The Auxiliary focus filter unit is a blue box mounted on at the Auxiliary port, which contains a 6 position filter wheel similar to the INT and JKT filter wheels. The maximum size of the filters to fit in this wheel is 51mm square or 55mm diameter. Adaptors for smaller filters can be made if requested well in advance. The filter unit also contains a CCD shutter 25mm in diameter, which is operated as normal by the CCD controllers. The filter wheel is controlled by the Cassegrain A&G box 4ms microprocessor.
At the f/11 Cassegrain focus, the telescope scale is 4.51 arcsec mm at the detector, which gives a scale of 0.10 arcsecs per 22 m CCD pixel. At present GEC P8603, EEV P88300 and Tek 1024 square CCDs are offered at the Auxiliary focus; Table 2 summarises the pixel and field sizes given by these CCDs.
Table 2: CCD pixel and field sizes at the Auxiliary port
The unvignetted field at the Auxiliary port is currently limited by the shutter to 2.0 arcmin diameter, thus the final unvignetted field is the intersection of this circle with the rectangle given in the table.