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Catadioptric telescopes are a hybrid design of refractor and reflector telescopes. They combine the benefits of both types of telescope – wide field of view with low distortion and high magnification – into one compact, affordable model. Catadioptric telescopes differ from other types of optical telescopes in having two lenses combined with a concave mirror as their primary optics. These three elements together form the catadioptric system. Catadioptric refers to light that is focused by two lenses at once with one being concave and the other convex while also being combined with a concave mirror. There are different variations of this general design, but they all have those same three components in some combination.

How do Catadioptric Telescopes Work?

Catadioptric telescopes use the catadioptric system to focus light from celestial objects into an image that can be seen by the telescope’s user. The catadioptric system uses two lenses in combination with a concave mirror to focus light from the sky. The primary mirror of a catadioptric telescope is usually spherical, hyperboloidal, or paraboloidal. The second lens is convex, and the third lens is concave. Because of the way the light is focused through the lenses and the mirror, catadioptric telescopes have the wide-field advantages of refractor telescopes with the high magnification of reflector telescopes. With the two lenses and one concave mirror, the catadioptric system is more compact than other optical telescopes.

Three Main Types of Catadioptric Telescopes

Cassegrain Telescope – The traditional model of the catadioptric telescope was first created by a French engineer named George Marie de la Cassegrain. It is called a Cassegrain telescope because its long focal length is in the same configuration as a Cassegrain reflecting telescope since the primary and secondary mirrors are in the same plane. Using two lenses instead of one, the Cassegrain design has a shorter tube length and a smaller primary mirror diameter than a similarly powerful refractor telescope. A Cassegrain telescope has a smaller aperture size than a comparable refractor design. This makes it less expensive to build, but also limits its aperture size and the amount of light it can collect, which restricts its magnification power.

Schmidt-Cassegrain Telescope (SCT) – The Schmidt-Cassegrain telescope was invented by Russian engineer Sergei Ivanovich Schmidt. It is an adaptation of the Cassegrain design in which the primary and secondary mirrors are tilted with respect to one another to allow the secondary mirror to be placed behind the primary mirror. This change of orientation allows the SCT telescope to be shorter than an equivalent Cassegrain telescope, and it allows a large secondary mirror to be placed farther away from the primary mirror, permitting more light to be reflected toward the secondary mirror.

Maksutov-Cassegrain Telescope (MCT) – The Maksutov-Cassegrain telescope was invented by a Russian optical engineer named Vladimir Maksutov. It has a combination of the advantages of both the Cassegrain and the Schmidt-Cassegrain telescope. Like the Cassegrain, it has a short length and a large aperture, and like the Schmidt-Cassegrain, it has a large secondary mirror. Advantages and Disadvantages of Catadioptric TelescopesCatadioptric telescopes have the benefits of both refractor and reflector telescopes rolled into one. They have wide fields of view and low distortion like refractors, and they have high powers of magnification like reflectors. With catadioptric telescopes, you don’t have to sacrifice one benefit for the other. Standard Optical Advantages for Catadioptric TelescopesWide fields of view are a benefit of refractors, and low distortion is a benefit of reflectors. The catadioptric telescope combines both into one compact design.

Key Things to Look for When Shopping for a Catadioptric Telescope

Focusing Screens and Eyepieces: Essential Pieces in all TelescopesBoth focusing screens and eyepieces are important in any telescope design. The focusing screen is a screen placed in the light path between the primary lens and the eyepiece. It allows you to focus the telescope for a clear image. The telescope eyepiece is placed at the top of the telescope, where the light path enters the telescope. The eyepiece magnifies the image and focuses it into your eye. Focusing Screens – There are three types of focusing screens used in catadioptric telescopes: Eyepieces – Catadioptric telescopes come with two types of eyepieces:


The main advantages of catadioptric telescopes are their wide fields of view, low distortion, and high magnification. Their main disadvantages are that they are more expensive than other types of telescopes, and they are heavier and larger than refractors and reflectors. For those who want one telescope for all uses, this hybrid design is an excellent choice. It is the best of both worlds, combining the wide field of view and low distortion of refractors with the high magnification power of reflectors.

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