The JCMT antenna is driven by four rollers, one at the bottom of each A-frame leg - together these take 10% of the load of the antenna, the remainder being supported by the central bearing. These rollers travel on a track composed of 14 sections. As the antenna rolls around on the track, the elevation axis experiences 3 independent motions (yaw, pitch and roll) which combine to produce pointing errors in azimuth and elevation on the sky. The track profile deviates from the horizontal plane by as much as 0.4mm in each sense, translating to 10 arcseconds of pointing error, and shows both large scale effects (i.e. some contiguous sections being above the average level, some below) and small scale effects (i.e. rapid changes in level over small ranges of azimuth - often where two sections abut each other).
Currently, the track profile is measured at a resolution of 1 degree, and to an accuracy of order one arcsecond (is this correct) - this process is referred to as ``inclinometry''.
The current inclinometry system comprises two single-axis inclinometers and their customized signal amplifiers and displays. There are a number of problems with the this system:
The above problems have conspired in the past to make it difficult to obtain a complete, consistent set of inclinometry data in a short period of time. This, together with the need for the antenna to remain completely unmolested during measurements, means that obtaining inclinometry data is a difficult chore.
Significant improvements have been made to the system in recent months, and undoubtably more could be made. However, the importance of this system to allow a new track model to be derived on demand leads us to consider it worthwhile to put effort into a significant upgrade. The main features of this improved system would be as follows:
Some thought has also been given to the possibility of performing ``real-time'' inclinometry. This is in use at CSO, for example, and would have the benefit of measuring the corrections under precisely the conditions for which they are required. This is a considerably more ambitious project, and it's benefits should be considered further. In any case, an accurate ``static'' track model is required as a first step, since any real-time system might fail. We should proceed on the basis however that real-time corrections would not be ruled out by any choices made for the static system.
Limitations of the current pointing procedures (basically, the fact that a complete fivepoint observation is logged at a single azimuth) makes it extremely difficult to deconvolve after the fact the relative contributions of the track and pointing models on pointing. This means that we cannot test the effect of a new track model before installation, by applying it to existing pointing data. This problem could probably be circumvented were more data logged while pointing was being performed, and this possibility will also be investigated further.
Current plans call for the purchase of additional inclinometers, to augment (or replace) the current ones; their interface to an ``Optomux'' data-acquisition system, and modifications to the current Vax ``Weather task'' to allow it to log the data. Further details of this aspect of the JCMT pointing project are described in the associated document ``Upgrades to the JCMT Inclinometry system''.