1996 JCMT Pointing Report

The year started with a failure of one of the antenna drive wheels. This introduced a 1"-2" ripple into inclinometry from the right A-frame, and radial-arm inclinometry eventually isolated the fault as residing with the rear right wheel. This was replaced in April. It is probable that the failure occurred on or about Christmas Day 1995.

New customized BEI 24-bit encoders were installed in May without any adverse effects.

August saw the most ambitious part of the track-levelling project, with half the 14 track joints being welded. The pointing and inclinometry project work during most of the year became subsumed within this project, Attempts in 1995 through 1996 to quantify the efficiency with which the track model predicted pointing offsets were regularly plagued by pointing glitches that seemed more than likely due to interactions between the antenna and track joints. The rationale behind the welding included the general opinion that removing the larger joint steps might at least negate these joint effects. Welding of all track joints will be complete in April 1997, whereafter we can return to the efficiency estimates.

Last year also saw several changes in the principal instrument used for pointing. Our workhorse, UKT14, was displaced by SCUBA in June of 1996. Excepting the use of SCUBA in its only dedicated pointing run of the year on 11 Oct 1996 , maintenance of the pointing model required using the heterodyne receivers with the continuum backend (CBE). The poorer sensistivity of this mode, the loss of the A-band receiver for substantial periods, and poor winter weather, made this task difficult. In the second half of the year rms residuals in (daz,del) were typically closer to (2.0",2.0") rather than the canonical (1.5",1.5") of the previous year.

However, the October SCUBA run - two months after the track-welding - showed the pointing model was in good shape with rms scatters in azimuth and elevation of 1.1" and 1.5", respectively, and that the results with the heterodyne receivers might have been misleadingly pessimistic. SCUBA was not used again for pointing until April 1997, but again produced a model with expected performance figures of (1.0",1.6") .

SCUBA is expected to become our predominant instrument for pointing henceforth.

imc@jach.hawaii.edu   970414