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The Cassegrain A&G Unit
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The autoguider consists of a CCD detector head fed by a right-angled prism and focal reducing optical system, with a field diameter of 1.8 arcmin. The centre of the autoguider field rotates about the centre of the main field at a radius of 110 to 150 mm (8.2 to 11.2 arcmin) and the entire probe assembly has a radial displacement of 40 mm. The extreme edge of the autoguider field is partially vignetted, but only by about 5%. The autoguider has an azimuthal scan of 180 degrees, so the total area scanned at a field scale of 4.51 arcsec mm equals 152 square arcmin or 0.04 square degrees. This gives a good chance of finding a star brighter than 11th magnitude at the galactic equator or 13th magnitude at the galactic pole (C.W.Allen, Astrophysical Quantities, publ. Athlone Press, 1976).
The limiting magnitude of the autoguider, for guiding at 1 Hz, is about V=17.5 in good conditions (that is, 1-arcsec seeing and a dark sky. When the telescope is being servoed on the tracking errors being sent from the autoguider, these are reduced to the 0.1 arcsec level.
The autoguider is provided with a filter wheel with six filter positions. The available filters are CLEAR (UBK7), EMPTY (no filter; different focus), OPAQUE (blanking disk), B (BG 28), V (BG 38), and I (RG 630). These filters do not give a standard photometric system and the Johnson letters are given for guidance only.
The autoguider can be focussed independently to compensate for different filter thicknesses.