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.