Summary
Systematic pointing errors were reported over the weekend, especially in the way the azimuth pointing residual varied as a function of elevation. The pointing data recorded were of sufficent number, distribution and quality to allow the model to be substantially updated for the first time since the installation of the new TCS.

Data were obtained as follows :

      Dataset      N        mean uaz +-     mean uel +-
 
     pt001118_15  47           3.7  2.5      0.0  2.4
     pt001119_14  41           3.2  2.9     -1.2  1.9      

The datasets were pared of those data taken before the formal start of first shift (17:30 HST) and after the formal end of 2nd shift (09:30), and were combined, leaving 67 data points. The data showed a particularly strong trend of the azimuth residual -vs- elevation (or zenith distance) as was reported by the observing team. This trend is shown below left :

(Click on image for better view)

After allowing all 7 of the pointing model parameters to float to new, optimal values the same data behave as shown above right; the systematic trend has been removed, and the residuals behave 'normally'.

Vector RMS residuals (ie the quadratic sum of the residuals in azimuth and elevation) are expected to be 3.0" with this new model. If equally allocated, this would correspond to errors of 2.1" in each coordinate. This is not yet as good as we have come to expect, but a dedicated pointing run in the future will hope fully improve upon this.

The new model was installed at 11:30 HST on Tuesday 21 November, 2000.