SUMMARY
Uranus was tracked for several hours to check the pointing model.
Changes are suggested to the thermal algorithms and to the pointing model that may yield an rms performance of (1.1",1.2").

Some recent pointing datasets, eg 990524-26 and 990616 , show strong systematic variations of the azimuith residual as a function of azimuth. In the azimuth range 90 - 270 this effect has an amplitude of 10 arcseconds. Such trends are not seen in all data so it is possible that the dominant problem is thermal, revealing itself as a systematic variation with temperature, rather than a defect in the model. The data of 990616 most clearly show the thermal explanation as being a viable alternative.

To determine which scenario is true, Uranus was tracked through most of this azimuth range. Seven hours of data was collected with SCUBA, using the map16 procedure, on a night when tau was too high (> 0.2) to do the scheduled observing. Three observations of 200 integrations each were executed :

• from (az,el) = (128,35) at HST 01:03 to (166,53) at 03:19
• from (az,el) = (167,53) at HST 03:20 to (216,46) at 05:35
• from (az,el) = (219,44) at HST 05:44 to (243,19) at 08:00

The resulting pointing residuals are shown below, both separately and together :

• The rms scatters in (daz,del) in each 2 hour subset are (1.2",0.8"), (1.4",1.7"), (1.4", 1.7"), while the totality of data has raw residuals of (1.6",2.4"). The seeing during this period is not known (the SAO phase meter is not working).

• There are interesting systematics at several places (particularly azimuths 145 for daz, 185 for del). It would be of great value to know if these, and the variations in general, repeat from one run such as this to another.

• The specific daz -vs- az systematic mentioned above is not clearly present, although peak to peak excursions in daz in this range reach 8". Despite the timing of this experiment to avoid the periods of rapid thermal change at JCMT, a thermal explanation still seems more likely - as we shall see below.

• The elevation residuals are plotted below against the (front - back) leg temperature difference :
  

  (Sorry about the poor visibility of these plot at this scale - click on the images for a better view).
  This plot shows a significant slope, and reveals clearly the discontinuity between the early data in the lower left and the second set of data in the middle-left. Note also that the rms scatter of the data about the best fitting straight line (ill-fitting though it is) is reduced to 1.8". An expected variation of 10"/degree is currently being accounted for. The change suggested here (-8"/degree) is clearly very large by comparison with previous adjustments to this figure and should best be confirmed over a larger temperature range before being incorporated into the pointing algorithms.

• The daz residuals are also a function of temperature : the mean leg temperature in this case :
  

  The rms scatter about the line is reduced to 1.1 arcseconds.

• The temperature-corrected data are shown below :
  

The subset of 521 data lying above 30 degrees elevation were run through the FIT7 program to deduce changes to the 7 parameters of the model. The suggested changes are given (in arcseconds) in the 5th column below :

 DT990618_HI.DAT                                                                 
 model #   489
 SCUBA
 No. of points  = 521

  N           OLD    FIXED=1      +          +            NEW

  1          11.2       0        0.0       -0.3          10.9
  2          17.8       0        0.0       -2.1          15.7
  3          50.9       0        0.0        6.6          57.5
  4         158.8       0        0.0        7.3         166.1
  5   229 55 26.1       0        0.0        5.6   229 55 31.7
  6   351 19 21.5       0        0.0        3.9   351 19 25.4
  7         -13.1       0        0.0       -6.0         -19.1
  8           0.0       1        0.0        0.0           0.0
  9           0.0       1        0.0        0.0           0.0

 daz =   0.0 +-  1.1     del =   0.0 +-  1.2     vec =   1.5 +-  0.7

Systematics remain, and this analysis has been done over only one third of the sky (in azimuth). However, it seems possible that we might achieve this performance (rms scatters in daz,del of 1.1",1.2") everywhere, if we make the changes suggested above - both to the algorithms controlling thermal factors and to the model itself.