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The image reflected from the centre of the slit jaws can be viewed on the CCD autoguider by means of a fibre-optic image guide, identical to the fine guide probe discussed in section 16.4 (i.e. field of view of 4.4 arcsec, scale of 0.13 arcsec/pixel, UV transmitting), except that this fibre is fixed in position. The light is filtered by the autoguider filter wheel, exactly as for the two guide probes.
The optical path which transfers the image to the CCD autoguider is fairly complex. It is summarised here for information only; see the nasmyth A&G Unit optics manual for a more detailed description. A mirror supported by a spider is used to view the central part of the slit jaws. The image is relayed by this mirror and a UV--triplet lens to a prism on the input end of the fibre image guide. This whole assembly is supported by the slit--unit, so moves with the slit, and so can be used at any slit--angle. The output of the slitviewer fibre image guide is combined with the output from the guide fibres, and re-imaged onto the CCD autoguider, as summarised in section 16.4.