A Cassegrain Reflector telescope is a Reflector Telescope with a convex secondary mirror that reflects the image through a hole in the Primary Mirror. Or, often in the case of a Radio Telescope, the Focal Plane can be located in front of the primary reflector. Also, the primary reflector may be asymmetric, e.g., just a fraction of a full circle, so as to avoid both a central hole and obstruction.
The classical design is with a parabolic primary mirror and a hyperbolic convex secondary mirror. The advantage of the Cassegrain design over a simple flat mirror is that the focal length can be set so the secondary mirror is small and blocks little incoming light while the focal plane is still behind the primary mirror, e.g., to be used in a hand-held telescope or, for large telescopes, to handle large instruments.
Herschel Space Observatory