GENERAL
The pointing of JCMT is determined by a 7-parameter model describing the non-orthogonality of the axes and telescope beam, the encoder zero-points and flexure of the secondary mirror support structure. These parameters are checked and adjusted by dedicated pointing runs.

The antenna track is irregular, causing offsets of the beam in azimuth and elevation. The size of these irregularities is 0.01 inches and translates to pointing errors of ~10" in each coordinate. The profile is measured by inclinometry and is fed into the 'telescope' task as a look-up table.

Elevation pointing is also corrected for refraction , calculated as a function of ambient atmospheric parameters, for the difference in temperature between the front and back legs, and for their absolute temperature. However, such parameterisation will not correct for all possible, or particularly rapid, thermal changes - see here.

Pointing errors will also increase with increased atmospheric turbulence, i.e. poor seeing, CLOCK errors , failure of or errors from the weather station or leg temperature probes, misalignments along the optical path (eg TMU), or from poor S/N during FIVEPOINTS.

Collimation offsets for each Front End are used in order to aid switching between FEs. It is recommended that pointing checks follow FE-switches and in any case at least once per hour; more frequently during times of poor weather or thermal change.

NEWS & UPDATES

You can find regular reports documenting pointing updates and analyses here

2009
A new track model and RxA pointing model were installed on the 19th May. This followed a central bearing adjustemnt which was prompted by the reduced loading on the weheels and subsequent spikes seen in the inclinometry data.

A new model has been installed on the 4th April. This was determined in excellent weather and gave good residuals.

A slight adjustment to the model was carried out on 20090304 to remove a minor systematic in elevation.

A new RxA pointing model has been installed on 27th February 2009. This was necessitated by adjustments to the central bearing and the removal of SCUBA2 from the antenna.

2008
The RxA pointing model has been installed on 28 May 2008. It produces residuals (dS, dZ) of 1.7" and 1.8" respectively.

2007
The current track model is a hybrid model from 08 Apr 2007, and was created from data taken with the 'fixed' TMU inclinometer, and with the antenna at ~elevation=90. The latest pointing model was installed on 12 Apr 2007.

The RMS scatters in (dS,dZ) are with the new model are (1.5",1.8").

2006
A 6.7 mag earthquake on 15 October 2006 caused a 2" tilt in the plane of the antenna track, but has otherwise little affected the telescope pointing.