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A reflecting telescope (also called a reflector) is a
telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observ ...
that uses a single or a combination of
curved mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
s that reflect
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
and form an image. The reflecting telescope was invented in the 17th century by
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...
as an alternative to the
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...
which, at that time, was a design that suffered from severe
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use
mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
s, the design is sometimes referred to as a
catoptric Catoptrics (from grc-gre, κατοπτρικός ''katoptrikós'', "specular", from grc-gre, κάτοπτρον ''katoptron'' "mirror") deals with the phenomena of reflected light and image-forming optical systems using mirrors. A catoptric ...
telescope. From the time of Newton to the 1800s, the mirror itself was made of metal usually
speculum metal Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make different kinds of mirrors from personal grooming ...
. This type included Newton's first designs and even the largest telescopes of the 19th century, the Leviathan of Parsonstown with a 1.8 meter wide metal mirror. In the 19th century a new method using a block of glass coated with very thin layer of silver began to become more popular by the turn of the century. Common telescopes which led to the
Crossley Crossley, based in Manchester, United Kingdom, was a pioneering company in the production of internal combustion engines. Since 1988 it has been part of the Rolls-Royce Power Engineering group. More than 100,000 Crossley oil and gas engines ...
and Harvard reflecting telescopes, which helped establish a better reputation for reflecting telescopes as the metal mirror designs were noted for their drawbacks. Chiefly the metal mirrors only reflected about of the light and the metal would
tarnish Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in ...
. After multiple polishings and tarnishings, the mirror could lose its precise figuring needed. Reflecting telescopes became extraordinarily popular for astronomy and many famous telescopes, such as the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...
, and popular amateur models use this design. In addition, the reflection telescope principle was applied to other electromagnetic wavelengths, and for example,
X-ray telescope An X-ray telescope (XRT) is a telescope that is designed to observe remote objects in the X-ray spectrum. In order to get above the Earth's atmosphere, which is opaque to X-rays, X-ray telescopes must be mounted on high altitude rockets, balloon ...
s also use the reflection principle to make image-forming optics.


History

The idea that
curved mirrors A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
behave like lenses dates back at least to Alhazen's 11th century treatise on optics, works that had been widely disseminated in Latin translations in early modern Europe. Soon after the invention of the
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...
, Galileo,
Giovanni Francesco Sagredo Giovanni Francesco Sagredo (1571– 5 March 1620) was a Venetian mathematician and close friend of Galileo. He was also a friend and correspondent of English scientist William Gilbert.S. P. Thompson (1903) ''The Geographical Journal'' vol 2 ...
, and others, spurred on by their knowledge of the principles of curved mirrors, discussed the idea of building a telescope using a mirror as the image forming objective. There were reports that the Bolognese
Cesare Caravaggi Cesare, the Italian version of the given name Caesar, may refer to: Given name * Cesare, Marquis of Beccaria (1738–1794), an Italian philosopher and politician * Cesare Airaghi (1840–1896), Italian colonel * Cesare Arzelà (1847–1912), ...
had constructed one around 1626 and the Italian professor
Niccolò Zucchi Niccolò Zucchi (; December 6, 1586 – May 21, 1670) was an Italian Jesuit, astronomer, and physicist. As an astronomer he may have been the first to see the belts on the planet Jupiter (on May 17, 1630), and reported spots on Mars in 1640. H ...
, in a later work, wrote that he had experimented with a concave bronze mirror in 1616, but said it did not produce a satisfactory image. The potential advantages of using parabolic mirrors, primarily reduction of spherical aberration with no
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
, led to many proposed designs for reflecting telescopes. The most notable being James Gregory, who published an innovative design for a ‘reflecting’ telescope in 1663. It would be ten years (1673), before the experimental scientist Robert Hooke was able to build this type of telescope, which became known as the Gregorian telescope. Five years after Gregory designed his telescope and five years before Hooke built the first such Gregorian telescope,
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...
in 1668 built his own reflecting telescope, which is generally acknowledged as the first reflecting telescope. It used a spherically ground metal
primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...
and a small diagonal mirror in an optical configuration that has come to be known as the
Newtonian telescope The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newto ...
. Despite the theoretical advantages of the reflector design, the difficulty of construction and the poor performance of the
speculum metal Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make different kinds of mirrors from personal grooming ...
mirrors being used at the time meant it took over 100 years for them to become popular. Many of the advances in reflecting telescopes included the perfection of
parabolic mirror A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a Mirror, reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface ge ...
fabrication in the 18th century, silver coated glass mirrors in the 19th century (built by
Léon Foucault Jean Bernard Léon Foucault (, ; ; 18 September 1819 – 11 February 1868) was a French physicist best known for his demonstration of the Foucault pendulum, a device demonstrating the effect of Earth's rotation. He also made an early measurement ...
in 1858), long-lasting aluminum coatings in the 20th century,
segmented mirror A segmented mirror is an array of smaller mirrors designed to act as segments of a single large curved mirror. The segments can be either spherical or asymmetric (if they are part of a larger parabolic reflector). They are used as objectives for ...
s to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was
catadioptric 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, ...
telescopes such as the
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. The design was invented by Bernhard Schmidt in 1930. Some notable e ...
, which use both a spherical mirror and a lens (called a corrector plate) as primary optical elements, mainly used for wide-field imaging without spherical aberration. The late 20th century has seen the development of
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical tele ...
and
lucky imaging Lucky imaging (also called lucky exposures) is one form of speckle imaging used for astrophotography. Speckle imaging techniques use a high-speed camera with exposure times short enough (100 ms or less) so that the changes in the Earth's a ...
to overcome the problems of seeing, and reflecting telescopes are ubiquitous on
space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first operational telescopes were the American Orbiting Astronomical Observatory, OAO-2 launch ...
s and many types of
spacecraft A spacecraft is a vehicle or machine designed to fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, p ...
imaging devices.


Technical considerations

A curved
primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...
is the reflector telescope's basic optical element that 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 a
secondary mirror A secondary mirror (or secondary) 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 mirro ...
may be added to modify the optical characteristics and/or redirect the light to film, digital sensors, or an
eyepiece An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is named because it is usually the lens that is closest to the eye when someone looks through the device. The ...
for visual observation. The primary mirror in most modern telescopes is composed of a solid glass
cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infin ...
whose front surface has been ground to a
spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
or parabolic shape. A thin layer of
aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...
is vacuum deposited onto the mirror, forming a highly reflective
first surface mirror A first-surface mirror or front-surface mirror (also commonly abbreviated FS mirror or FSM) is a mirror with the reflective surface being above a backing, as opposed to the conventional, second-surface mirror with the reflective surface behind a ...
. Some telescopes use primary mirrors which are made differently. Molten glass is rotated to make its surface paraboloidal, and is kept rotating while it cools and solidifies. (See
Rotating furnace A rotating furnace is a device for making solid objects which have concave surfaces that are segments of axial symmetry, axially symmetrical paraboloids. Usually, the objects are made of glass. The furnace makes use of the fact, which was known alr ...
.) The resulting mirror shape approximates a desired paraboloid shape that requires minimal grinding and polishing to reach the exact figure needed.


Optical errors

Reflecting telescopes, just like any other optical system, do not produce "perfect" images. The need to image objects at distances up to infinity, view them at different wavelengths of light, along with the requirement to have some way to view the image the primary mirror produces, means there is always some compromise in a reflecting telescope's optical design. Because the primary mirror focuses light to a common point in front of its own reflecting surface almost all reflecting telescope designs have a
secondary mirror A secondary mirror (or secondary) 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 mirro ...
, film holder, or detector near that focal point partially obstructing the light from reaching the primary mirror. Not only does this cause some reduction in the amount of light the system collects, it also causes a loss in contrast in the image due to diffraction effects of the obstruction as well as
diffraction spike Diffraction spikes are lines radiating from bright light sources, causing what is known as the starburst effect or sunstars in photographs and in vision. They are artifacts caused by light diffracting around the support vanes of the secondary m ...
s caused by most secondary support structures. The use of mirrors avoids
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
but they produce other types of aberrations. A simple
spherical mirror A curved mirror is a mirror with a curved reflecting surface. The surface may be either ''convex'' (bulging outward) or ''concave'' (recessed inward). Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are ...
cannot bring light from a distant object to a common focus since the reflection of light rays striking the mirror near its edge do not converge with those that reflect from nearer the center of the mirror, a defect called spherical aberration. To avoid this problem most reflecting telescopes use parabolic shaped mirrors, a shape that can focus all the light to a common focus. Parabolic mirrors work well with objects near the center of the image they produce, (light traveling parallel to the mirror's
optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propaga ...
), but towards the edge of that same field of view they suffer from off axis aberrations: * Coma – an aberration where point sources (stars) at the center of the image are focused to a point but typically appears as "comet-like" radial smudges that get worse towards the edges of the image. *
Field curvature Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis (or a non-flat object past the hyperfocal distance) cannot be brought properly into focus on a flat image pl ...
– 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. It is sometimes corrected by a field flattening lens. *
Astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. This results in distorted or blurred vision at any distance. Other symptoms can include eyestrain, headaches, and trouble driving at n ...
– an
azimuth An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north. Mathematical ...
al variation of focus around the aperture causing point source images off-axis to appear elliptical. Astigmatism is not usually a problem in a narrow
field of view The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation. Human ...
, but in a wide field image it gets rapidly worse and varies quadratically with field angle. *
Distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...
– Distortion does not affect image quality (sharpness) but does affect object shapes. It is sometimes corrected by image processing. There are reflecting telescope designs that use modified mirror surfaces (such as the Ritchey–Chrétien telescope) or some form of correcting lens (such as catadioptric telescopes) that correct some of these aberrations.


Use in astronomical research

Nearly all large research-grade astronomical telescopes are reflectors. There are several reasons for this: * Reflectors work in a wider
spectrum of light The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...
since certain wavelengths are absorbed when passing through glass elements like those found in a
refractor A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...
or in a
catadioptric telescope 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 ...
. * In a
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
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
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
s travels through a medium other than
vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
at different speeds. This causes
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the w ...
. Reducing this to acceptable levels usually involves a combination of two or three aperture sized lenses (see
achromat An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane. The most comm ...
and
apochromat An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses. Explanation Chromatic aberration is the phenomenon of differen ...
for more details). The cost of such systems therefore scales significantly with aperture size. An image obtained from a mirror does not suffer from chromatic aberration to begin with, and the cost of the mirror scales much more modestly with its size. * 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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stro ...
, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter. 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.


Reflecting telescope designs


Gregorian

The Gregorian telescope, described by Scottish astronomer and mathematician James Gregory in his 1663 book ''Optica Promota'', employs a concave secondary mirror that reflects the image back through a hole in the primary mirror. This produces an upright image, useful for terrestrial observations. Some small
spotting scope A spotting scope is a compact high-power telescope optimized for detailed observation of distant objects. They are used as portable optical enhancement devices for various outdoor activities such as birdwatching, skygazing and other naturali ...
s are still built this way. There are several large modern telescopes that use a Gregorian configuration such as the
Vatican Advanced Technology Telescope The 1.8 meter Alice P. Lennon Telescope and its Thomas J. Bannan Astrophysics Facility, known together as the Vatican Advanced Technology Telescope (VATT), is a Gregorian telescope observing in the optical and infrared situated on Mount Graham i ...
, the
Magellan telescopes The Magellan Telescopes are a pair of optical telescopes located at Las Campanas Observatory in Chile. The two telescopes are named after the astronomer Walter Baade and the philanthropist Landon T. Clay. First light for the telescopes was on ...
, the
Large Binocular Telescope The Large Binocular Telescope (LBT) is an optical telescope for astronomy located on Mount Graham, in the Pinaleno Mountains of southeastern Arizona, United States. It is a part of the Mount Graham International Observatory. When using both ...
, and the
Giant Magellan Telescope The Giant Magellan Telescope (GMT) is a ground-based extremely large telescope under construction, as part of the US Extremely Large Telescope Program (US-ELTP), . It will consist of seven 8.4 m (27.6 ft) diameter primary segments, that ...
.


Newtonian

The
Newtonian telescope The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newto ...
was the first successful reflecting telescope, completed by
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, theologian, and author (described in his time as a " natural philosopher"), widely recognised as one of the grea ...
in 1668. It usually has a paraboloid primary mirror but at
focal ratio In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
s of about f/10 or longer a spherical primary mirror can be 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.


The Cassegrain design and its variations

The cassegrain telescope (sometimes called the "Classic Cassegrain") was first published in a 1672 design attributed to Laurent Cassegrain. It has a parabolic primary mirror, and a hyperbolic secondary mirror that reflects the light back down through a hole in the primary. The folding and diverging effect of the secondary mirror creates a telescope with a long focal length while having a short tube length.


Ritchey–Chrétien

The Ritchey–Chrétien telescope, invented by George Willis Ritchey and Henri Chrétien in the early 1910s, 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 nearly flat focal plane if the primary and secondary curvature are properly figured, making it well suited for wide field and photographic observations. Almost every professional reflector telescope in the world is of the Ritchey–Chrétien design.


Three-mirror anastigmat

Including a third curved mirror allows correction of the remaining distortion, astigmatism, from the Ritchey–Chrétien design. This allows much larger fields of view.


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 ''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it ...
'' 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 A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...
secondary. While this system is easier to grind than a classic Cassegrain or Ritchey–Chrétien system, it does not correct for off-axis coma. Field curvature is actually less than a classical Cassegrain. Because this is less noticeable at longer
focal ratio In optics, the f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil ("clear aperture").Smith, Warren ''Modern Optical Engineering'', 4th Ed., 2007 McGraw-Hill Pro ...
s, Dall–Kirkhams are seldom faster than f/15.


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 An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propaga ...
, commonly called
off-axis optical system An off-axis optical system is an optical system in which the optical axis of the aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system det ...
s.


Herschelian

The Herschelian reflector is named after
William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...
, who used this design to build very large telescopes including the
40-foot telescope William Herschel's 40-foot telescope, also known as the Great Forty-Foot telescope, was a reflecting telescope constructed between 1785 and 1789 at Observatory House in Slough, England. It used a diameter primary mirror with a focal length ...
in 1789. In the Herschelian reflector the primary mirror is tilted so the observer's 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
speculum metal Speculum metal is a mixture of around two-thirds copper and one-third tin, making a white brittle alloy that can be polished to make a highly reflective surface. It was used historically to make different kinds of mirrors from personal grooming ...
mirrors of that time
tarnish Tarnish is a thin layer of corrosion that forms over copper, brass, aluminum, magnesium, neodymium and other similar metals as their outermost layer undergoes a chemical reaction. Tarnish does not always result from the sole effects of oxygen in ...
ed quickly and could only achieve 60% reflectivity.


Schiefspiegler

A variant of the Cassegrain, the Schiefspiegler telescope ("skewed" or "oblique reflector") 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. A number of variations are common, with varying numbers of mirrors of different types. The Kutter (named after its inventor Anton Kutter) style uses a single concave primary, a convex secondary and a plano-convex lens between the secondary mirror and the focal plane, when needed (this is the case of the ''catadioptric Schiefspiegler''). 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.


Stevick-Paul

Stevick-Paul telescopes are off-axis versions of Paul 3-mirror systems with an added flat diagonal mirror. A convex secondary mirror is placed just to the side of the light entering the telescope, and positioned afocally so as to send parallel light on to the tertiary. The concave tertiary mirror is positioned exactly twice as far to the side of the entering beam as was the convex secondary, and its own radius of curvature distant from the secondary. Because the tertiary mirror receives parallel light from the secondary, it forms an image at its focus. The focal plane lies within the system of mirrors, but is accessible to the eye with the inclusion of a flat diagonal. The Stevick-Paul configuration results in all optical aberrations totaling zero to the third-order, except for the Petzval surface which is gently curved.


Yolo

The Yolo was developed by Arthur S. Leonard in the mid-1960s. Like the Schiefspiegler, it is an unobstructed, tilted reflector telescope. The original Yolo consists of a primary and secondary concave mirror, with the same curvature, and the same tilt to the main axis. Most Yolos use
toroidal reflector A toroidal mirror is a reflector whose surface is a section of a torus, defined by two radii of curvature. Such reflectors are easier to manufacture than mirrors with a surface described by a paraboloid or ellipsoid. They suffer from spherical abe ...
s. 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. Like Schiefspieglers, many Yolo variations have been pursued. The needed amount of toroidal shape can be transferred entirely or partially to the primary mirror. In large focal ratios optical assemblies, both primary and secondary mirror can be left spherical and a spectacle correcting lens is added between the secondary mirror and the focal plane (''catadioptric Yolo''). The addition of a convex, long focus tertiary mirror leads to Leonard's ''Solano'' configuration. The Solano telescope doesn't contain any toric surfaces.


Liquid-mirror telescopes

One design of telescope uses a rotating mirror consisting of a liquid metal in a tray that is spun at constant speed. As the tray spins, the liquid forms a paraboloidal surface of essentially unlimited size. This allows making very big telescope mirrors (over 6 metres), but unfortunately they cannot be steered, as they always point vertically.


Focal planes


Prime focus

In a ''prime focus'' design no secondary optics are used, the image is accessed at the focal point of the
primary mirror A primary mirror (or primary) is the principal light-gathering surface (the objective) of a reflecting telescope. Description The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective meta ...
. At the focal point is some type of structure for holding a film plate or electronic detector. In the past, in very large telescopes, an observer would sit inside the telescope in an "observing cage" to directly view the image or operate a camera. Nowadays CCD cameras allow for remote operation of the telescope from almost anywhere in the world. The space available at prime focus is severely limited by the need to avoid obstructing the incoming light. 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.


Cassegrain focus

For telescopes built to the Cassegrain design or other related designs, the image is formed behind the primary mirror, at the focal point of the
secondary mirror A secondary mirror (or secondary) 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 mirro ...
. An observer views through the rear of the telescope, or a camera or other instrument is mounted on the rear. Cassegrain focus is commonly used for amateur telescopes or smaller research telescopes. However, for large telescopes with correspondingly large instruments, an instrument at Cassegrain focus must move with the telescope as it slews; this places additional requirements on the strength of the instrument support structure, and potentially limits the movement of the telescope in order to avoid collision with obstacles such as walls or equipment inside the observatory.


Nasmyth and coudé focus


Nasmyth

The Nasmyth design is similar to the Cassegrain except the light is not directed through a hole in the primary mirror; instead, a third mirror reflects the light to the side of the telescope to allow for the mounting of heavy instruments. This is a very common design in large research telescopes.


Coudé

Adding further optics to a Nasmyth-style telescope to deliver the light (usually through the declination axis) to a fixed focus point that does not move as the telescope is reoriented gives a coudé focus (from the French word for elbow). The coudé focus gives a narrower field of view than a Nasmyth focus and is used with very heavy instruments that do not need a wide field of view. One such application is high-resolution spectrographs that have large collimating mirrors (ideally with the same diameter as the telescope's primary mirror) and very long focal lengths. Such instruments could not withstand being moved, and adding mirrors to the light path to form a ''coudé train'', diverting the light to a fixed position to such an instrument housed on or below the observing floor (and usually built as an unmoving integral part of the observatory building) was the only option. The
60-inch Hale telescope The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observat ...
(1.5 m),
Hooker Telescope The Mount Wilson Observatory (MWO) is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles. The observat ...
,
200-inch Hale Telescope The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in San Diego County, California, US, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation in 1928, he orchestrated the planning, de ...
,
Shane Telescope The C. Donald Shane telescope is a 120-inch (3.05-meter) reflecting telescope located at the Lick Observatory in San Jose, California, San Jose, California. It was named after astronomer C. Donald Shane in 1978, who led the effort to acquire the ne ...
, and Harlan J. Smith Telescope all were built with coudé foci instrumentation. The development of
echelle An echelle grating (from French ''échelle'', meaning "ladder") is a type of diffraction grating characterised by a relatively low groove density, but a groove shape which is optimized for use at high incidence angles and therefore in high diffract ...
spectrometers allowed high-resolution spectroscopy with a much more compact instrument, one which can sometimes be successfully mounted on the Cassegrain focus. Since inexpensive and adequately stable computer-controlled alt-az telescope mounts were developed in the 1980s, the Nasmyth design has generally supplanted the coudé focus for large telescopes.


Fibre-fed spectrographs

For instruments requiring very high stability, or that are very large and cumbersome, it is desirable to mount the instrument on a rigid structure, rather than moving it with the telescope. Whilst transmission of the full field of view would require a standard coudé focus, spectroscopy typically involves the measurement of only a few discrete objects, such as stars or galaxies. It is therefore feasible to collect light from these objects with
optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparent fiber made by drawing glass ( silica) or plastic to a diameter slightly thicker than that of a human hair. Optical fibers are used most often as a mea ...
s at the telescope, placing the instrument at an arbitrary distance from the telescope. Examples of fiber-fed spectrographs include the planet-hunting spectrographs
HARPS The High Accuracy Radial Velocity Planet Searcher (HARPS) is a high-precision echelle planet-finding spectrograph installed in 2002 on the ESO's 3.6m telescope at La Silla Observatory in Chile. The first light was achieved in February 2003. ...
or
ESPRESSO Espresso (, ) is a coffee-brewing method of Italian origin, in which a small amount of nearly boiling water (about ) is forced under of pressure through finely-ground coffee beans. Espresso can be made with a wide variety of coffee beans a ...
. Additionally, the flexibility of optical fibers allow light to be collected from any focal plane; for example, the HARPS spectrograph utilises the Cassegrain focus of the
ESO 3.6 m Telescope The ESO 3.6 m Telescope is an optical reflecting telescope run by the European Southern Observatory at La Silla Observatory, Chile since 1977, with a clear aperture of about and area. The telescopes uses the HARPS instrument and has discovered ...
, whilst the Prime Focus Spectrograph is connected to the prime focus of the Subaru telescope.


See also

* Catadioptric telescopes *
Honeycomb mirror A honeycomb mirror is a large mirror usually used as the primary mirror in astronomical reflecting telescopes whose face is supported by a ribbed structure that resembles a honeycomb. The design provides sufficient rigidity for ultra-high-precision ...
*
List of largest optical reflecting telescopes A ''list'' is any set of items in a row. List or lists may also refer to: People * List (surname) Organizations * List College, an undergraduate division of the Jewish Theological Seminary of America * SC Germania List, German rugby unio ...
*
List of largest optical telescopes historically Telescopes have grown in size since they first appeared around 1608. The following tables list the increase in size over the years. Different technologies can and have been used to build telescopes, which are used to magnify distant views especia ...
*
List of telescope types The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
*
Mirror support cell In astronomy, a mirror support cell - more commonly mirror cell - is a component of a reflecting telescope that supports the mirror in place to hold optical alignment, allow collimation adjustment, and protect it from falling out. The common usa ...
*
PLate OPtimizer PLate OPtimizer, or PLOP is a CAD program used by amateur telescope makers to design primary mirror support cells for reflecting telescopes. It was developed by telescope maker David Lewis, first described in 1999, and used to simplify calculati ...
*
Refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and a ...


References


External links


Who was James Gregory? Reflecting Telescopes, Explore, National Museums Scotland
{{DEFAULTSORT:Reflecting Telescope Telescope types 1663 introductions 1663 in science 17th-century inventions