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The CCD Autoguider is described in detail in The WHT Autoguider System User Guide, by P.J. Rudd. 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 autoguider utilises the off-axis field. 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 autoguider head is mounted inside the Cassegrain A&G box, and contains a frame-transfer CCD having 385 by 288 active pixels, with 22 micron pixels, with a peak quantum efficiency of 50% at 7500Å. This is mounted on a copper block connected to a two stage Peltier thermo-electric cooler. This cools the CCD to around -35 degrees C to reduce the dark current. The CCD must be flushed with Nitrogen when the cooler is in operation to prevent the formation of condensation and ice crystals on the CCD surface. If this is not done the CCD could be badly damaged. There is a flowmeter on the nitrogen outlet line from the CCD Autoguider, the user should check that nitrogen is flowing through this as soon as the CCD begins cooling.
The limiting magnitude of the autoguider, for guiding at 1 Hz, is about V=17.5 in good conditions (i.e. 1-arcsec seeing and a dark sky). Autoguider errors are sent to the telescope control system. When the telescope is servoing on these errors, the telescope tracking errors 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.