Report to the JCMT Board - April 2003

The actual submision to the Board was reduced to this, but if space had allowed might have been as shown below . . .

Inclinometry

• General
  Measurements of the antenna track profile are made every week or so, and have shown no substantial changes during the past 6 months.
  
• Inception of a hybrid track model
  It may be recalled that we usually make these measurements with the antenna moved in a CW direction, pausing every degreee or so in azimuth to record the appropriate tilts. The resulting 'CW' track model is nonetheless invoked by the TCS irrespective of the direction of motion of the targetted celestial source, which potentially introduces a systematic error for sources moving CCW. The occasional CCW profile is measured, however, and the difference between CW & CCW models has been seen to remain very stable over the years, so the use of a hybrid track model (part CW & part CCW) has been anticipated for some time. On 12 April 2003 such a model was installed, and may be contributing to the overall improvement seen in the pointing recently.
  
• Application error
  In April 2003, it was discovered that the application within the TCS of the pointing corrections due to the track irregularities was in error, resulting in azimuth pointing excursions at azimuths corresponding to track joints of as much as 6 arcsecs for sources at the highest operational elevations (83 degrees). This presumably has affected all data to some degree since the TCS was introduced in August 2000. Only two tracking data sets taken in the interim show the effect at this high level, and the excursions were duly noted. However, being isolated problems they were not understood at the time and were not fully pursued.

Transit step
Few measures of the transit step size have been made during the reporting period; the problem has remained of low amplitude and is controlled by the TCS. However, tracking experiments have occurred more frequently recently in response to the error discussed above and those that 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 of astronomical sources, particularly through transit, and typically performed in hour-long sessions, show stability of <1" rms both in azimuth and (away from the transit event) in elevation.

All-sky pointing
Last time I wrote :

   "This period has seen continued disappointing all-sky performance with 
    rms errors in (daz,del) being more typically 2" in each coordinate (or 
    more!) . . . Partly this may be due to the poor weather pertaining 
    during 2002 . . . "

This situation persisted through the first half of this reporting period. Much effort was expended by several staff in seeking technical causes of the poor pointing - including re-examination of the Telescope Control System functionality, searches for possible unregarded thermal effects, and searches for and studies of periodicities within the pointing data.

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 typical rms scatters in azimuth pointing of 1.4". The performance in elevation is more susceptible to atmospheric instabilities but is often as good when conditions stabilize. 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.

Focus
Default focus positions and elevation- and temperature- dependencies are functioning well.

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