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What Are Catadioptric Telescopes?

Meg Higa
Meg Higa

Catadioptric telescopes are optical instruments that bounce an incoming image off of mirrors within the telescope’s housing, in order to increase the instrument’s focal length and magnification. This image forming optical system has also been applied to other equipment such as microscopes and photographic telephoto lenses. Comparable quality and capacity can be obtained with modern models for nearly half the cost and a third the size compared to equivalent models made only of glass lenses. It is an especially popular choice for amateur astronomers.

As light, reflecting from or emitted by a distant object, enters a telescope, a system of both convex and concave glass lenses called dioptrics bends the light to strike a ring-shaped mirror near the rear of the scope. The mirror reflects the light back out of the telescope. Then, it is intercepted by another small, circular mirror built near the front of the scope that re-reflects it through the center hole of the rear mirror. The use of such curved mirrors to direct light is called catoptrics. Catadioptric telescopes both refract, or bend, light with glass lenses and reflect, or redirect, light with mirrored surfaces.

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The manipulation of light in this way had been practiced for a long time, but precision applications began to be invented in the early 19th century. The earliest catadioptric telescopes were called dialytes for their use of multiple glass lenses to correct for some of the natural aberrations of light when it refracts through crystalline structures. In 1876 a French engineer improved on this and invented the namesake “Mangin mirror,” which marries a glass lens with a silver mirror to correct for light’s reflective aberrations.

Subsequent improvements addressed efficiencies of design, the minimizing of obstructions from the path of light, and other technical issues. Most efforts at improvement have been to correct for one type of light aberration or another without resorting to the inefficient remedy of adding another element of glass dioptrics. One of the most popular commercial designs of catadioptric telescopes is called the Schmidt-Cassegrain.

In this type of telescope, light reflects and traverses the length of the telescope three times before coming to a single point of focus. A telescope with a straight line of focus would have to be three times as long, and conical in shape. It would be heavy and expensive with all the precision glass required to maintain the integrity of the light rays. Catadioptric telescopes, in contrast, are compact cylinders of large diameter. Heavy glass is minimized and replaced with thin, inexpensive sheets of silver mirrors polished to high precision.

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