Report to the JCMT Board - April 2003

Inclinometry
Measurements of the antenna track profile are made every week or so, and have shown no substantial changes during the past 6 months.

In April 2003 a hybrid (part CW, part CCW) track model was installed, that should avoid potential systematic errors when observing in the north.

Also in April 2003 a coding error was discovered in the way the TCS applied the track corrections. Pointing errors of 6 arcsecs result at some azimuths and at the highest elevations. This presumably has affected all data since August 2000.

Transit step
The 'transit' (elevation motion reversal) problem remains of low practical impact and is effectively corrected within the TCS. However, tracking experiments performed recently in response to the TCS error above, and which cover source transit, show a broad structure that challenge the current (relatively abrupt) correction either in its application or in its conceptualization. Tests are ongoing at the time of writing.

Tracking
With the above caveats, tracking stability is better than 1" rms in azimuth and (away from the transit event) in elevation.

All-sky pointing
The poor pointing reported last time persisted through the first half of this reporting period too. Much effort was expended by several staff in seeking possible technical (mechanical, software and thermal) causes. However, these concerns were dispelled following the scheduled pointing run of 18 Feb 2003, which, unusually, coincided with excellent weather. High S/N data were obtained on more than thirty blazars, evenly distributed in (azimuth, elevation) space. The resulting model has performed very well since, with rms scatters in each coordinate of 1.5" or better. This improved performance aids greatly in identifying low-amplitude errors such as the TCS coding error and the broad transit effect mentioned above. The overarching concern about the poor pointing has been replaced by these specific, hopefully tractable, problems. This is the more preferable state.

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