Schmidt corrector plate
Encyclopedia
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 (sometimes called the Schmidt-Väisälä camera
). Schmidt originally designed it as part of a wide field photographic catadioptric telescope, the Schmidt camera
, and is also used in other telescope designs, camera lenses and image projection systems.
and just behind the prime focus for a Schmidt-Cassegrain
. The Schmidt corrector is thicker in the middle and the edge. This corrects the light paths so light reflected from the outer part of the mirror and light reflected from the inner portion of the mirror is brought to the same common focus "F". The Schmidt corrector only corrects for spherical aberration. It does not change the focal length of the system.
the corrector by grinding and polishing the aspherical shape into a flat glass blank using specially shaped and sized tools. This method requires a high degree of skill and training on the part of the optical engineer creating the corrector.
Schmidt himself worked out a second more elegant scheme for producing the complex figure needed for the correcting plate. A thin glass disk with a perfectly polished accurate flat form was placed on a heavy metal pan. The upper edge of the pan was ground at a precise angle or bevel
based on the coefficient of elasticity
of the particular type of glass plate that was being used. The glass plate was sealed to the ground edge of the pan, then a vacuum pump
was used to exhaust the air until a particular negative pressure had been achieved. This caused the glass plate to warp slightly. The exposed side was then ground and polished to a perfect flat. When the vacuum was released, the plate sprang back until its bottom surface was again plane, while the upper surface had the correct figure. Schmidt's vacuum figuring method is rarely used today. The glass plate will usually break if bent enough to generate a curve for telescopes of focal ratio f/2.5 or faster. Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.
A third method, invented in 1970 for Celestron
by Tom Johnson and John O'rourke uses a vacuum pan with the correct shape of the curve pre-shaped into the bottom of the pan, called a "master block". This removes the need to have to hold a shape by applying an exact vacuum and allows for the mass production of corrector plates of the same exact shape.
The technical difficulties associated with the production of Schmidt corrector plates led to some designers, such as Dmitri Dmitrievich Maksutov
and Albert Bouwers
, to come up with an alternative designs using a more conventional Meniscus corrector
lens.
Aspheric lens
An aspheric lens or asphere is a lens whose surface profiles are not portions of a sphere or cylinder. In photography, a lens assembly that includes an aspheric element is often called an aspherical lens....
which is designed to correct the spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|optic axis]]. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the...
in the spherical primary mirror it is combined with. It was invented by 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 1931, although it may have been independently invented by Finnish astronomer Yrjö Väisälä in 1924 (sometimes called the 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:...
). Schmidt originally designed it as part of a wide field photographic catadioptric telescope, the Schmidt camera
Schmidt camera
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....
, and is also used in other telescope designs, camera lenses and image projection systems.
Function
Schmidt corrector plates work because they are aspheric lenses with spherical aberration that is equal to but opposite of the spherical primary mirror they are placed in front of. It is placed at the center of curvature "C" of the mirror for a pure Schmidt cameraSchmidt camera
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....
and just behind the prime focus for a Schmidt-Cassegrain
Schmidt-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 Schmidt corrector is thicker in the middle and the edge. This corrects the light paths so light reflected from the outer part of the mirror and light reflected from the inner portion of the mirror is brought to the same common focus "F". The Schmidt corrector only corrects for spherical aberration. It does not change the focal length of the system.
Manufacture
Schmidt corrector plates can be manufactured in many ways. The most basic method, called the "classical approach", involves directly figuringFiguring
Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...
the corrector by grinding and polishing the aspherical shape into a flat glass blank using specially shaped and sized tools. This method requires a high degree of skill and training on the part of the optical engineer creating the corrector.
Schmidt himself worked out a second more elegant scheme for producing the complex figure needed for the correcting plate. A thin glass disk with a perfectly polished accurate flat form was placed on a heavy metal pan. The upper edge of the pan was ground at a precise angle or bevel
Bevel
A beveled edge refers to an edge of a structure that is not perpendicular to the faces of the piece. The words bevel and chamfer overlap in usage; in general usage they are often interchanged, while in technical usage they may sometimes be differentiated as shown in the image at right.-Cutting...
based on the coefficient of elasticity
Elasticity Coefficient
Elasticity Coefficients are used in Physics, Economics, Chemistry, or more generally in mathematics as a definition of point elasticity: the article below applies to Chemical/Biochemical Elasticity Coefficients....
of the particular type of glass plate that was being used. The glass plate was sealed to the ground edge of the pan, then a vacuum pump
Vacuum pump
A vacuum pump is a device that removes gas molecules from a sealed volume in order to leave behind a partial vacuum. The first vacuum pump was invented in 1650 by Otto von Guericke.- Types :Pumps can be broadly categorized according to three techniques:...
was used to exhaust the air until a particular negative pressure had been achieved. This caused the glass plate to warp slightly. The exposed side was then ground and polished to a perfect flat. When the vacuum was released, the plate sprang back until its bottom surface was again plane, while the upper surface had the correct figure. Schmidt's vacuum figuring method is rarely used today. The glass plate will usually break if bent enough to generate a curve for telescopes of focal ratio f/2.5 or faster. Also, for fast focal ratios, the curve obtained is not sufficiently exact and requires additional hand correction.
A third method, invented in 1970 for Celestron
Celestron
Celestron is a company that manufactures and imports telescopes, binoculars, spotting scopes, microscopes, and accessories for their products.-Origins and History:...
by Tom Johnson and John O'rourke uses a vacuum pan with the correct shape of the curve pre-shaped into the bottom of the pan, called a "master block". This removes the need to have to hold a shape by applying an exact vacuum and allows for the mass production of corrector plates of the same exact shape.
The technical difficulties associated with the production of Schmidt corrector plates led to some designers, such as Dmitri Dmitrievich Maksutov
Dmitri Dmitrievich Maksutov
Dmitry Dmitrievich Maksutov was a Russian / Soviet optical engineer and amateur astronomer. He is best known as the inventor of the Maksutov telescope.-Biography:...
and Albert Bouwers
Albert Bouwers
Albert A. Bouwers was a Dutch optical engineer. He is known for developing and working with X-Rays and various optical technologies as a high-level researcher at Philips research labs...
, to come up with an alternative designs using a more conventional Meniscus corrector
Meniscus corrector
A meniscus corrector is a negative meniscus lens that is used to correct spherical aberration in image-forming optical systems such as catadioptric telescopes...
lens.