There are two 8 position filter wheels located immediately after the slit.
The filter types held in these wheels are shown in HELP FILTERS.
Table 
gives the set as at February 1995.
Table: Slit Filter Specifications.
Because these filters are positioned after the slit they will affect the calibration light as well as the starlight. If they are left in during setup, there is a danger of seriously over-illuminating the detector subsequently.
Main commands: SF1 n; SF2 n