IRC+10216 was tracked (using the'map16' technique) for 3 hours between 06:19 HST and 09:23. It traversed the sky from (az,el) = (100,64) through transit at 08:06 HST at an elevation of 83.5^o, ending at (az,el) = (253,71). Plots of the residuals against azimuth are shown below :

The azimuth tracking performance is good, with rms errors of 1.1". The elevation residuals show systematic structures of several arcseconds that cannot be significantly removed by adjusting any of the 7 parameters of the pointing model. Nor indeed does including the additional parameters u3 & u4 (for TMU orientation) help in any way.

The raw elevation residuals of 2.3" are substantially worse than those routinely seen in allsky pointing data (usually it's the other way around !) - suggesting, first, poor atmospheric conditions. We shall return to this theme later. The rms value can be reduced to 1.8" by correcting for the variation with T_air (a 10-sigma determination : del = (1.0 + 0.1) * T_air), and subsequent optimization of the 6 pointing parameters (excluding the flexure) yields rms's for (daz,del) of (1.0", 1.6"), although the ripple through transit persists, albeit at a lower level :

However, the behaviour of the elevation residuals seem qualitatively different from (worse than) that seen in lower elevation (58^o) data of Uranus taken on 23 Aug and that seen on IRC+10216 last time (14 May). Any resemblance to the 'transit error' must be superficial; the expected amplitude at these elevations is minimal. Put another way: if this is the 'transit error' then the pointing errors at lower elevations should be enormous - and we don't see that in the general run of things.

With azimuth tracking seemingly nominal one must consider always the conditions that affect only elevation, and this often means the weather in its many guises. An uncorrected temperature effect has already been described, but there are also the general sky condition and anomalous refraction issues to worry about; these data were taken with tau_cso = 0.2 or so, although with a sky clear of visible cloud.

Conclusion
The tracking behaviour in elevation may be erratic, but the major systematic form of the error is not totally correctible by adjustment of the pointing model. An uncorrected thermal effect in these latest data exists but is insufficient to account for the poorer performance, and it remains possible that other atmospheric effects are at play. There is no justification on the basis of these data alone for changing the pointing model without also incorporating a correction to the elevation pointing for T_air - and that has not been consistently supported historically by allsky pointing data.

If the weather and schedule allow, I shall (hereby) request

• a further repeat with SCUBA on IRC+10216 in better conditions,
• a repeat of the experiment using RxA - just to eliminate any possible Nasmyth-specific geometric effects, and
• a complimentary experiment through the north (eg SCUBA tracking of BLLac in the evening)

24 Oct 2003
The tracking of IRC+10216 was repeated using RxA :

The data is a lot rattier all round. While the elevation residuals again possibly show a trend from -ve to +ve through transit, their behaviour elsewhere does not well match that of the earlier SCUBA data. And the azimuth residuals are considerably strange and systematic with a distinct +ve peak at transit : some +5" more positive than the residuals at the start and end of the track. These systematic trends are, as above, not convincingly removed by adjustment of the pointing model.

Contemplation is on-going.