The Cassegrain A & G Box

The layout of the A&G unit is shown in Figure  5. It is described in some detail by P.A. Ellis in ING La Palma Technical Note no. 56. The unit provides acquisition, guiding and calibration facilities for all Cassegrain instruments. In the upper half of the A&G box a series of 45 degree mirrors direct light to the various ports around the circumference of the A&G box. Three of these are relevant for imaging.

• The large feed mirror, which directs the entire 15 arcmin diameter field of the A&G box to the Auxiliary port. This mirror is usually used when fibre optic aperture plates are mounted at the auxiliary focus.
• The small feed mirror, which directs the central 4 arcmin diameter region of the Cassegrain field to the Auxiliary port, leaving the periphery available for the autoguider. This mirror is normally used for CCD imaging.
• The acquisition and comparison mirror, which can direct the central part of the field to the Westinghouse ISEC TV camera for object acquisition. When used direct this provides a 1.5 arcmin field at 4.51 arcsec mm . It is possible to insert a focal reducer which provides a field of 4 arcmin at a scale of 12 arcsec mm . The TV camera is provided with a filter wheel with six filter positions, normally loaded with CLEAR (UBK7), B (BG28), V (BG38),

  
Figure 5: Cutaway view of the Cassegrain A&G box