The weather has been borderline grade 3/4 the past few nights and the following trackings of irc+10216 through transit using SCUBA and RxA have been obtained:

The SCUBA data (above left)show small but significant steps at transit in both coordinates : -1" in the azimuth residual, +2" in elevation. The RxA data (above right) confirm the elevation step but not the azimuth step. The formal rms's in the data are as follows :

                        N     az     el
     RxA    all data   52    0.8    1.2
    SCUBA   all data  148    0.6    1.0
    SCUBA   az < 175   58    0.4    0.4
    SCUBA   az > 185   83    0.4    0.8  

In the past the RxA performance would have been deemed most satisfactory, and that of the totality of SCUBA data met with rapture. However, it is clear that a new tracking performance standard is established with these data (0.4" rms) and that the steps seen in these data are a real impediment to this goal. The cause is unknown at this stage, but two possibilities present themselves :

1. The transit step embedded in the 'empirical correction' has, in the past, been disabled prior to these tracking experiments, but this was intentionally omitted from the instruction set (MSB note) since it ought to have been quite negligible at these elevations. In the aftermath of the TCS tan error the sense with which this correction is being applied must be checked.

   Note 1 : an incorrect (horizon) value of the transit step size (its current value has been assumed as 1.5" for an awfully long time now) should impact the elevation pointing at el=83^o in the most negligible way, its amplitude on the sky varying as cos(elevation).

   Note 2 : This applies only to the elevation residuals: the source of the step in azimuth pointing at transit (and its non-corroboration by the RxA data) is a mystery in this context.

2. The second possibility that occurred is that the telescope pointing model is incorrect. The SCUBA data alone were run through the FIT9 routine (the data not being amenable to analysis by TPOINT) and the result of allowing optimization of all 7 pointing model parameters is shown below :
   Click on image for better view

   Clearly - the rms's have been reduced, but the steps remain unaffected. This second possibility now seems remote.

20040120/21
The tracking experiments were repeated with the transit step correction disabled:

The elevation steps persist and at the same amplitudes as before. The formal rms in these data are :

                        N     az     el
     RxA   all data    62    0.8    1.3
    SCUBA  all data   150    0.7    1.0
    SCUBA  el < 175    76    0.6    0.7
    SCUBA  el > 185    67    0.6    0.7

Disabling the transit step correction seems to have had no impact - so (with mixed feelings) bang! goes theory #1 above, too. To review :

• Transit steps of -1" in daz and +2" in del are seen in SCUBA data.
• Only the step in del is seen in tracking with RxA.
• The steps are not correctable using the telescope pointing parameters.
• They are not caused by (an erroneous application of) the transit step correction.

So what's left ? Speculatively :

• The unused (old) telescope model terms p8 & p9 : for SCUBA these define the alignment errors of the array w.r.t the beam. Perhaps at transit the image on the central pixel of SCUBA rotates quickly but offset about the central pixel . . .

  The data were run through the FIT program with the following result:

  Click on image for better view

  Overall rms's are rduced to (0.5",0.7"), but the steps persist while additional excursions appear in the data away from transit. It is not the cure, and the changes to all (9) parameters of the model are unrealistically large (200" for p3 p4 p5 p8). Allowing p8 and p9 alone to change to accommodate this effect does not reduce the rms's. (For completeness : p8 and p9 change by -1.9" and +0.9", resp. in this solution).

I shall request SCUBA trackings of 3c279 (el = 64 at transit) to establish the variation of transit step size with elevation at transit (el*).