" can occur due to temperature changes of only a few degrees. These changes are as yet unpredictable.

SCUBA has been the primary instrument for obtaining pointing data in this period, and its sensitivity and speed have proven most useful.

Thermal effects upon the elevation pointing became obvious as a result of daytime openings of the building, often noticeable during, but not exclusively caused by, daytime observing of comet Hale-Bopp. A simple correction to nighttime pointing, based upon the mean temperature of the antenna, was sometimes possible, although the problem was clearly more complicated, possibly being a function of the specific recent thermal history of the antenna.

Aside from the elevation pointing drifts sometimes seen during such periods, pointing errors in this period have typically been (1.5",1.5") rms in (azimuth, elevation).

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