A convex mirror is known to produce a virtual image. How do telescopes of the Cassegrain system use a secondary mirror in the form of a convex hyperboloid to obtain actual images of space objects?
A converging beam of rays falls on the secondary convex mirror from the main mirror of the telescope, which remains converging after reflection from it. The equivalent focal length of the Cassegrain optical system is greater than the focal length of the main mirror. A ghost image in a convex mirror arises from a light source that produces a parallel or diverging beam of rays.
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