FOS-2 has equivalent modes of operation to FOS-1 modes B,C and D, although the dimensions of the apertures and separations do differ slightly. These are shown in Table . Again, the most common way of operating FOS-2 is to use Mode D -- that is, a central 20 arcsecond aperture, giving both orders, without any field lens. For most applications, there is sufficient sky as well as the target spectrum in the 20 arcsecond aperture for sky subtraction, especially for point like sources.
Table: Operating modes for FOS-2
As with ISIS, it is possible to use FOS-2 with any of the following observing modes -- that is slitless, multi-slit, fibre-fed and polarimetry modes. See section for details.