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.
Catadioptric telescopes
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:
- They employ spherical surfaces that are easier to manufacture.
- When used in a Cassegrain configuration it results in a long focal length instrument that is "folded" into a much smaller package.
- Catadioptric designs are low maintenance and rugged since some or all of their elements are fixed in alignment (collimation).
- Combining a moving primary mirror with a cassegrain configuration allow for large movements in the focal plane to accommodate cameras and CCDs.
- The corrector plates seal the tube assembly from dust and dirt. They also block air currents from the interior of the tube, thereby increasing image stability.
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
Schmidt
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 40 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
Maksutov
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.
The Maksutov-Cassegrain design was popularized in the United States by the Questar Corporation, which began production in 1954.
Photographic catadioptric lenses
Catadioptric lenses are also used for photography, where they are known as 'reflex' or 'mirror' lenses. They are much lighter, smaller, and cheaper than refractive lenses with comparably long focal lengths (> 300-400mm), but at the cost of some optical compromises.
Refractive-design lenses with focal lengths above 300 mm may use as many as twenty optical elements in a housing of a length comparable to the focal length. Catadioptric designs folding the optical path, greatly reducing the size and weight of the lens, and making longer focal lengths such as 500 and 1000 mm more easily accessible. Moreover, chromatic aberration, a major problem with long refractive lenses, is almost completely eliminated.
An example of the Iris Blur produced by a catadioptric lens, behind an in-focus light.
Catadioptric lenses do have several drawbacks. They do not have adjustable diaphragms. Their aperture is typically f/8 (for 500mm designs) or f/11. Their modulation transfer function shows low contrast at low spatial frequencies. The folded optical path does reduce the length of the lens, but increases its width.
Finally, their most salient characteristic in the final image is the annular shape of defocused areas of the image, giving a doughnut-shaped 'iris blur' or bokeh, caused by the shape of the entrance pupil.
Several companies made catadioptric lenses throughout the later part of the 20th century. Nikon (under the Mirror-Nikkor and later Reflex-Nikkor) and Canon both offered several designs, such as 500mm 1:8 and 1000mm 1:11. Smaller companies such as Tamron also offered their own versions. Currently only Sony (formerly Minolta) offers a 500mm lens for their Alpha range of cameras. The Sony lens has the distinction of being the only reflex lens manufactured by a major brand to feature auto-focus.
See also
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