Three Cassegrain focal ratios are available; the f/8 Ritchey-
Chrétien, f/15 and the f/36 chopping secondary (which mounts in
the prime focus cage once the prime focus camera is removed).
Dimensional data for the Cassegrain mirrors are shown in Figure
2.8, while Figure 2.9 shows spot diagrams for various
configurations. Figure 2.8: Nominal optical data for Cassegrain
foci Table 2.2: Nominal optical data for Cassegrain and coudé
foci Figure 2.9: Spot diagrams for Cassegrain foci. The bottom
pair of images shows the effect of moving away from the nominal
focus, as discussed in § 2.3.5. The obscuration with the f/8 top
end is 1680mm diameter (19%) and with the f/15-36, 1450mm
diameter (14%). Note that for the f/15-36 top end, the central
baffle obscuration is greater than that of the top end. Support
vanes obstruct about a further 1% of the primary area. With the
exception of the chopping secondary, the baffling is designed to
exclude direct paths from the sky to the working fields. For
f/15, it is necessary to c