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There are four 2-position remotely driven filter slides: two in the blue arm beam just after the blue fold; and two below the blue fold but above the red fold; the latter two are for use with both the red arm of ISIS and FOS-II. There is a range of neutral density and colour filters for use in these slides. The blue arm filter slides are normally used to hold neutral density filters for when the IPCS is in use; the red arm filter slides are used to hold long pass filters to cut out second order blue light. The red arm filters slides cal also hold a coloured field lens for use with FOS-II in multi-slit mode.
Procedures for changing the below-slit filters are described in Section 8.2.3.