SUMMARY
A further test was carried out of the 's-correction' - the lookup table that corrects the (4" amplitude) elevation pointing step across transit.
The results possibly show a residual 2" step, but symmetric about azimuth 185. Alternately, the data show only noise of rms errors 0.7" in each coordinate.
All-sky pointing show the current model performing satisfactorily, although some improvement may result from the suggested parameter adjustments.

Transit Tracking
The 'transit' problem was noticed first in April . A correction was installed on 16 May, but recent checks in May, June and July have been inconclusive for various reasons.

This latest test was done during conditions of poor transparency (tau_cso=0.24) but stable seeing. Uranus was tracked for 2 hours, using the SCUBA map16 method, with the EMPIRICAL correction activated :

Transit occurred at elevation 55 degrees, and the seeing throughout was 0.2". The raw residuals are 0.7" in each coordinate. There is a possible 2" step in del across 'transit', but the axis of symmetry is actually at azimuth 185 degrees. Similar 'transit-asymmetric' residuals were noted in the previous Uranus runs and, if real, must be considered a function of either the telescope or the correction. Alternately, the 'step' is just part of the noise, despite the low formal seeing value : note, for instance, the correlated excursions in both daz and del at about azimuth 174.

The situation remains somewhat unclear due to the possible atmospheric components to all the recent tests, but I would place more weight on these data than on the last lot, and the residual errors appear at least to be of the order of +1".

It had been my intention to do this experiment with the empirical correction disabled - but I forgot : next time . . . maybe with conventional weather.

All-sky pointing
38 data were collected before and after the above transit experiment. The raw residuals are shown below :

The performance is nominal, although running these data through the FIT9 routine allows adjustments that may improve the rms scatters in (daz,del) to (1.2",1.3"). These adjustments were made at 13:00 HST 17 July 2000, together with the regular update to the (UT1-UTC) correction. For the record - this is the 500^th version of radio_telescope.dat.