Report to the JCMT Board; April 2008

Introduction
This report builds on its rather lengthy predecessor and shall not repeat the issues and formulations involving the HARP K-mirror, for instance, that are described in detail therein.

Pointing with Receiver A
. . remains nominal, with rms scatters in (azimuth,elevation) remaining in the 1.5"-2.0" range. An adjustment on March 23 to the (Earthly) coordinates used for JCMT stimulated the most recent, matching, pointing model adjustment, and recovery of the model was again at this nominal level.

Pointing with HARP
All-sky pointing with HARP remains comparatively poor, with rms scatters reaching 3" or more on any given night. This is due to the lack, still, of a model for the K-mirror misalignments. In addition to the 'noise' in the basic (RxA) model, the impact upon science observations with HARP depends less upon this 'all-sky' characterization however, and more upon the relative pointing errors between 'RA/Dec' science observations and the accompanying 'AZ/EL' pointing observations: the phrases in quotes describing how the sky is oriented on the HARP array, with the orientation being determined by the K-mirror position. Much of the experimentation in the new year has been devoted to serendipitous collection of relevant data. In the absence of a K-mirror model, the data show mean differences and scatters in (az,el) of (-0.3"+2.0",-0.0"+2.5"). Optimization analysis for values for the K-mirror coefficients yields (xo,yo,x',y')=(-0.1",-1.2",-0.5",0.9"), a solution which potentially reduces the scatters to (1.8",1.3"). [These values reveal, by the way, the precision of the K-mirror construction and installation]. The K-mirror model is to be tested once the telescope is returned to operation (April 11), and future plans to perform the pre-science pointing observations in a RA/DEC frame would eliminate even this component of the HARP pointing error budget.

While the absence so far of a working model for the K-mirror has had little impact upon the quality of science produced by HARP, it is acknowledged that the project is proceeding slower than may be desired.

Inclinometry
Measurements of the antenna track profile are made every couple of weeks or so, and updates of the track model have been routine and uneventful during the reporting period.

Transit step and central bearing race defect
Neither defect has been characterized during the period.

Temperature corrections
Routine checks are made on the various thermal algorithms within the TCS code, and no significant and consistent changes have been noted during the reporting period.