Schmidt camera
Encyclopedia
A Schmidt camera, also referred to as the Schmidt telescope, is a catadioptric
astrophotographic
telescope
designed to provide wide fields of view
with limited aberrations
. Other similar designs are the Wright Camera
and Lurie-Houghton telescope.
n optician Bernhard Schmidt
in 1930. Its optical components are an easy-to-make spherical
primary mirror
, and an aspherical correcting lens
, known as a Schmidt corrector plate
, located at the center of curvature of the primary mirror. The film or other detector is placed inside the camera, at the prime focus. The design is noted for allowing very fast focal ratios, while controlling coma
and astigmatism
.
Schmidt cameras have very strongly curved focal planes, thus requiring that the film, plate, or other detector be correspondingly curved. In some cases the detector is made curved; in others flat media is mechanically conformed to the shape of the focal plane through the use of retaining clips or bolts, or by the application of a vacuum
. A field flattener
, in its simplest form a planoconvex lens in front of the film plate or detector, is sometimes used. Since the corrector plate is at the center of curvature of the primary mirror in this design the tube length can be very long for a wide-field telescope. There are also the drawbacks of having the obstruction of the film holder or detector mounted at the focus half way up the tube assembly, a small amount of light is blocked and there is a loss in contrast in the image due to diffraction
effects of the obstruction and its support structure.
Because of its wide field of view, the Schmidt camera is typically used as a survey instrument, for research programs in which a large amount of sky must be covered. These include astronomical surveys, comet
and asteroid
searches, and nova
patrols.
In addition, Schmidt cameras and derivative designs are frequently used for tracking artificial earth satellite
s.
shortly before the Second World War. Between 1945 and 1980, about 8 more large (1 meter or larger) Schmidt telescopes were built around the world.
One particularly famous and productive Schmidt camera is the Oschin Schmidt Telescope
at Palomar Observatory
, completed in 1948. This instrument was used in the National Geographic Society - Palomar Observatory Sky Survey
(POSS, 1958), the POSS-II survey, the Palomar-Leiden (asteroid) Surveys, and other projects.
The European Southern Observatory
with a 1 meter Schmidt telescope at La Silla and the UK Science Research Council with a 1.2 meter Schmidt telescope at Siding Springs Observatory engaged in a collaborative sky survey to complement the first Palomar Sky Survey, but focusing on the southern hemisphere. The technical improvements developed during this survey encouraged the development of the second Palomar Sky Survey.
The telescope used in the Lowell Observatory Near-Earth-Object Search
(LONEOS) is also a Schmidt camera. The Schmidt telescope of the Karl Schwarzschild Observatory
is the largest Schmidt camera of the world.
satellite from the European Space Agency
(1989–1993). This was used in the Hipparcos Survey which mapped the distances of more than a million stars with unprecedented accuracy - this included 99% of all stars up to magnitude 11.
The spherical mirror used in this telescope was extremely accurate; if scaled up to the size of the Atlantic Ocean
, bumps on its surface would be about 10 cm high.
The Kepler photometer
, mounted on the Kepler probe
, and orbiting since march 2009, is the largest Schmidt camera launched into space.
marketed an 8-inch Schmidt Camera. The camera was focused in the factory and was made of materials with low expansion coefficients so it would never need to be focused in the field. Early models required the photographer to cut and develop individual frames of 35mm film as the film holder could only hold one frame of film. About 300 Celestron Schmidt Cameras were produced.
The Schmidt system was popular, used in reverse, for television projection
systems. Large Schmidt projectors were used in theaters but systems as small as 8-inches were made for home use and other small venues.
"lensless Schmidt".
originally designed an "astronomical camera" similar to Bernhard Schmidt's "Schmidt camera", but the design was unpublished. Väisälä did mention it in lecture notes in 1924 with a footnote: "problematic spherical focal plane". Once Väisälä saw Schmidt's publication, he promptly went ahead and solved the field-flattening problem in Schmidt's design by placing a doubly convex lens slightly in front of the film holder. This resulting system is known as: Schmidt-Väisälä camera
or sometimes as Väisälä camera.
of Harvard University
modified the Schmidt camera design to include a convex secondary mirror, which reflected light back toward the primary. The photographic plate was then installed near the primary, facing the sky. This variant is called the Baker-Schmidt camera.
, replaced the Baker-Schmidt camera's corrector plate with a small triplet corrector lens closer to the focus of the camera. It used 55 mm Cinemascope film. A dozen f/0.75 Baker-Nunn cameras with 20-inch apertures – each weighing 3.5 tons including a multiple axis mount allowing it to follow satellites in the sky – were used by the Smithsonian Astrophysical Observatory
to track artificial satellites from the late 1950s to mid 1970s.
at 45° to the optical axis of the Schmidt design creates a Schmidt-Newtonian telescope
.
.
The last two designs are popular with telescope manufacturers because they are compact and use simple spherical optics.
Catadioptric
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors . Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes,...
astrophotographic
Astrophotography
Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...
telescope
Optical telescope
An optical telescope is a telescope which is used to gather and focus light mainly from the visible part of the electromagnetic spectrum for directly viewing a magnified image for making a photograph, or collecting data through electronic image sensors....
designed to provide wide fields of view
Field of view
The field of view is the extent of the observable world that is seen at any given moment....
with limited aberrations
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...
. Other similar designs are the Wright Camera
Wright Camera
In astrophotography, the Wright Camera design, presented by Franklin Wright in 1935, just a few years after the introduction of the Schmidt camera, was his "short" alternative to the original arrangement.-Design:...
and Lurie-Houghton telescope.
Invention and design
The Schmidt camera was invented by EstoniaEstonia
Estonia , officially the Republic of Estonia , is a state in the Baltic region of Northern Europe. It is bordered to the north by the Gulf of Finland, to the west by the Baltic Sea, to the south by Latvia , and to the east by Lake Peipsi and the Russian Federation . Across the Baltic Sea lies...
n optician Bernhard Schmidt
Bernhard Schmidt
Bernhard Woldemar Schmidt was a German optician. In 1930 he invented the Schmidt telescope which corrected for the optical errors of spherical aberration, coma, and astigmatism, making possible for the first time the construction of very large, wide-angled reflective cameras of short exposure time...
in 1930. Its optical components are an easy-to-make spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
primary mirror
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...
, and an aspherical correcting lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
, known as a Schmidt corrector plate
Schmidt corrector plate
A Schmidt corrector plate is an aspheric lens which is designed to correct the spherical aberration in the spherical primary mirror it is combined with. It was invented by Bernhard Schmidt in 1931, although it may have been independently invented by Finnish astronomer Yrjö Väisälä in 1924...
, located at the center of curvature of the primary mirror. The film or other detector is placed inside the camera, at the prime focus. The design is noted for allowing very fast focal ratios, while controlling coma
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...
and astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...
.
Schmidt cameras have very strongly curved focal planes, thus requiring that the film, plate, or other detector be correspondingly curved. In some cases the detector is made curved; in others flat media is mechanically conformed to the shape of the focal plane through the use of retaining clips or bolts, or by the application of a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. A field flattener
Field Flattener Lens
Field flattener lens is a type of lens used in modern binocular designs and in astronomic telescopes....
, in its simplest form a planoconvex lens in front of the film plate or detector, is sometimes used. Since the corrector plate is at the center of curvature of the primary mirror in this design the tube length can be very long for a wide-field telescope. There are also the drawbacks of having the obstruction of the film holder or detector mounted at the focus half way up the tube assembly, a small amount of light is blocked and there is a loss in contrast in the image due to diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
effects of the obstruction and its support structure.
Applications
Comet
A comet is an icy small Solar System body that, when close enough to the Sun, displays a visible coma and sometimes also a tail. These phenomena are both due to the effects of solar radiation and the solar wind upon the nucleus of the comet...
and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
searches, and nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
patrols.
In addition, Schmidt cameras and derivative designs are frequently used for tracking artificial earth satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s.
Ground Based
The first relatively large Schmidt telescopes were built at Hamburg Observatory and Palomar ObservatoryPalomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
shortly before the Second World War. Between 1945 and 1980, about 8 more large (1 meter or larger) Schmidt telescopes were built around the world.
One particularly famous and productive Schmidt camera is the Oschin Schmidt Telescope
Samuel Oschin telescope
The Samuel Oschin telescope is a 48-inch aperture Schmidt camera at the Palomar Observatory in northern San Diego County, California. It consists of a 49.75-inch Schmidt corrector plate and a 72-inch mirror. The instrument is strictly a camera; there is no provision for an eyepiece to look...
at Palomar Observatory
Palomar Observatory
Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...
, completed in 1948. This instrument was used in the National Geographic Society - Palomar Observatory Sky Survey
National Geographic Society - Palomar Observatory Sky Survey
The National Geographic Society – Palomar Observatory Sky Survey is a major photographic survey of the night sky that was completed at Palomar Observatory in 1958.-Observations:...
(POSS, 1958), the POSS-II survey, the Palomar-Leiden (asteroid) Surveys, and other projects.
The European Southern Observatory
European Southern Observatory
The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...
with a 1 meter Schmidt telescope at La Silla and the UK Science Research Council with a 1.2 meter Schmidt telescope at Siding Springs Observatory engaged in a collaborative sky survey to complement the first Palomar Sky Survey, but focusing on the southern hemisphere. The technical improvements developed during this survey encouraged the development of the second Palomar Sky Survey.
The telescope used in the Lowell Observatory Near-Earth-Object Search
Lowell Observatory Near-Earth-Object Search
Lowell Observatory Near-Earth-Object Search ' was a project designed to discover asteroids and comets that orbit near the Earth. The project, funded by NASA, was directed by Dr. Ted Bowell of Lowell Observatory in Flagstaff, Arizona...
(LONEOS) is also a Schmidt camera. The Schmidt telescope of the Karl Schwarzschild Observatory
Karl Schwarzschild Observatory
The Karl-Schwarzschild-Observatorium is an astronomical observatory owned and operated by the institute of Thüringer Landessternwarte ’Karl Schwarzschild’ Tautenburg. In 1992 it was acquired by the state of Thuringia...
is the largest Schmidt camera of the world.
Space Based
A Schmidt telescope was at the heart of the HipparcosHipparcos
Hipparcos was a scientific mission of the European Space Agency , launched in 1989 and operated between 1989 and 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky...
satellite from the European Space Agency
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
(1989–1993). This was used in the Hipparcos Survey which mapped the distances of more than a million stars with unprecedented accuracy - this included 99% of all stars up to magnitude 11.
The spherical mirror used in this telescope was extremely accurate; if scaled up to the size of the Atlantic Ocean
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
, bumps on its surface would be about 10 cm high.
The Kepler photometer
Kepler photometer
The Kepler photometer is the main instrument on NASA's Kepler spacecraft.It is a Schmidt telescope with an array of 42 2200x1024 CCDs in the focal plane; each CCD is flat, but they are mounted on a curved structure to account for the curved focal plane...
, mounted on the Kepler probe
Kepler Mission
The Kepler spacecraft is an American space observatory, the space-based portion of NASA's Kepler Mission to discover Earth-like planets orbiting other stars. The spacecraft is named in honor of the 17th-century German astronomer Johannes Kepler...
, and orbiting since march 2009, is the largest Schmidt camera launched into space.
Other Applications
Starting in the early 1970s, CelestronCelestron
Celestron is a company that manufactures and imports telescopes, binoculars, spotting scopes, microscopes, and accessories for their products.-Origins and History:...
marketed an 8-inch Schmidt Camera. The camera was focused in the factory and was made of materials with low expansion coefficients so it would never need to be focused in the field. Early models required the photographer to cut and develop individual frames of 35mm film as the film holder could only hold one frame of film. About 300 Celestron Schmidt Cameras were produced.
The Schmidt system was popular, used in reverse, for television projection
Video projector
A video projector is an image projector that receives a video signal and projects the corresponding image on a projection screen using a lens system. All video projectors use a very bright light to project the image, and most modern ones can correct any curves, blurriness, and other...
systems. Large Schmidt projectors were used in theaters but systems as small as 8-inches were made for home use and other small venues.
Lensless Schmidt
Prior to Schmidt's design, the solution to spherical aberration was to place an aperture stop at the center of curvature of the mirror, stopping the aperture to f/10. This removes spherical aberration while preserving the wide field of the short focal-length mirror. However, it does so at the cost of light-gathering ability. Although this solution was well-known long before Bernhard Schmidt invented his corrector plate, the design is sometimes given the retronymRetronym
A retronym is a type of neologism that provides a new name for an object or concept to differentiate the original form or version of it from a more recent form or version. The original name is most often augmented with an adjective to account for later developments of the object or concept itself...
"lensless Schmidt".
Schmidt-Väisälä
Prof. Yrjö VäisäläYrjö Väisälä
Yrjö Väisälä was a Finnish astronomer and physicist.His main contributions were in the field of optics, but he was also very active in geodetics, astronomy and optical metrology...
originally designed an "astronomical camera" similar to Bernhard Schmidt's "Schmidt camera", but the design was unpublished. Väisälä did mention it in lecture notes in 1924 with a footnote: "problematic spherical focal plane". Once Väisälä saw Schmidt's publication, he promptly went ahead and solved the field-flattening problem in Schmidt's design by placing a doubly convex lens slightly in front of the film holder. This resulting system is known as: Schmidt-Väisälä camera
Schmidt-Väisälä camera
The Schmidt-Väisälä camera is a type of astronomical telescope intended for wide-field photographic work. It was designed by Yrjö Väisälä.-Invention and design:...
or sometimes as Väisälä camera.
Baker-Schmidt
In 1940, James BakerJames Gilbert Baker
James Gilbert Baker was an American astronomer and designer of optics systems.-Biography:He was born in Louisville, Kentucky to Jesse B. Baker and Hattie M. Stallard, the fourth child of that couple. He attended Louisville duPont Manual High School then majored in mathematics at the University of...
of Harvard University
Harvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
modified the Schmidt camera design to include a convex secondary mirror, which reflected light back toward the primary. The photographic plate was then installed near the primary, facing the sky. This variant is called the Baker-Schmidt camera.
Baker-Nunn
The Baker-Nunn design, by Dr. Baker and Joseph NunnJoseph Nunn
Joseph Nunn was an American engineer.In 1956 he worked in collaboration with Dr. James G. Baker to design and manufacture a series of satellite tracking cameras. These were called Baker-Nunn cameras after their designers, and consisted of a very precise tracking system combined with an unusually...
, replaced the Baker-Schmidt camera's corrector plate with a small triplet corrector lens closer to the focus of the camera. It used 55 mm Cinemascope film. A dozen f/0.75 Baker-Nunn cameras with 20-inch apertures – each weighing 3.5 tons including a multiple axis mount allowing it to follow satellites in the sky – were used by the Smithsonian Astrophysical Observatory
Smithsonian Astrophysical Observatory
The Smithsonian Astrophysical Observatory is a research institute of the Smithsonian Institution headquartered in Cambridge, Massachusetts, where it is joined with the Harvard College Observatory to form the Harvard-Smithsonian Center for Astrophysics .-History:The SAO was founded in 1890 by...
to track artificial satellites from the late 1950s to mid 1970s.
Mersenne-Schmidt
The Mersenne-Schmidt camera consists of a concave paraboloidal primary mirror, a convex spherical secondary mirror, and a concave spherical tertiary mirror.Schmidt-Newtonian
The addition of a flat secondary mirrorSecondary mirror
A secondary mirror is the second deflecting or focusing mirror element in a reflecting telescope. Light gathered by the primary mirror is directed towards a focal point typically past the location of the secondary. Secondary mirrors in the form of an optically flat diagonal mirror are used to...
at 45° to the optical axis of the Schmidt design creates a Schmidt-Newtonian telescope
Schmidt-Newton telescope
A Schmidt–Newton telescope is a catadioptric telescope that combines elements from both the Schmidt camera and the Newtonian reflector. In this telescope design a spherical primary mirror is combined with a Schmidt corrector plate, making system that has less spherical aberration than a reflecting...
.
Schmidt-Cassegrain
The addition of a convex secondary mirror to the Schmidt design directing light through a hole in the primary mirror creates a Schmidt-Cassegrain telescopeSchmidt-Cassegrain telescope
The Schmidt–Cassegrain is a catadioptric telescope that combines a cassegrain reflector's optical path with a Schmidt corrector plate to make a compact astronomical instrument that uses simple spherical surfaces.-Invention and design:...
.
The last two designs are popular with telescope manufacturers because they are compact and use simple spherical optics.