Caption: A diagram of a Cassegrain reflector, a standard reflector telescope design.

Features:

1. The primary mirror is a concave mirror with a hole.

2. The secondary mirror is a convex mirror.

3. The use of the secondary mirror and the hole in the primary mirror allows the image formation and observation to be done behind the primary mirror which is a design convenience.

4. The light rays from two point sources of light (which may be two distinct point sources of light or two points on an extended object) at optical infinity are represented by two sets of two parallel light rays.

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