
88 5. Fermat's Principle and Abberrations
Further discussion of the image characteristics of a paraboloid is presented in
Chapter 6.
For a conic mirror with b = 0, the condition for zero spherical aberration fixes
K in terms of
m.
Setting ^3 = 0 in Table 5.2 gives the relation in Eq. (3.5.4), a
result used in Section 4.5 to establish the conic constant for the secondary mirror
of a classical Cassegrain telescope. Note that such a mirror has coma and
astigmatism.
From the entries in Table 5.2 we also see that a sphere (K = 0) in a
configuration with m = —I, thus s' = s = R, has zero spherical aberration and
coma, hence is an aplanat.
As a final ...