Why is the secondary mirror in telescopes of the Newtonian system flat, and in the Cassegrain system it has the shape of a convex hyperboloid?
In a Newtonian telescope, the secondary mirror only turns the light beams (90 °). In the Cassegrain system, the purpose of the secondary mirror is different – to increase the focus. Here, the shape of the secondary mirror should be symmetrical with respect to the main optical axis of the primary mirror.
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