shows that coma is almost always the aberration that sets the limiting field size for
good images, as the reader can verify by constructing a diagram similar to that of
Fig. 6.1. Spot diagrams for an//10 Cassegrain with m = 4 and j8 = 0.25 are
shown in Fig. 6.2. The images for a classical Gregorian are very similar.
Looking at the curvature of the median image surface in the case j8
==
0, we
see that it is approximately 2(m + l)/Ri. This relation is not exact, but it
illustrates three features of the image surface. First, the sign of K^ is opposite
for the Cassegrain and Gregorian types; the surface of best image ...
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