Uranus was tracked using SCUBA and the map16 method. On UT20041220, tracking started at HST 19:14 at (az,el) = (232,42) and ended at (246,26) at HST 20:33. The pointing residuals are shown below:

The formal rms of the residuals in elevation is an impressive 0.5", while that in azimuth is 1.3", dominated by the steady decrease throughout the dataset.

The defect was expected at (az,el) = (240,35), but is not seen at levels greater than 0.5". In retrospect I realize that the instructions for the experiment requested the disabling of the transit track corrections rather than the 'empirical' file that corrects for the race defect - so the correction is still in effect here. The lack of obvious problems at az=241 imply the correction is working OK to the level described by the residuals.

The experiment really needs repeating with the correct setup, but for now we can accept that the defect amplitude is unchanged from the current value of 3.0" at the horizon.

On UT20041221, tracking started at HST 19:06 at (az,el) = (231,43) and ended at (246,26) at HST 20:30. The pointing residuals are shown below:

The formal rms of the residuals in elevation is again an impressive 0.6", while that in azimuth is 1.2", again, one might argue, dominated by a steady decrease throughout the dataset.

The defect manifests itself at many azimuths separated by 16.4degs or so, and was expected to be seen in the azimuth residuals of this dataset at (az,el) = (240,35). The plot shows (in green) the current empirical correction - constructed using an amplitude at the horizon of 3" - superimposed on the data. Despite the baseline offsets on either side of the defect there is no apparent need to adjust the empirical correction at this time.