A Cassegrain is a type of reflective telescope. The telescope has two mirrors, a primary and a secondary. The primary mirror is a concave parabolic mirror, while the secondary is a convex hyperbolic mirror. The primary mirror focuses light onto the secondary mirror, which is then reflected through a hole in the primary mirror into the eyepiece for viewing.

The Cassegrain was first discussed in 1672 and was the apparent invention of its namesake, the priest Laurent Cassegrain. Papers uncovered after his death showed the basic design for the telescope. It is unsure if he was the original inventor or if the design came from another first, but the telescope still bears his name as it did when it was first introduced.

An object is viewed through the far end of the telescope and reflects onto the parabolic secondary mirror which is at the opposite end of the telescope. This mirror reflects the image back onto the smaller parabolic primary mirror, which is placed toward the open end facing the secondary. This mirror then sends the image directly to the eyepiece through a hole in the secondary mirror. The two mirrors are curved in the opposite direction, one convex and one concave, which would make them a whole circle if the angles continued. The curved mirrors allow for greater focal ability in a smaller space than allowed by a Newtonian reflector.

There are several variations on the Cassegrain design. The Ritchey-Chrétien telescope was invented around 1910 by the two astronomers George Willis Ritchey and Henri Chrétien. Instead of a parabolic mirror, the primary mirror is hyperbolic like the secondary. The Ritchey-Chrétien design naturally cuts out spherical aberration, an optical effect that causes blurring and rings around objects, without the use of a corrector plate. The Ritchey-Chrétien is by far the most popular research telescope.

There are other less successful variations. The Dall-Kirkham telescope was invented in 1928 by Horace Dall. The primary mirror of a Dall-Kirkham is concave elliptical, while the secondary is convex spherical. This telescope is prone to coma, a ring around the object being viewed, and blurring. While it is cheaper to produce, the lack of focal range makes it a fairly unpopular choice.

There are three types of telescopes that combine the Cassegrain style with a catadioptric mirror system. These systems use the mirrors in combination with lenses to reduce optical irregularities. The Schmidt Cassegrain Telescope (SCT) adds a special corrector plate to limit spherical aberration. A Maksutov-Cassegrain, invented in 1957, applies a similar concept but with two spherical mirrors where primary mirror functions doubly as a full view corrector plate. An Argunov Cassegrain also uses spherical optics, but with three stacked lenses replacing the secondary mirror.