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In surveillance "catadioptric" refers to catadioptric sensors.
A catadioptric optical system is one which contains both lenses and mirrors. Catadioptric systems are commonly used in telescopes and in lightweight, long focal length lenses for cameras.
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Catadioptric telescopes are designs that combine specifically shaped mirrors and lenses to allow very fast focal ratios (when used at the prime focus), while controlling coma and astigmatism.
Telescope makers also use catadioptric designs for any or all of the following reasons:
A disadvantage to this design is that the secondary mirror blocks a portion of the light entering the tube.
Light path in a Schmidt-Cassegrain
This design is seen in instruments such as the Schmidt camera and the Schmidt-Cassegrain. The first optical element is a Schmidt corrector plate. The plate is figured by placing a vacuum on one side, and grinding the exact correction required to correct the spherical aberration caused by the primary mirror.
Thousands of amateur astronomers have purchased and used Schmidt-Cassegrain telescopes, with diameters from 20 cm (8 in.) to 48 cm (16 in.), since this type of telescope was introduced by Celestron in the 1960s. Now many companies mass-produce this type of telescope, at prices that make them quite affordable for many amateurs.
Light path in a Maksutov-Cassegrain
The Maksutov design was invented by Dmitri Maksutov. It uses an optically transparent "meniscus corrector shell" that is a section of a hollow sphere. It has a spherical primary mirror, and, in the cassegrain configuration, uses a spherical secondary that is often just a mirrored section of the corrector lens. Maksutov-cassegrains tend to have a narrower field of view than Schmidt-Cassegrains due to their longer focal length and are generally heavier as well. However, their small secondary mirror gives them better resolution than a Schmidt-Cassegrain.
500mm catadioptric lens mounted on a Yashica FX-3.
Photographic catadioptric lenses are similar to astronomical catadioptric designs and are used for some of the same reasons (with added modifications to accommodate photographic use). In order to make catadioptric "mirror lenses" less susceptible to blurring from internal air currents caused by external differential heating the internal space is sometimes filled with glass (referred to as a "solid-cat"). The refractive surfaces of the glass combined with added reflective (mirror) coatings are shaped to contribute to the optical properties of the whole mirror assembly, and so such devices are catadioptric. This has the added advantage of making them extremely rugged.
An example of the bokeh produced by a catadioptric lens, behind an in-focus light.
500mm catadioptric lenses are far lighter than a normal 500mm lens, but typically lack an adjustable diaphragm and operate at a high fixed f-number equal to the focal ratio of the total optical system. Mirror lenses produce a "doughnut" bokeh that is generally considered unpleasant. Mirrors do not suffer from chromatic aberration, thus mirror lenses do not require focus adjustment when used for infrared photography.
As of 2007, the only catadioptric lens capable of autofocusing is the Minolta AF 500mm Reflex lens.
This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia
