SUMMARY
3c273 was tracked for several hours to check the pointing & tracking models. Tracking accuracy is better than 1" rms in azimuth, 2" in elevation, over an interval of >3^h.

Three 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 100 integrations each were executed :

• from (az,el) = (109,47) at HST 03:45 to (214,69) at 07:10.

- with a transit at elevation 72 degrees at HST 06:25. The resulting pointing residuals are shown below, both separately (note the different scales) and together :

• The rms scatters in (daz,del) in each subset are (0.5",0.8"), (0.5",0.5") (0.4",1.3").
  The seeing during these 1^h periods was approximately 0.7", 0.4", 0.5" respectively.
  The totality of data has scatters of (0.7",1.8").

• This is not the perfect test of the track model, for which we should do more at lower elevations (not possible in these condition), but it does indicate that the general tracking performance can be better than 1" rms (total vector) over 1^h intervals, and better than 2" over 3^h.

• The performance, especially in azimuth, is a considerable improvement on similar tests done on 990618 , and justifies the change in the yaw factor (1.00 from 1.21) introduced on 991105 in the production of track models from inclinometry data. (Yaw affects the azimuth pointing only).