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 parameterization 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.

LATEST NEWS

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.

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 lastest pointing model was installed on UT 12 Apr 2007.

The plot below shows the latest data displayed as if taken with the new model. RMS scatters in (dS,dZ) are expected to be (1.5",1.8").