Catadioptric system
From Wikipedia, the free encyclopedia
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.
Contents |
[edit] 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.
[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 longer focal length and are generally heavier as well. However, their small secondary mirror gives them better resolution than a Schmidt-Cassegrain.
[edit] Photographic catadioptric lenses
The catadioptric lens concept can also be applied to photography, where it can be impractical to manufacture and use refractive lenses with focal lengths above approximately 300 to 400 millimetres. Catadioptric lenses for camera are often known as 'reflex' or 'mirror' lenses.
Photographic catadioptric designs are similar to those used for astronomy, with some modifications to accommodate direct use with cameras. The simplified optical construction allows for a much smaller and lighter lens that was favoured over much larger, and significantly more expensive, refractive lenses. This is particularly so for lenses with focal lengths above 300 mm, by which stage a refractive design may be using as many as twenty optical elements in a housing of a length almost equal to that of the focal length.
A catadioptric design solves this problem by 'folding' the optical path in on itself, greatly reducing the size and weight of the lens, and making longer focal lengths such as 500 and 1000 mm more easily accessible. Chromatic aberration, a major annoyance with refractive designs that requires further optical correction, is almost completely eliminated.
The catadioptric design does, however, have several drawbacks that are of particular relevance to imaging. By folding the optical path with mirrors, the length of the lens is reduced, but the width is increased. This also precludes the use of an adjustable diaphragm, which is a severe impediment to exposure control. Furthermore, the generally large focal lengths mean that the effective apertures of such lenses are often no larger than f/8 or even f/11, limiting the lens' use in all but very bright situations.
The other characteristic of catadioptric lenses that is generally considered undesirable is the annular shape in the defocused areas of the image. This doughnut-shaped bokeh is caused by the placement of the mirror arrangement in the middle of the lens.
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.
[edit] See also
- Dioptrics
- Catoptrics
- Image-forming optical system
- Lurie Houghton
- Optical telescope
- List of telescope types
[edit] External links
- Learning to love your Mirror Lens - from olympuszuiko.com
