Report to the JCMT Board; April 2009

Introduction
To a large extent this report echos the previous one: RxA continues to perform nominally whilst HARP shows a comparatively worse performance. RxW has been pointed successfully and shows performance in line with RxA. The major task remains the optimisation of HARP pointing and the determination of an all-sky model for the k-mirror misalignments. This task remains on-going but we are leading into a period of concentrated effort in which we expect to make good progress.

Pointing with Receiver A
Pointing performance remains essentially nominal. A new RxA pointing model was required following major engineering work along with the need for a robust RxA model on which to base the development of a solution to the HARP pointing issues. Before observations resumed following the removal of SCUBA2 from the antenna and the adjustment of the central bearing, a dedicated pointing run was carried out. This new model gave rms scatters in (azimuth, elevation) of 1.7", 2.0" and data collected since show this model to be performing well.

Pointing with HARP
All-sky pointing with HARP remains relatively poor. This period has particularly suffered due to a problem associated with the k-mirror rotation. To minimise the impact of the four dead receptors on HARP, the array was rotated between consecutive scans to produce the best coverage. It transpired that this rotation generated large pointing offsets of ~10" between the two array orientations or k-mirror positions. With the HARP upgrade this forced rotation is no longer necessary. As is standard procedure for all receivers, but with special importance for HARP, we continue to encourage observers to do frequent, local pointing. However this is only effective if the k-mirror does not flip between the pointing source and the target.

Until SCUBA2 requires pointing work, the priority remains the determination of a model to describe the k-mirror misalignments.

Pointing with Receiver W
Receiver WD has been extensively utilised during recent months. In all but the best weather a differential pointing method for RxW where pointing is performed with RxA and a fixed offset is subsequently applied. So far this method has proved successful. Tests show the applied offsets to be relatively constant throughout AZ/EL space with approximate errors in azimuth and elevation of 1.6" and 0.8". Direct pointing with RxWD has yielded residuals in line with those achieved with RxA, e.g. rms scatters (az,el) during March of 2.6", 3.0" (38 datapoints).

Inclinometry
Measurements of the antenna track profile are made every couple of weeks or so, and updates of the track model have been routine during the reporting period. The recent engineering work (the removal of SCUBA2 from the antenna and central bearing adjustment) suggested a large shift in the track profile, however analysis of the new model showed rms scatters in the differences between the 'before' and 'after' track profiles to be minimal (~1").

Transit step and central bearing race defect
The central bearing was adjusted during the removal of SCUBA2 on Feb 26th 2009. This introduced pointing offsets of the order of 25" in azimuth and 15" in elevation which were immediately corrected for with the new pointing model. The transit step has not been characterized during the period.

Temperature corrections
Routine checks are made on the various algorithms within the TCS code relating pointing and focus settings with temperature and elevation. No significant and consistent changes have been noted during the reporting period.