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The calibration unit is a separate unit mounted to one side of the main A&G unit. Light from the calibration unit enters the A&G unit via a bellow--like interface, and is injected into the main beam to the spectrograph by means of a 45 degree mirror mounted on the end of the autoguider mirror.
At the end of the calibration unit furthest from the A&G is a set of six lamphouses arranged in a fan. Four of these lamphouses currently contain lamps; ThAr and FeAr lamps for wavelength calibration, and Tungsten and Deuterium lamps for flatfields. In normal use the ThAr is the lamp of choice for wavelength calibration, and the Tungsten lamp for flatfields. The lamps are driven by the same type of constant--current power supply as used at Cassegrain; these are mounted in the A&G electronics rack.
Each lamphouse has a shutter, so that it is possible in principle to leave the lamp on for it to stabilise before using the lamp. In practice this is not done, due to the limited lifetime of the lamps.
In normal operation, the light from each lamp is fed via condensor lenses into a 6--input, 1--output fibre--optic lightguide, the output from which is randomised. This fibre-optic mixer makes it possible to use as many lamps as desired simultaneously. The output of the fibre-optic mixer is re-imaged by a transfer lens to form the system pupil. At this pupil it is possible to locate a diffuser and an aperture mask, which mimic the pattern of illumination of the telescope pupil (in particular, the secondary obstruction). Another lens re-images this pupil model to form a virtual image at the same distance as the telescope pupil.
There is a choice of two diffusers, one for UV work and one for broadband work. In practice, the diffuser greatly reduces the throughput of the calibration system, and is often moved out of the beam completely. Under these circumstances the pupil of the calibration system does not fully mimic that of the telecope. Note that the diffuser cannot be moved under computer control; it must be moved manually, and only by a properly trained member of staff.
There are two other means, less frequently used, for increasing the intensity of the calibration source. It is not possible to select either of these options under computer control; they must be set--up by a properly trained member of staff.
Just before the diffuser are two filter wheels, for colour and neutral--density filtering of the calibration light. The filters contained within these wheels are summarised in Tabs. 10 and 11. Transmission curves are shown in Figs. 14 and 15.