The pre-slit filter wheel (figure 1 ) contains six 2" x 2" (50.8 x 50.8mm) filters, used mainly for cross disperser order separation. Their use may be preferred to the post-slit filters as a collimator focus change may be required for the latter. Table 1 contains a list of these filters and figure 2 illustrates their transmission curves. A spare filter wheel is available for special filters or filters supplied by the observer.
Table 1: Pre-slit and post-slit filters