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After passing through the autoguider mirror, the main beam from the telescope passes through three filter wheels before reaching the slit--unit of the spectrograph. The three filter wheels are intended for colour filters, neutral density filters, and auxiliary components such as polarisers.
Each filter wheel contains 11 positions, of which position 1 is always empty. The maximum filter diameter is 80 mm. The rear filter wheel, intended for colour filters, can accomodate filters up to 25 mm in thickness, the front wheel, intended for auxiliary components, can accomodate components up to 15 mm in thickness, whilst the central wheel, intended for neutral density filters, can accomodate filters up to 10 mm in thickness.
The filters normally loaded in the colour and neutral density filter wheels are summarised in Tabs. 7 and 8. Transmission curves for the filters are shown in Figs. 11 and 12. These filters are of differing thickness and hence the WHT telescope focus must be adjusted according to the filter combinations inserted into the beam, and also the autoguider probe focus must be adjusted to compensate for the this change in the reflected focal plane.
At present (1994) the main beam neutral density filters introduce an unacceptable amount of ripple and hence their use is discouraged.
It is possible for observers to bring non-standard filters for 'special' applications, but at least one months advance notice is required, since it will generally be necessary to manufacture special adapters in order to be able to mount the filters.
Filters are loaded by means of an access hatch in the side of the A&G unit; however, this is a delicate operation and must only be carried out by properly trained staff-this is a daytime operation. Having changed a filter, the name of filter must be input to the system computer using the UNAMES command. (It is eventually intended that the system computer will automatically be able to determine the filters in use by means of the bar-coding the filters-check with your Support Astronomer for the progress on this feature.)
At the point where the main beam enters the A&G unit from the derotation optics is a dark slide. This should be kept closed except when observing, to protect the optics from dust.
N.B. Remember-the WHT focus must be adjusted to compensate for the total thickness of glass of the filter combinations in use.