Pointing with JCMT involves the following process :
- The astronomical coordinates are made known to the telescope computer;
- These are converted from their natural reference frame into azimuth and elevation, taking into account the observer’s latitude and longitude, the time, refraction, aberration, precession, and other astronomical corrections.
- the current azimuth and elevation are read by on-axis encoders
- every 50 milliseconds the telescope drives servo system is commanded to close the gap between the demanded and current positions.
This would be enough for accurate pointing if it were not for the fact that the antenna is structurally imperfect : viz, the azimuth axis is not quite vertical, the elevation axis is not perpendicular to the azimuth axis, etc.. Each of these mis-alignments generates pointing errors that vary in systematic, predictable ways across the sky. A 7-parameter pointing model quantifies these imperfections and is used by the telescope software to correct the pointing. It is checked nightly or periodically by analyzing the residual pointing errors.
There are other factors affecting the pointing precision, including:
- the uneven nature of the track (which would cause movements of the telescope beam of as much as 20arcseconds), and the consequent need to measure the track profile using ‘Inclinometry’.
- the elevation is corrected for the temperature difference between the front and back legs. This correction is of the order 6″/oC of difference. The temperature of the dish suffers little in the way of gradients and so does not impact telescope pointing.
- the HARP K-mirror installation suffered small mis-alignments that generated small additional pointing errors. The misalignments and the sign conventions in the corrective algorithm proved difficult to determine. Removal and replacement of the K-mirror in April 2014 generated slightly larger misalignments, but, ironically, this allowed the sign conventions to be established and a corrective algorithm to be applied. (See report). HARP pointing has been nominal (2″ rms in each of az & el)) since then.