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Cassegrain Focus
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system. In a symmetrical Cassegrain both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the center, thus permitting the light to reach an eyepiece, a camera, or an image sensor. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or to avoid the need for a hole in the primary mirror (or both). The classic Cassegrain ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cassegrain , a Catholic priest and teacher and probably the inventor of the Cassegrain Reflector
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Cassegrain may refer to * Cassegrain reflector, a design used in telescopes * Cassegrain antenna, a type of parabolic antenna * Cassegrain (crater), on the Moon * a Belgian canned vegetables producer now part of Bonduelle S.A. People : * Guillaume Cassegrain, a French sculptor * Giovanni Cassegrain, a French sculptor * Jean Cassegrain, a French businessman, founder of Longchamp in 1948 * Laurent Cassegrain Laurent Cassegrain (; – 1 September 1693) was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector, a folded two-mirror reflecting telescope design. Biography Laurent Cassegrain was born in the region of Char ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Journal Des Sçavans
The ''Journal des sçavans'' (later renamed ''Journal des savans'' and then ''Journal des savants,'' lit. ''Journal of the Learned''), established by Denis de Sallo, is the earliest academic journal published in Europe. It is thought to be the earliest published scientific journal. It currently focuses on European history and premodern literature. History The first issue appeared as a twelve-page quarto pamphlet on Monday, 5 January 1665. This was shortly before the first appearance of the '' Philosophical Transactions of the Royal Society,'' on 6 March 1665. The 18th-century French physician and encyclopédiste Louis-Anne La Virotte (1725–1759) was introduced to the journal through the protection of chancellor Henri François d'Aguesseau. Its content originally included obituaries of famous men, church history, the findings of other's in a scientific manner, and legal reports. Natural philosophy was part of its original scope. It is thought to be the first published scienti ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Henri Chrétien
Henri Jacques Chrétien (1 February 1879, Paris – 6 February 1956, Washington, D.C.) was a French astronomer and an inventor. Born in Paris, France, his most famous inventions are: - the anamorphic widescreen process, using an anamorphic lens system called Hypergonar, that resulted in the CinemaScope widescreen technique, and - the co-invention, with George Willis Ritchey, of the Ritchey–Chrétien telescope, an improved type of astronomical telescope, employing a system now used in virtually all large research telescopes. He spent part of his early astronomical career at the Nice Observatory, which was close to his house, the Villa Paradou. The Villa was built by famous French architect Charles Garnier who also built the Opera of Paris. In 1995, the abandoned villa was acquired by the artist Rainer Maria Latzke, who restored it and added new murals to the existing frescoes. Chrétien was one of the founders of the ''Institut d'optique théorique et appliquée'' and pr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
George Willis Ritchey
George Willis Ritchey (December 31, 1864 – November 4, 1945) was an American optician and telescope maker and astronomer born at Tuppers Plains, Ohio. Ritchey was educated as a furniture maker. He coinvented the Ritchey–Chrétien (R–C) reflecting telescope along with Henri Chrétien. The R-C prescription remains the predominant optical design for telescopes and has since been used for the majority of major ground-based and space-based telescopes. He worked closely with George Ellery Hale, first at Yerkes Observatory and later at Mt. Wilson Observatory. He played a major role in designing the mountings and making the mirrors of the Mt. Wilson and telescopes. Hale and Ritchey had a falling out in 1919, and Ritchey eventually went to Paris where he promoted the construction of very large telescopes. He returned to America in 1930 and obtained a contract to build a Ritchey-Chrétien telescope for the U.S. Naval Observatory. This last telescope produced by Ritchey rem ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Coma (optics)
In optics (especially telescopes), the coma (), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration. Overview Coma is an inherent property of telescopes using parabolic mirrors. Unlike a spherical mirror, a bundle of parallel rays parallel to the optical axis will be perfectly focused to a point (the mirror is free of spherical aberration), no matter where they strike the mirror. However, this is only true if the rays are parallel to the axis of the parabola. When the incoming rays strike t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Spherical Aberration
In optics, spherical aberration (SA) is a type of aberration found in optical systems that have elements with spherical surfaces. Lenses and curved mirrors are prime examples, because this shape is easier to manufacture. Light rays that strike a spherical surface off-centre are refracted or reflected more or less than those that strike close to the centre. This deviation reduces the quality of images produced by optical systems. Overview A spherical lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction of the lens material. A typical value of refractive index for crown glass is 1.5 (see list), which indicates that only about 43% of the area (67% of diameter) of a spherical lens is useful. It is often considered to be an imperfection of telescopes and other instruments which makes their focusing less than ideal due to the spherical shape of lenses and mirrors. This is an important ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Conic Constant
In geometry, the conic constant (or Schwarzschild constant, after Karl Schwarzschild) is a quantity describing conic sections, and is represented by the letter ''K''. The constant is given by K = -e^2, where is the eccentricity of the conic section. The equation for a conic section with apex at the origin and tangent to the y axis is y^2-2Rx+(K+1)x^2 = 0 alternately x = \dfrac where ''R'' is the radius of curvature at . This formulation is used in geometric optics to specify oblate elliptical (), spherical (), prolate elliptical (), parabolic (), and hyperbolic () lens and mirror surfaces. When the paraxial approximation In geometric optics, the paraxial approximation is a small-angle approximation used in Gaussian optics and ray tracing of light through an optical system (such as a lens). A paraxial ray is a ray which makes a small angle (''θ'') to the optica ... is valid, the optical surface can be treated as a spherical surface with the same radius. Some ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Radius Of Curvature (optics)
Radius of curvature (ROC) has specific meaning and sign convention in optical design. A spherical lens or mirror surface has a center of curvature located either along or decentered from the system local optical axis. The vertex of the lens surface is located on the local optical axis. The distance from the vertex to the center of curvature is the radius of curvature of the surface. The sign convention for the optical radius of curvature is as follows: * If the vertex lies to the left of the center of curvature, the radius of curvature is positive. * If the vertex lies to the right of the center of curvature, the radius of curvature is negative. Thus when viewing a biconvex lens from the side, the left surface radius of curvature is positive, and the right radius of curvature is negative. Note however that ''in areas of optics other than design'', other sign conventions are sometimes used. In particular, many undergraduate physics textbooks use the Gaussian sign conve ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Modulation Transfer Function
The optical transfer function (OTF) of an optical system such as a camera, microscope, human eye, or projector specifies how different spatial frequencies are captured or transmitted. It is used by optical engineers to describe how the optics project light from the object or scene onto a photographic film, detector array, retina, screen, or simply the next item in the optical transmission chain. A variant, the modulation transfer function (MTF), neglects phase effects, but is equivalent to the OTF in many situations. Either transfer function specifies the response to a periodic sine-wave pattern passing through the lens system, as a function of its spatial frequency or period, and its orientation. Formally, the OTF is defined as the Fourier transform of the point spread function (PSF, that is, the impulse response of the optics, the image of a point source). As a Fourier transform, the OTF is complex-valued; but it will be real-valued in the common case of a PSF that is symmetr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cassegrain Telescope
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas, the main characteristic being that the optical path folds back onto itself, relative to the optical system's primary mirror entrance aperture. This design puts the focal point at a convenient location behind the primary mirror and the convex secondary adds a telephoto effect creating a much longer focal length in a mechanically short system. In a symmetrical Cassegrain both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the center, thus permitting the light to reach an eyepiece, a camera, or an image sensor. Alternatively, as in many radio telescopes, the final focus may be in front of the primary. In an asymmetrical Cassegrain, the mirror(s) may be tilted to avoid obscuration of the primary or to avoid the need for a hole in the primary mirror (or both). The classic Cassegrain ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Catadioptric System
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 searchlights, 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 for many early optical systems. In the 1820s, Augustin-Jean Fresnel developed several catadioptric lighthouse reflectors. Léon Foucault developed a catadioptric microscope in 1859 to counteract aberrations of using a lens to image objects at high power. 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 t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
James Gregory (mathematician)
James Gregory FRS (November 1638 – October 1675) was a Scottish mathematician and astronomer. His surname is sometimes spelt as Gregorie, the original Scottish spelling. He described an early practical design for the reflecting telescope – the Gregorian telescope – and made advances in trigonometry, discovering infinite series representations for several trigonometric functions. In his book ''Geometriae Pars Universalis'' (1668) Gregory gave both the first published statement and proof of the fundamental theorem of the calculus (stated from a geometric point of view, and only for a special class of the curves considered by later versions of the theorem), for which he was acknowledged by Isaac Barrow. Biography Gregory was born in 1638. His mother Janet was the daughter of Jean and David Anderson and his father was John Gregory, an Episcopalian Church of Scotland minister, James was youngest of their three children and he was born in the manse at Drumoak, Aberdeenshire ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
