Catadioptric system
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A catadioptric optical system is one where lenses and curved mirrors are used to form the Image-forming optical system. Catadioptric systems are commonly used in optical telescopes and in lightweight, long focal length lenses for cameras, where the term mirror lens is often used for them.
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[edit] Catadioptric telescopes
Catadioptric telescopes are designs that combine specifically shaped mirrors and lenses in a design that takes advantage of all the surfaces being "spherically symmetrical"[1]. This allows the designer create optical systems with very fast focal ratios with little coma or astigmatism.
Catadioptric designs have the following advantages:
- When used in a prime focus configuration they can have very wide coma free field of view (e.g. Schmidt camera).
- When used in a Cassegrain configuration it results in a long focal length instrument that is "folded" into a much smaller package.
- They employ spherical surfaces that are easier to manufacture.
- 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.
Catadioptric designs have the following disadvantages:
- The secondary structure blocks a portion of the light entering the tube.
- The secondary obstruction causes some image degradation due to introduced diffraction effects[2].
- When used in the standard commercial Cassegrain configuration, the secondary is not at the optimal position. Instead it is mounted on, or part of the corrector, causing the optical design to deviate from the theoretical "coma free" configuration[3][4].
[edit] 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.
[edit] 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 higher focal ratio[citation needed] 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.
[edit] 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 mm), 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 fold the optical path, greatly reducing the size and weight of the lens, and making longer focal lengths such as 500 mm and 1000 mm more easily accessible. Moreover, chromatic aberration, a major problem with long refractive lenses, is almost completely eliminated.
Catadioptric lenses do however have several drawbacks. The fact that they have a central obstruction means they cannot use an adjustable diaphragm to control light transmission[5]. This means the lens's F-number is fixed to the overall focal ratio of the optical system (typically f/8 for 500 mm 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 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 names) and Canon both offered several designs, such as 500 mm 1:8 and 1000 mm 1:11. Smaller companies such as Tamron also offered their own versions. Of the major manufacturers, currently only Sony (formerly Minolta) offers a 500 mm catadioptric 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.
[edit] See also
- Dioptrics
- Catoptrics
- Image-forming optical system
- Optical telescope
- List of telescope types
- Astrograph
[edit] References
[edit] External links
- Learning to love your Mirror Lens - from olympuszuiko.com
