Report to the JCMT Board - October 2000

The track profile, as measured by inclinometry, remains stable. Data taken before and after the central bearing load adjustment on 06 June showed not-unexpected track-segment-related changes of amplitude 2 arcseconds, but before and after such events the measured changes are dominated by the overall movement (settling) of the mean track plane.

The transit tracking problem reported last time persists. A lookup table has been incorporated into the pointing model to correct empirically for the observed 4" elevation pointing shift across transit, and much E&C time allocated to 'pointing' has been spent collecting data to analyse the efficacy of this method. Surprisingly, we found that the effect is not symmetric about azimuths 180 and 360 (the azimuths of the meridian) but at points about 3 degrees westward, with some hint of variation with time. (The lookup table currently assumes a 3 degrees westward offset). With each transit experiment requiring at least 2 hours of telescope time, it has been difficult to monitor this possible temporal change, or to confirm or deny that the offset varies with elevation, for instance.

A new telescope control software system was installed on 24 August, and with it the pointing is now modelled using TPOINT. Generally, the accuracy of pointing remains at the 1.5" rms level, but we are still experiencing some teething troubles in adjusting the collimations of each instrument and so pointing has shown inadequacies recently, particularly at elevations above 80 degrees.

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