Cassegrain is a reflecting telescope with a concave parabolic primary mirror and a convex hyperbolic secondary mirror. The design was first mentioned in 1672 and is named after its possible inventor, Laurent Cassegrain. The telescope reflects light onto the secondary mirror and then through a hole in the primary mirror to the eyepiece. There are variations of the design, including the popular Ritchey-Chrétien telescope, which eliminates spherical aberration. Combination types include the Schmidt Cassegrain Telescope, Maksutov-Cassegrain, and Argunov Cassegrain.
Cassegrain is the name of a type of reflecting 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 on the secondary mirror and the light is then reflected through a hole in the primary mirror into the eyepiece for viewing.
Invented in the 17th century
The first known mention of Cassegrain dates back to 1672 and is believed to have been the invention of its namesake, the clergyman Laurent Cassegrain. Papers uncovered after his death showed the basic design of the telescope. It’s not sure if he was the original inventor or if the design first came from another person, but the telescope still bears his name, as it did when it was first introduced.
How does it work
An object is viewed through the opposite end of the telescope and reflects off the secondary parabolic mirror, which is at the opposite end of the telescope. This mirror reflects the image onto the smaller parabolic primary mirror, which is positioned towards the open end opposite 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 opposite directions, one convex and one concave, which would make them a full circle if the angles continued. Curved mirrors allow for a greater focal length in a smaller space than a Newtonian reflector allows.
Design variants
There are several variations of 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 eliminates spherical aberration, an optical effect that causes blurring and ringing around objects, without the use of a correction plate. The Ritchey-Chrétien is by far the most popular research telescope.
There are other less successful variants. 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 spherical convex. This telescope is prone to coma, a ring around the object being viewed, and blurring. While cheaper to manufacture, the lack of focal range makes it a fairly unpopular choice.
Combination types
There are three types of telescopes that combine the Cassegrain style with a catadioptric mirror system. These systems use mirrors in combination with lenses to reduce optical irregularities. The Schmidt Cassegrain Telescope (SCT) adds a special correction 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 overlapping lenses replacing the secondary mirror.
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