SUMMARY
Transit tracking data through the north show a similar 4" step to those taken through transit in the south, with a similar displacement of the event towards the west.

The 'transit' problem was noticed first in April . A correction was installed on 16 May, but recent checks in May, June, twice in July, in August, and again in September with RxA, have shown an apparent change in location of the event to a point between zero and 5 degrees of azimuth towards the west. This attempt to provide matching data in the north used SCUBA in its map16 mode, tracking the compact HII region w75n from azimuth 380, through transit at azimuth 360 towards a scheduled end at azimuth 340 degrees. Unfortunately, an electrical fault towards the end caused a premature termination but the data up to that point were recovered successfully. The data cover the (az,el) domain from = (381,65) at 20:14, through transit at an elevation of 67 degrees, to (345,66) at 21:35 HST.

• The seeing during the observation was reported as 3.7", but the monitor was not working properly at that time. From the relatively featureless parts of the data one might estimate the seeing to have been no worse than 1".

• The ~+4" step in the elevation pointing residuals is clearly seen (recall that time runs right to left through the diagram above), although the behaviour beyond azimuth 350 hints at further structure previously unsuspected.

• Re-reading my own instructions for acquiring these data, I see that I make no mention of the status of the s-correction. The data suggest that it was not enabled at the time, but I shall attempt to confirm this. (I shall also clarify the instructions before any further data are taken).

• The data are not symmetric about azimuth 360 but about azimuth 357, a displacement to the west essentially equal to that seen most recently in southern transit tracking data.

• The structure in the azimuth pointing residuals at azimuths 373-376 (13-16) coincides with a cluster of wheel/track joint interactions at azimuths 13, 15, 16. We note that inclinometry has not been taken since 31 July, and not since the installation of the new TCS.

• The possible structure in the elevation residuals beyond azimuth 350 means that a simple reconstruction of the s-correction is not yet possible : the current version comprises purely the 4" steps at azimuths 180 and 360. Larger datasets, covering larger azimuth domains, seem necessary. A complicating issue is that the TPOINT pointing model within the new TCS is still iterating towards a stable solution as more data becomes available and large scale error waves from an imperfect pointing model may also permeate such larger datasets.

• new inclinometry in order to generate an updated track model
• new allsky pointing in order to generate a new TPOINT pointing model, and
• new transit tracking data in the north and south, covering large azimuth domains.