
6.2. Two-Mirror Telescopes
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telescope, the so-called Couder, therefore suffers from the problem that the focal
surface is relatively inaccessible. For a reasonable choice of m, say 0.25, it also
has a relatively large secondary obscuration compared to the Ritchey-Chretien.
One final thing to note is that the telescope focal length, in general, is one-half the
distance between the primary and secondary mirrors. A diagram of a Couder
design is shown in Fig. 6.4.
Another type of anastigmatic aplanat is found if the primary is convex and
A:
> 1. From Eq. (6.2.6) we find m < 0, hence there is a real final focus and the
configuration is that o ...