What is Cassegrain?

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

The first known mention of the Cassegrain was in 1672, and it is believed to have been the invention of its namesake, the priest Laurent Cassegrain. Papers uncovered after his death showed the basic design for the telescope. It is unsure whether he was the original inventor or if the design came from another person 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 mirror. 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 is allowed by a Newtonian reflector.

There are several variations on the Cassegrain design. The Ritchey-Chrétien telescope was invented around 1910 by 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, and the secondary is convex spherical. This telescope is prone to coma, a ring around the object being viewed, and blurring. Although 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, and the primary mirror also functions as a full-view corrector plate. An Argunov Cassegrain also uses spherical optics, but with three stacked lenses replacing the secondary mirror.