Report to the JCMT Board; April 2010

Introduction
The focus during this reporting period has been firmly on SCUBA-2. Fortunately during this time the pointing performance of our other instruments has remained stable and nominal.

Pointing with Receiver A
RxA continues to preform nominally with rms scatters in azimuth and elevation falling in the 1.5"-2.0" range.

Pointing with HARP
The pointing performance of HARP remains an high priority issue with the K-mirror terms still undescribed. The rms residuals consistently look poor being of the order of 2.0"-3.5". However these numbers can be misleading for as long as the K-mirror does not undergo a rotation between the pointing source and the science target then the quality of the pointing is on a par with the other receivers. Attention will return to this issue during the upcoming break from SCUBA-2 on-sky commissioning.

Pointing with Receiver W
Conditions have been good enough to allow direct pointing with RxW on a fair number of nights during the past six months. Overall the pointing performance of both RxWD and RxWB has been very satisfactory with residuals of 1.5"-2.0" on any given night.

Pointing with SCUBA-2
The initial pointing model for SCUBA-2 involved azimuth and elevation offset terms from the RxA pointing model as is in place for HARP and RxW. We saw initial rms scatters in azimuth and elevation of approximately 3.5" - 5.0" which were considered reasonable numbers for the early stages of commissioning. As the fitting techniques in the software, along with the observing strategies themselves developed, these numbers improved to better than 3.0". Despite this improvement the pointing residuals were displaying a strong elevation systematic; this was attributed to a focal plane offset, though may also be contributed to by its position on the Nasmyth platform. As the focal plane offset was undetermined we carried out an independent SCUBA-2 pointing run at the end of February and installed model with (azimuth, elevation) rms scatters of 1.6", 1.7". Data taken subsequently have shown residuals of 1.9", 2.9".

Inclinometry
Measurements of the antenna track profile are routinely made every couple of weeks and this reporting period has proved extremely stable, a state which can attributed to the presence of SCUBA-2 on the antenna. Following the removal of SCUBA-2 we will resort to weekly inclinometry runs to more closely monitor the wheel loadings.

Transit step and central bearing race defect
Neither defect has been characterised during the period. Plans to address the transit step issue by utilising SCUBA-2 proved infeasible under existing observing modes. As an alternative we are turning to transit inclinometry to monitor the transit step in the near future. Following the removal of SCUBA-2, upcoming engineering work will include the installation of a tiltmeter in the receiver cabin, once in place this issue can be properly addressed..

Temperature corrections
Routine checks are made on the various algorithms within the TCS code relating pointing and focus settings with temperature and elevation. At the start of this reporting period a special effort was made to characterise and remove the long standing Z-focus systematic with respect to temperature. Subsequent updates have led to a reduction in this effect, however it seems our current model is unable to remove it completely. This does not adversely affect observing as the TSSs routinely check the focus more regaulrly at the start of the night to compensate.