A fixed rectangular flat with a circular hole set at 45 degrees
to the optical axis which directs light to a standard RGO
autoguider. The field of view is annular with an outer radius of
about 17 arcmin and an elliptical inner radius of 10-13
arcmin. The PA of the major axis is usually east-west. Stars as
faint as V=13 can be used for guiding but brighter stars are
preferred. The autoguider moves on x-y slides and can examine
the offset field from the flat mirror. It has an acquisition
field of 2x2 arcmin.
The autoguider has two sets of filters: ND and colour. For the
highest accuracy at large zenith distances, the colour filter
should be similar to that of the CCD camera. This minimizes the
effects of atmospheric differential refraction.