One of the factors degrading the pointing accuracy of JCMT is the
irregularity of the azimuth track. The up-down motions of the antenna wheels
cause the left and right A-frames to tilt, and thus cause the elevation axis
to experience PITCH, ROLL and YAW. The first of these directly affects the
elevation pointing, while the other two combine with functions of the elevation
to affect azimuth pointing. The contribution of the track irregularities to
the pointing error budget is calculated by Jamie Scobbie in MTPPN004 and
has a residual value between about 0.5
and 1.0
rms in each
coordinate, after the current "track model" corrections are applied.
In addition to affecting the pointing, irregularities in the track displace
the phase centre of the telescope, and so affect the JCMT-CSO Interferometer.
These movements are of the order of 250-400 
m, and are not currently
corrected for (although corrections can be made after the fact in the
off-line data-reduction software).
The Azimuth Track Improvement Project aims to mechanically remove some of the track irregularities by modifications to the track joints and/or segments. We propose herein to derive the astronomical specification required of the azimuth track, in terms of pointing errors in arcseconds on the sky, and residual phase errors in microns.