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-focus 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-Cassegrain
The Schmidt-Cassegrain is a classic wide-field telescope. 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. One of the major advantages of the Schmidt-Cassegrain is that its folded light path makes the optical tube very short and squat, thus increasing its portability. It also has optics that are good for both planetary and deep sky observing.
[edit] Maksutov-Cassegrain
The Maksutov-Cassegrain is a variation of the Maksutov telescope, invented by Dmitri Maksutov. It starts with an optically transparent corrector lens that is a section of a hollow sphere. It has a spherical primary mirror, and a spherical secondary that is often just a mirrored section of the corrector lens. Maksutovs are mechanically simpler than small Cassegrains, have a closed tube and all-spherical optics. The key difference from the similar Schmidt telescope design is the meniscus-shaped corrector plate, that has easy-to-make spherical surfaces, and not the complex aspherical form of the Schmidt design. Maksutovs 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
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 filled with glass (sometimes called 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. A 500mm catadioptric lens is far lighter than a normal 500mm lens, but typically lack an adjustable iris 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.
