At Prime focus the A&G unit is mainly used with the prime focus CCD camera. The unit is mounted on a rotator, which allows the detector to be rotated to any position angle, although it is normally operated at one of the cardinal points. The default orientation is 180, for which position angle spillage of Liquid Nitrogen is lowest.
Note that no direct viewing facility exists at prime. The telescope points sufficiently accurately that any object with known position can be acquired close to the centre of the CCD detector. Further, there are no calibration lamps provided, but there is a filter wheel which can carry up to 6 filters, for use with the CCD detector.
The prime focus autoguider is identical to the one at Cassegrain focus.
However, because there is insufficient space to accommodate a filter wheel or
slide, a single fixed broad-band green filter is used. The choice of this
filter is a compromise between a match to the FW-130 response and to the
spectral response of the CCD detectors used at prime focus. Note that
the autoguider at prime does not look through the same colour filter as the
CCD, which has implications for focal adjustments, in particular on changing
(CCD) filter (see section 
).
An important feature of the geometry at Prime is that both the guide probe
and the CCD detector are mounted off-axis. This means that guide stars can
only be selected in an area located about 30 away from the field viewed
by the CCD, on the opposite side of the rotator axis. This feature makes some
special procedures necessary for the set up of geometry parameters at prime
(see section ) to enable easy acquisition of guide stars.
Figure shows the geometry and the area in which
guide stars can be selected. As for Cassegrain,
stars with magnitudes in the range 714 are suitable.