Catadioptric system

A 150mm aperture catadioptric telescope (Maksutov)

A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors (catoptrics). Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes, microscopes, and telephoto lenses. Other optical systems that use lenses and mirrors are also referred to as "catadioptric" such as surveillance catadioptric sensors.

Early catadioptric systems

Catadioptric combinations have been used in many early optical systems. In the 1820’s Augustin-Jean Fresnel developed several catadioptric lighthouse reflectors.^[1] Léon Foucault developed a catadioptric microscope 1859 to counteract aberrations of using a lens to image objects at high power.^[2] In 1876 a French engineer, A. Mangin, invented what has come to be called the Mangin mirror, a concave glass reflector with the silver surface on the rear side of the glass. The two surfaces of the reflector have different radii to correct the aberration of the spherical mirror. Light passes through the glass twice making the over all system act like a triplet lens^[3]. Mangin mirrors were used in searchlights where they produced a nearly true parallel beam. Many Catadioptric telescopes use negative lenses with a reflective coating on the backside that are referred to as “mangin mirrors”, although they are not single element objectives like the original mangin and some even predate the mangins invention.^[4]

Catadioptric telescopes

Catadioptric telescopes are optical telescopes that combine specifically shaped mirrors and lenses in designs that have all spherical surfaces that are easier to manufacture, have an overall greater degree of error correction than their all lens or mirror counterparts, have a wide field of view, take advantage of a folded optical path, or a combination of any or all of these attributes. Many types employ “correctors”, a lens or curved mirror in a combined image-forming optical system so that the reflective or refractive element can correct the aberrations produced by its counterpart.

Catadioptric dialytes

Catadioptric dialytes are the earliest type of catadioptric telescope. They consist of a single element refractor objective combined with a silver backed negative lens (similar to a Mangin mirror). The first of these was the Hamiltonian telescope patented by W. F. Hamilton in 1814. The Schupmann medial telescope designed by German optician Ludwig Schupmann near the end of the 19th century placed the catadioptric mirror beyond the focus of the refractor primary and added a 3rd correcting/focusing lens to the system.

Full aperture Correctors

There are several telescope designs that take advantage of placing full diameter lens (commonly called a "corrector plate") in front of a spherical primary mirror. These designs take advantage of all the surfaces being "spherically symmetrical"^[5] and were originally invented to create optical systems with very fast focal ratios (wide fields of view) with little coma or astigmatism for use as astrographic cameras. They work by combining a spherical mirrors ability reflect light back to the same point with large lens at the front of the system (a corrector) that slightly bends the incoming light, allowing the spherical mirror to image objects at infinity. Some of these designs have been adapted to create compact long focal length catadioptric cassegrains.

The Schmidt corrector plate

The Schmidt corrector, the first full diameter corrector plate, was used in Bernhard Schmidt's 1931 Schmidt camera. The Schmidt camera is a wide field photographic telescope, with the corrector plate at the center of curvature of the primary mirror, producing an image at a focus inside the tube assembly where a curved film plate or detector is mounted. The relatively thin light weight corrector allows Schmidt cameras to be constructed in diameters up to 1.3 m. The corrector's complex shape takes several processes to make, starting with a flat piece of optical glass, placing a vacuum on one side of it to curve the whole piece, then grinding and polishing the other side flat to achieve the exact shape required to correct the spherical aberration caused by the primary mirror. The design has lent its self to many Schmidt variants.

Popular sub-types

Light path in a Schmidt-Cassegrain

• Schmidt-Cassegrain telescopes are one of the most popular commercial designs on the amateur astronomical market^[6], having been mass-produced since the 1960’s. The design replaces the Schmidt Camera film holder with a Cassegrain secondary mirror making a folded optical path with a long focal length and a narrow field of view.

The meniscus corrector shell

The idea of replacing the complicated Schmidt corrector plate with an easy to manufacture full aperture spherical meniscus lens (a meniscus corrector shell) to create a wide field telescope occurred to at least 4 optical designers in early 1940's war torn Europe, including Albert Bouwers (1940), Dmitri Dmitrievich Maksutov(1941), K. Penning, and Dennis Gabor (1941).^[7][8] War time secrecy kept these inventors from knowing about each others designs leading to each being an independent invention. Albert Bouwers built a prototype meniscus telescope in August of 1940 and patented it in February of 1941. It used a spherically concentric meniscus and was only suitable as a monochromatic astronomical camera. In a later design he added a cemented doublet to correct chromatic aberration. Dmitri Maksutov built a prototype for a similar type of meniscus telescope, the Maksutov telescope, in October of 1941 and patented in November of that same year. ^[9] His design corrected spherical and chromatic aberrations by placing a slightly positive shaped meniscus corrector closer to the primary mirror.

Popular sub-types

Light path in a meniscus telescope (Maksutov-Cassegrain)

• Maksutov Cassegrain telescopes are the most commonly seen design that uses a meniscus corrector, a variant of the Maksutov telescope. It has a silvered "spot" secondary on the corrector making a long focal length but compact (folded optical path) telescope with a narrow field of view. This design idea appeared in Dmitri Maksutov's 1941 notes and was original developed in commercial designs by Lawrence Braymer (Questar, 1954), and John Gregory (1955 patent^[10]). The combination of the corrector with the silvered secondary spot makes Maksutov Cassegrains low maintenance and ruggedized since they can be air sealed and fixed in alignment (collimation).

Sub-aperture correctors

In Sub-aperture corrector designs, the corrector elements are usually at the focus of a much larger objective. These elements can be both lenses and mirrors, but since multiple surfaces are involved, achieving good aberration correction in these systems can be very complex.^[11] Examples of Sub-aperture corrector catadioptric telescopes include the Argunov-Cassegrain telescope and the Klevtsov-Cassegrain telescope, both of which use as a secondary mirror an optical group consisting of several lens elements and a “Mangin mirror”.

Photographic catadioptric lenses

500mm catadioptric lens mounted on a Yashica FX-3.

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.

An example of the Iris Blur produced by a catadioptric lens, behind an in-focus light.

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^[12]. 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 (aside from the identical Minolta-manufactured lens that preceded Sony's production).

References

See also

External links

• telescope-optics.net, CATADIOPTRIC TELESCOPES
• Learning to love your Mirror Lens - from olympuszuiko.com