In the Cassegrain reflector, parallel rays of light entering the telescope are reflected from a large concave mirror toward the focal point of that mirror, which is called the prime focus of the telescope. Before reaching the prime focus, the light rays are reflected again by a small convex mirror that brings them to a focus near a small hole in the centre of the main mirror.

The value of the Cassegrain reflector was not fully appreciated until a century later, when the English optician Jesse Ramsden (Ramsden, Jesse) found that this design reduces blurring of the image caused by the sphericity of the lenses or mirrors. This blurring (spherical aberration (aberration)) may be entirely corrected by making the large concave mirror paraboloidal and the small convex mirror hyperboloidal. The Cassegrain reflector has been employed in radio transmitters and receivers.