 24 inch convertible Newtonian/Cassegrain reflecting telescope on display at the Franklin Institute. A reflecting telescope (also called a reflector) is an optical telescope which uses a single or combination of curved mirrors that reflect light and form an image. The Reflecting telescope was invented in the 17th century as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives and is used almost exclusively in major research telescopes. Reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes uses mirrors, the design is sometimes referred to as a "catoptric" telescope. Image File history File linksMetadata Download high-resolution version (900x696, 87 KB) 24 inch convertable Newtonian/Cassegrain reflecting telescope on display in the Franklin Institute rooftop observatory (has since been moved to another location). ...
Image File history File linksMetadata Download high-resolution version (900x696, 87 KB) 24 inch convertable Newtonian/Cassegrain reflecting telescope on display in the Franklin Institute rooftop observatory (has since been moved to another location). ...
Franklin Institute Front steps as seen from the adjacent Moore College This article is about the science museum in Philadelphia. ...
Eight Inch refracting telescope. ...
Reflections in a spherical convex mirror. ...
For other uses, see Light (disambiguation). ...
Image of a refracting telescope from the Cincinnati Observatory in 1848 A refracting or refractor telescope is a dioptric telescope that uses a lens as its objective to form an image. ...
On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing visible chromatic aberration (especially at the dark edges on the right). ...
Several objective lenses on a microscope. ...
This article is about wave reflectors (mainly, specular reflection of visible light). ...
This article belongs in one or more categories. ...
History
The Italian professor Niccolò Zucchi is credited with making the first reflector in 1616. With it, in 1630 he observed two bands on Jupiter, and in 1640 observed spots on Mars. But his inability to shape the concave mirror accurately, and the lack of means of viewing the image without blocking the mirror, meant others did not adopt Zucchi's design. In 1663 James Gregory published Optica Promota which described the first practical design of a reflector using two concave mirrors. A working example was not built until 10 years later by Robert Hooke. Sir Isaac Newton is credited with constructing the first "practical" reflecting telescope after his own design in 1668.[1] Newton's added a smaller "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle. This allowed the user to view the image without obstructing the incoming light. Newton invented his reflector to solve the problem of chromatic aberration, a serious degradation in all refracting telescopes before the perfection of achromatic lenses. Niccolò Zucchi (December 6, 1586 - May 21, 1670) was an Italian Jesuit astronomer and physicist. ...
For other uses, see Jupiter (disambiguation). ...
This article is about the planet. ...
James Gregory For other people with the same name, see James Gregory. ...
Robert Hooke, FRS (July 18, 1635 – March 3, 1703) was an English polymath who played an important role in the scientific revolution, through both experimental and theoretical work. ...
Sir Isaac Newton FRS (4 January 1643 – 31 March 1727) [ OS: 25 December 1642 – 20 March 1727][1] was an English physicist, mathematician, astronomer, natural philosopher, and alchemist. ...
On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing visible chromatic aberration (especially at the dark edges on the right). ...
Image of a refracting telescope from the Cincinnati Observatory in 1848 A refracting or refractor telescope is a dioptric telescope that uses a lens as its objective to form an image. ...
Diagram of an achromatic lens (doublet). ...
Technical considerations A curved primary mirror is the reflector telescope's basic optical element and creates an image at the focal plane. The distance from the mirror to the focal plane is called the focal length. Film or a digital sensor may be located here to record the image, or an eyepiece for visual observation or a mirror that reflects the image to an eyepiece. This article is about focal length related to lenses and systems of lenses. ...
A collection of different types of eyepieces. ...
The primary mirror in most modern telescopes is composed of a solid glass cylinder whose front surface has been ground to a spherical or parabolic shape. A thin layer of aluminum is vacuum deposited onto the mirror, forming a highly reflective front surface. Early reflecting telescopes used a metal objective called a speculum. Look up cylinder in Wiktionary, the free dictionary. ...
A spherical reflector is a reflector shaped like a portion of a sphere, so that light (or other electromagnetic radiation such as radio waves) from a point-source located at the virtual centre point of the sphere will reflect off the mirrored surface and back through the centre point of...
A parabolic reflector (also known as a parabolic dish or a parabolic mirror) is a reflective device, formed in the shape of a paraboloid of revolution. ...
Aluminum is a soft and lightweight metal with a dull silvery appearance, due to a thin layer of oxidation that forms quickly when it is exposed to air. ...
Vacuum deposition is a process used to create a thin layer of a substance (a coating) on a solid object (the substrate). ...
Speculum metal is an alloy of four parts copper to one part tin. ...
Mirrors eliminate the risk of chromatic aberration but may still produce other types of aberrations: In general, on axis they may produce spherical aberration, in which case the outer and inner zones of the telescope do not share a common focus. This was the construction flaw in the Hubble Space Telescope mirrors. Spherical aberration can be eliminated with aspheric (non-spherical) mirrors. Off axis, additional aberrations may become apparent: On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing visible chromatic aberration (especially at the dark edges on the right). ...
This article or section is in need of attention from an expert on the subject. ...
The Hubble Space Telescope (HST; also known colloquially as the Hubble or just Hubble) is a space telescope that was carried into Earth orbit by the Space Shuttle in April 1990. ...
- Coma - a variation of telescope magnification with radial zone on the mirror typically appears as a radial smudging of the images which gets worse at the edges of the field. Spherical aberration and coma are eliminated in two mirror Ritchey Chretien designs.
- The best image plane is in general curved, which may not correspond to the detector's shape and leads to a focus error across the field.
- Astigmatism, an azimuthal variation of focus around the aperture. Near the center of the field astigmatism is not usually a problem, but it gets rapidly worse once it becomes apparent - it varies quadratically with field angle.
- Distortion over the field of view. Distortion does not affect image quality (sharpness) but does affect object shapes. It can be corrected by image processing.
There are reflector designs and modifications such as catadioptrics that correct some of these aberrations. In optics (especially telescopes), the coma in an optical system refers to monochromatic 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. ...
In optics, astigmatism (from Greek: α- a- without + στίγματος stigmatos, gen. ...
Azimuth is the horizontal component of a direction (compass direction), measured around the horizon, from the north toward the east (i. ...
The field of view is the part of the observable world that is seen at any given moment. ...
Optical systems which involve both lenses and mirrors. ...
Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this:
- In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
- Light of different wavelengths travels through a medium other than vacuum at different speeds. This causes chromatic aberration in uncorrected lenses and creating an aberration-free large lens is a costly process. A mirror can eliminate this problem entirely.
- Reflectors work in a wider spectrum of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
- There are structural problems involved in manufacturing and manipulating large-aperture lenses. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter[2]. In contrast, a mirror can be supported by the whole side opposite its reflecting face, allowing for reflecting telescope designs that can overcome gravitational sag. The largest reflector designs currently exceed 10 meters in diameter.
While the Newtonian focus design is still used in amateur astronomy, professionals now tend to use prime focus, Cassegrain focus, and coudé focus designs. This article is about the optical device. ...
For other uses, see Wavelength (disambiguation). ...
Look up Vacuum in Wiktionary, the free dictionary. ...
On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing visible chromatic aberration (especially at the dark edges on the right). ...
The optical spectrum (light or visible spectrum) is the portion of the electromagnetic spectrum that is visible to the human eye. ...
Gravity is a force of attraction that acts between bodies that have mass. ...
It has been suggested that this article or section be merged with Skygazing. ...
Reflecting telescope designs
Newtonian -
The Newtonian usually has a paraboloid primary mirror but for small apertures, say 12 cm or less, if the focal ratio is f/8 or longer a spherical primary mirror is sufficient for high visual resolution. A flat secondary mirror reflects the light to a focal plane at the side of the top of the telescope tube. It is one of the simplest and least expensive designs for a given size of primary, and is popular with amateur telescope makers as a home-build project. Newtonian Telescope The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727), using a parabolic primary mirror and a flat diagonal secondary mirror. ...
Image File history File links Simple Diagram of a Newtonian Telescope created by TMoore using MSPaint. ...
Image File history File links Simple Diagram of a Newtonian Telescope created by TMoore using MSPaint. ...
Newtonian Telescope The Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton (1643-1727), using a parabolic primary mirror and a flat diagonal secondary mirror. ...
A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...
A 22 Newtonian reflector sits in front of the clubhouse at Stellafane, home of the Springfield Telescope Makers The field of amateur telescope making is considered an offshoot of the amateur astronomy community. ...
See also The optical design of this telescope combines elements from both the Schmidt camera and the Newtonian telescope. ...
The Cassegrain design and its variations -
 Light path in a Cassegrain The Cassegrain (sometimes called the "Classic Cassegrain") has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. Folding the optics makes this a compact design. On smaller telescopes, and camera lenses, the secondary is often mounted on an optically flat, optically clear glass plate that closes the telescope tube. This support eliminates the "star-shaped" diffraction effects caused by a straight-vaned support spider. The closed tube stays clean, and the primary is protected, at the cost of some loss of light-gathering power. Light path in a Cassegrain reflector Laurent Cassegrain was a catholic priest born in the region of Chartres around 1629 and died at Chaudon (Eure-et-Loir) on August 31, 1693. ...
Image File history File links No higher resolution available. ...
Image File history File links No higher resolution available. ...
Light path in a Cassegrain reflector Laurent Cassegrain was a catholic priest born in the region of Chartres around 1629 and died at Chaudon (Eure-et-Loir) on August 31, 1693. ...
Diffraction spikes seen from various stars in the Pleiades. ...
Ritchey-Chrétien -
Main article: Ritchey-Chrétien telescope The Ritchey-Chrétien is a specialized Cassegrain reflector which has two hyperbolic mirrors (instead of a parabolic primary). It is free of coma and spherical aberration at a flat focal plane if the primary and secondary curvature are equal[citation needed], making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey-Chrétien design. It was invented by George Willis Ritchey and Henri Chrétien in the early 1910s. The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a relatively large field of view as compared to a more conventional configuration. ...
The Ritchey-Chrétien telescope or RCT is a specialized Cassegrain telescope designed to eliminate coma, thus providing a relatively large field of view as compared to a more conventional configuration. ...
In optics (especially telescopes), the coma in an optical system refers to monochromatic 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. ...
Ritchey 24 reflecting Telescope George Willis Ritchey (December 31, 1864 – November 4, 1945) was an American optician and telescope maker and astronomer (educated as a furniture maker) born at Tuppers Plains, Ohio. ...
Henri Chrétien (February 1, 1879 – February 6, 1956) was a French astronomer and an inventor. ...
Dall-Kirkham The Dall-Kirkham cassegrain telescope's design was created by Horace Dall in 1928 and took on the name in an article published in Scientific American in 1930 following discussion between amateur astronomer Allan Kirkham and Albert G. Ingalls, the magazine editor at the time. It uses a concave elliptical primary mirror and a convex spherical secondary. While this system is easier to grind than a classic Cassegrain or Ritchey-Chretien system, it does not correct for off-axis coma and field curvature so the image degrades quickly off-axis. Because this is less noticeable at longer focal ratios, Dall-Kirkhams are seldom faster than f/15. Scientific American is a popular-science magazine, published (first weekly and later monthly) since August 28, 1845, making it the oldest continuously published magazine in the United States. ...
Elliptical redirects here. ...
For other uses, see sphere (disambiguation). ...
A 35mm lens set to f/11, as indicated by the white dot above the f-stop scale on the aperture ring In photography the f-number (focal ratio) expresses the diameter of the diaphragm aperture in terms of the effective focal length of the lens. ...
See also: A Schmidt-Cassegrain Telescope, invented by Bernhard Schmidt, is a catadioptric telescope. ...
A 150mm aperture Maksutov-Cassegrain telescope. ...
Gregorian -
 Light path in a Gregorian telescope The Gregorian telescope, invented by James Gregory, employs a concave, not convex, secondary mirror and in this way achieves an upright image, useful for terrestrial observations. Some small spotting scopes are still built this way. The Steward Observatory Mirror Lab has been making mirrors for large Gregorian telescopes since 1994. These telescopes are the 6.5 meter diameter Magellan telescopes, the Large Binocular Telescope, and the Giant Magellan Telescope. The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer, James Gregory in the 17th century. ...
Image File history File links Gregorian_Telescope_Lightpath. ...
Image File history File links Gregorian_Telescope_Lightpath. ...
The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer, James Gregory in the 17th century. ...
James Gregory For other people with the same name, see James Gregory. ...
Swarovski ATS80-HD, an angled, typical high-end 80 mm spotting scope A spotting scope is a portable telescope, optimized for the observation of terrestrial objects. ...
Steward Observatorys main office is located on the University of Arizona campus. ...
The Magellan Telescopes are 6. ...
The Large Binocular Telescope (LBT, originally named the Columbus Project) is located on 10,700-foot Mount Graham in the Pinaleno Mountains of southeastern Arizona and is a part of the Mount Graham International Observatory. ...
The Giant Magellan Telescope is a ground-based telescope planned for completion in 2016. ...
Off-axis designs There are several designs that try to avoid obstructing the incoming light by eliminating the secondary or moving any secondary element off the primary mirror's optical axis, commonly called off-axis optical systems. In telecommunication, the term optical axis has the following meanings: 1. ...
In telecommunication, an off-axis optical system is an optical system in which the optical axis of the aperture is not coincident with the mechanical center of the aperture. ...
Herschelian The Herschelian reflector is named after William Herschel, who used this design to build very large telescopes including a 49½ inche (126 cm) diameter telescope in 1789. The design is similar to Niccolò Zucchi's early attempts to build a reflector where the observer stands in front of the objective to view the image. In the Herschelian reflector the primary mirror is tilted so the observers head does not block the incoming light. Although this introduces geometrical aberrations, Herschel employed this design to avoid the use of a newtonian secondary mirror since the fast tarnishing speculum metal mirrors of that time could only achieve 60% reflectivity[3]. For other persons named William Herschel, see William Herschel (disambiguation). ...
Tarnish is a layer of corrosion that develops over copper, brass, silver, aluminum as well as a degree of semi-reactive metals as they undergo oxidation. ...
Speculum metal is an alloy of four parts copper to one part tin. ...
Schiefspiegler A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector"), which uses tilted mirrors to avoid the secondary mirror casting a shadow on the primary. However, while eliminating diffraction patterns this leads to an increase in coma and astigmatism. These defects become manageable at large focal ratios - most Schiefspieglers use f/15 or longer, which tends to restrict useful observation to the moon and planets.  The light path of Schiefspiegler and Yolo optical systems. A number of variations are common, with varying numbers of mirrors of different types. The Kutter style uses a single concave primary and a convex secondary. One variation of a multi-schiefspiegler uses a concave primary, convex secondary and a parabolic tertiary. One of the interesting aspects of some Schiefspieglers is that one of the mirrors can be involved in the light path twice - each light path reflects along a different meridional path.
Yolo The Yolo was developed by Arthur S. Leonard in the mid 1960's [1]. Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. The Yolo design eliminates coma, but leaves significant astigmatism, which is reduced by deformation of the secondary mirror by some form of warping harness, or alternatively, polishing a toroidal figure into the secondary.
Focal planes
Prime focus In a prime focus design in large observatory telescopes, the observer sits inside the telescope, at the focal point of the reflected light. In the past this would be the astronomer himself, but nowadays CCD cameras are used. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light. Child – 5:16 All I Need – 3:55 Drifting – 6:43 Hold On – 4:40 Open Me – 3:35 Beautiful – 5:44 Look In – 4:14 Without You – 4:55 Live It – 7:23 Dont Walk Away – 3:04 Lead Me On – 5:34 Rest – 5:06 Child [Piano...
A specially developed CCD used for ultraviolet imaging in a wire bonded package. ...
Radio telescopes often have a prime focus design. The mirror is replaced by a metal surface for reflecting radio waves, and the observer is an antenna. The 64 meter radio telescope at Parkes Observatory A radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. ...
Radio frequency, or RF, refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna. ...
A yagi antenna Most simply, an antenna is an electronic component designed to send or receive radio waves. ...
See also: Optical ray paths inside Schmidt camera 2m Schmidt Camera (Alfred-Jensch-Telescope Tautenburg, Thuringia, Germany A Schmidt camera is an astronomical camera designed to provide wide fields of view with limited aberrations. ...
Nasmyth and Coudé focus
 Nasmyth/Coudé light path. Image File history File links Nasmyth-Telescope. ...
Image File history File links Nasmyth-Telescope. ...
Nasmyth -
The Nasmyth design is similar to the Cassegrain except no hole is drilled in the primary mirror; instead, a third mirror reflects the light to the side. Simple Nasmyth telescope The Nasmyth telescope is a reflecting telescope developed by James Nasmyth. ...
Coudé Adding further optics to a Nasmyth style telescope that deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives you a Coudé focus. This design is often used on large observatory telescopes, as it allows heavy observation equipment, such as spectrographs, to be more easily used. In astronomy, declination (abbrev. ...
For Acoustic uses in spectrographs of sound waves, see below. ...
References - ^ Reflecting Telescope Optics I, R. N. Wilson, A&A Library, Springer, 1996 (ISBN 0941-7834)
- ^ "Physics Demystified" By Stan Gibilisco, ISBN 0071382011, page 515
- ^ brunelleschi.imss.fi.it - Institute and Museum of the History of Science - Florence, Italy, Telescope, glossary
See also The following is a list of the largest optical reflecting telescopes sorted by mirror diameter. ...
A Large liquid mirror telescope or (Large LMT) is a technology being pursued by NASA and the University of British Columbia (UBC). ...
Optical systems which involve both lenses and mirrors. ...
A 13-inch, f/5. ...
Image of a refracting telescope from the Cincinnati Observatory in 1848 A refracting or refractor telescope is a dioptric telescope that uses a lens as its objective to form an image. ...
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