The tracking experiments of 19 Jan showed a small, but significant step at transit in the azimuth pointing. Those data were of irc+10216, which transits particularly high up (el*=83^o). This repeat (with the transit step correction correctly disabled) is of 3c279, which is still just about bright enough to give good S/N in each 18^s integration with SCUBA, and which transits at 64^o.

The data are less definitive than before, showing slow changes in the elevation residuals of -2" prior to transit and of +3" across transit, and merely a steady decline in azimuth residuals; most unhelpful in the context of the sharp steps observed last time.

The formal rms's in the data are as follows :

                        N     az     el
    SCUBA   all data  166    0.8    1.2
    SCUBA   az < 175   69    0.5    0.7
    SCUBA   az > 185   73    0.7    0.5

The data are clearly 'better' either side of transit than taken as a whole, but they barely reach the 0.4" rms standard set previously (on a brighter source, it must be said).

NB :

• The spikes in the azimuth residuals do not correspond to the locations of track joints.
• The elevation residuals do not behave like any putative transit step.

Conclusion
These data of 3c279 do not help progress the understanding of the pointing across transit : in particular, they do not confirm or explain the small, sharp step in azimuth residuals seen in data for irc+10216 which transits particularly high up. RMS pointing errors over transit may be tolerable at the level observed here (~1" rms in each coordinate over 2 hours), but - erring on the side of caution - observers should, as always, be advised to point as locally as possible to minimize any systematic problems.