SMU & TMU

This document is intended to examine the impact of misalignments in the SMU & TMU upon telescope pointing. The results described below are based on analyses using my own 'optics(n)' packages (n=5 at time of writing) which are themselves based on a simple model of the antenna optics described here.

The nominal configuration of the SMU & TMU is optimized for instruments in the cassegrain cabin and generates a light path shown below :

with this detail around the first focus :

The scales on both diagrams are millimetres, so it can be seen that the focus is on-axis (my program puts it -0.05mm off axis, but this must be some rounding error or such, and is negligible at a plate-scale of 1.146"/mm - see below). It is also 'tight' in Z - i.e. not spread out along the axis.

The scale of this tightness in arcseconds is seen by running the program with a source 1 arcsecond off axis :

The 'plate-scale' at the nominal focus is approx 0.87mm per arcsec or 1.146"/mm (as shown in the table).

For SCUBA, the SMU Z-position was adjusted by -4mm to ensure that the beam gets through the elevation bearing invignetted :

I haven't included the SCUBA fore-optics mirrors in this analysis yet.

In the nominal (cassegrain) configuration, the impact of adjusting the location of the SMU in the other axes (X & Y) is illustrated by the following diagram for an adjustment of 0.1mm (in X):

The image is spread out in the both the dimension of adjustment and in Z, i.e. along the optical axis. The smallest image is still located at the nominal Z-focus position, and its centroid is located 0.70mm (0.8") off axis and has a total width of approximately 1.5mm, or 1.7". The centroid offset is insensitive to the Z-location of the image plane; changing by only 0.05mm at the upper and lower boundaries of the diagram above. The effect scales proportionately; at least to X-table adjustments of 1mm.

In the SCUBA configuration, the first focus (just beyond the elevation bearing) is less well defined, but a table error of 1mm moves the image in the (first) focal plane by about 11mm. The plate-scale here is approximately 0.77"/mm. (Again, behaviour at the SCUBA final focus in not incorporated into this model).

Future updates to this page should also include the impact of tilting the SMU.