Report to the JCMT Board; October 2009

Introduction
This has been an extremely busy period for pointing with 3 central bearing adjustments and no fewer than 5 new pointing models in the past 6 months. We have encountered a number of issues, some of which have been resolved with while others require the capabilities of SCUBA2 to fully resolve.

Inclinometry and Central Bearing Adjustment
On 15th May a routine inclinometry run showed spikes in the track model profile coinciding with the positions of the wheel-joint interactions. These spikes have been seen before, although not since 2002, and were determined to be the result of one of the wheels lifting off the track. This immediately prompted a central bearing adjustment (CBA) to redistribute the weight.

Following this engineering work the strain gauge measurements were monitored more frequently with inclinometry runs occurring at weekly rather than bi-weekly intervals. After a few weeks of relative stability the loadings once gain began to drop on wheels 1 and 4 (corresponding to the side of the telescope where SCUBA2 was mounted). Another CBA was carried out on 7th July as a preventative measure, with a final CBA on the 9th August after the replacement of SCUBA2 onto the telescope. The additional weight over wheels 1 and 4 improved the situation considerably and since then the loadings have been far more stable and the weight distribution more even.

With the exception of the track models immediately following a CBA, the difference between successive track models has remains minimal (<1") wiht only minor changes seen over the past 6 months.

Pointing with Receiver A
The inclinometry was addressed first to explain the implementation of 5 new pointing models during the reporting period. Three of these were required following a CBA which typically throws the pointing off by over 10". The two other models were prompted by a number of nights of sub-par RxA pointing, likely due to the post-CBA models having been carried out in poor weather. The rms residuals for these 5 models fell in the range 1.4"-2.1" in azimuth (uaz) and 1.7"-1.9" in elevation (uel).

Despite, or more likely because of, the constant changes, the pointing performance has remained close to nominal. However the long periods of poor weather over the summer months has meant that the rms of the residuals has been of the order of 1.8"-2.8".

Pointing with HARP
Over the summer we have carried out multiple tests to categorise the K-mirror problem by forcing it to rotate between consecutive pointings. We consistently see large offsets between the different position angles of the K-mirror however the magnitude and elevation profile of the offsets is not repeatable night to night.

HARP pointing as described by single rms values for the azimuth and elevation residuals remain poor - typically, (uaz, uel) = (3.0", 3.5"), but this does not accurately reflect the performance. Single rms values are calculated over all position angles for the K-mirror (as well as the nightly collimation differences) yet for any single position angle the pointing performance is much more satisfactory.

The key is ensuring the K-mirror does not rotate between the pointing source and the science target. To minimise the frequency of K-mirror rotations we switched from (azimuth, elevation) to (RA, Dec) pointing on 21st May. Additionally the operators are aware of the situation and will repeat a pointing if the K-mirror does flip spontaneously. These operational tactics have minimised the impact of the K-mirror misalignments on science data.

Transit step
K-mirror tests revealed a potential transit step effect as sources were tracked with continuous pointing measurements through transit. The effect is much broader than the narrow step function seen in back in 2002/2003 and seems to primarily affect northern sources. However an assessment of recent historical pointing data shows little evidence of this effect outside of test conditions. This will be something to investigate further with the increased time and azimuth resolution of SCUBA2.

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 changes have been noted during the reporting period although work is in progress to allow the opening of the louvres before the start of night-time observing to allow the temperature of the dish to stabilise prior to opening.