Introduction

JCMT pointing is defined using a model for the antenna and a model for the effects of the irregularities of the antenna azimuth track. The latter is created using measurements from a set of inclinometers located strategically on the antenna, and a set of assumptions about the behaviour of the antenna as it rolls around on this track.

The JCMT Inclinometry system was significantly upgraded at the end of 1994, (see MTPPN002) with the installation of new meters, and in June 1995, the TEL task was upgraded to accomodate track models with resolutions that are variable and as fine as 0.2. Fig.1 shows the tilt of the Right A-frame in a limited azimuthal range. It shows the effects of some wheel-joint interactions, in particular the 8 effect of the worst joint, that between track segments 14 & 1, due to a step in the track of height 270m. Such steps challenge the establishment of a pointing model, and add not only to the overall rms description of pointing errors, but also to localized peak-to-peak errors which often have more impact upon an observer.

The need to mechanically improve the track, eg to remove some or all of the large steps between these track segments has become an important engineering consideration. To justify the substantial effort, cost and associated downtime of such a major project, it is important to fully understand the problems produced by the current condition of the track, to quantify and confirm their effect upon telescope pointing, to evaluate the other contributions to pointing errors, to specify a set of possible desired conditions and associated pointing accuracy, to cost the effort of reaching each of these conditions, and to choose one as an optimal solution.

This document aims to present the work that has been done during 1995 which facilitates the important engineering decision that must be made later in the year. This must be based, to a large extent, on analysis of the astronomical implications. We describe the various experiments that have been performed as part of our attempts to more fully understand the behaviour of the antenna and track, and their interaction, and to justify or correct the assumptions in our models.

Much of the work described herein has been documented, less formally, elsewhere and further details may be found in those memos and reports. See IMC for details.