and / have the same sign and the neutral point lies to the left of the secondary
mirror in Fig. 6.6. For an aplanatic Gregorian the neutral point is to the right of
the concave secondary. Because the quantity multiplying
kf^
is less than unity, the
neutral point is between the secondary mirror and the focal point of the primary
for an aplanatic telescope. The existence of a neutral point can be used to
advantage if the secondary is deliberately displaced to bring a different source on
to a fixed detector as, fo ...
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