The last tracking exercise did not help explain the small step-features seen in the pointing residuals at transit in earlier data for irc+10216. The absence of SCUBA in recent times has necessitated that this next repeat be tried with RxA. The transit step correction was left enabled :

Formal rms residuals through the 2.5 hours of the experiment are (0.8", 0.9") in (daz,del), respectively.

Confirming the features seen in the previous data of Feb 2004, the azimuth data show 4 'spikes' separated by approx 16.4 degrees. The behaviour is identical to that caused by a central bearing race defect last seen in 1997, with the pointing excursions occurring at precisely the same azimuths as before. At that time empirical corrections were introduced to correct the azimuth pointing, although a mechanical replacement of the bearing was eventually required. The pointing excursions seen here near transit are about +-2", but the expectation is that they will be larger at lower elevations; they reached about 10" in 1997. However, cursory examination of all 2004 pointing data so far show no systematic excursions anywhere near as large as this. Further tracking experiments will be requested to properly quantify the amplitude of the current effect.

Tracking experiments in the past year have concentrated on irc+10216 for reasons to do with the TCS tan error. But in tracking experiments done at sufficiently low elevations that the pointing excursion might be larger than a couple of arcseconds the effect is seen in data of 23 Aug 2003 at similar amplitude, but not on 02 June 2003 or 05 Apr 2003.

The elevation residuals show a general decrease in elevation prior to transit and an increase after transit : nothing to help tweak the current pointing model or transit correction.