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A telescope is an
optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common examples include periscopes, microscopes, ...
using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. The first known practical telescopes were
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope de ...
s invented in the
Netherlands ) , national_anthem = ( en, "William of Nassau") , image_map = EU-Netherlands.svg , map_caption = , image_map2 = BES islands location map.svg , map_caption2 = , image_map3 ...

Netherlands
at the beginning of the 17th century, by using glass
lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simple ...

lenses
. They were used for both terrestrial applications and
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
. The reflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope. In the 20th century, many new types of telescopes were invented, including
radio telescopes A radio telescope is a specialized antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio) In radio engineering, an antenna or aerial is the interface between radio waves propagating through sp ...

radio telescopes
in the 1930s and
infrared telescopes An infrared telescope is a telescope A telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or ...
in the 1960s. The word ''telescope'' now refers to a wide range of instruments capable of detecting different regions of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

electromagnetic spectrum
, and in some cases other types of detectors.


Etymology

The word ''telescope'' (from the
Ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally referred to by speakers simply as Greek (, ), refers collectively to the diale ...
,
romanized Romanization or romanisation, in linguistics Linguistics is the scientific study of language A language is a structured system of communication used by humans, including speech (spoken language), gestures (Signed language, sign lan ...
'far' and , 'to look or see'; , 'far-seeing') was coined in 1611 by the Greek mathematician
Giovanni Demisiani Giovanni Demisiani ( el, Ἰωάννης Δημησιάνος; died 1614), a Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in S ...
for one of
Galileo Galilei Galileo di Vincenzo Bonaiuti de' Galilei (; 15 February 1564 – 8 January 1642) was an Italian astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...

Galileo Galilei
's instruments presented at a banquet at the
Accademia dei Lincei The Accademia dei Lincei (; literally the "Academy An academy (Attic Greek: Ἀκαδήμεια; Koine Greek Ἀκαδημία) is an institution of secondary education, secondary or tertiary education, tertiary higher education, higher learning, ...
. In the ''
Starry Messenger ''Sidereus Nuncius'' (usually ''Sidereal Messenger'', also ''Starry Messenger'' or ''Sidereal Message'') is a short astronomical treatise (or ''pamphlet'') published in New Latin New Latin (also called Neo-Latin or Modern Latin) is the rev ...
'', Galileo had used the
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became ...

Latin
term .


History

The earliest existing record of a telescope was a 1608 patent submitted to the government in the
Netherlands ) , national_anthem = ( en, "William of Nassau") , image_map = EU-Netherlands.svg , map_caption = , image_map2 = BES islands location map.svg , map_caption2 = , image_map3 ...

Netherlands
by Middelburg spectacle maker
Hans Lippershey Hans Lipperhey (circa 1570 – buried 29 September 1619), also known as Johann Lippershey or Lippershey, was a German-Dutch spectacle-maker. He is commonly associated with the invention of the telescope, because he was the first one who tried to ...
for a
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope de ...
. The actual inventor is unknown but word of it spread through Europe.
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific qu ...

Galileo
heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects. The idea that the
objective Objective may refer to: * Objective (optics), an element in a camera or microscope * ''The Objective'', a 2008 science fiction horror film * Objective pronoun, a personal pronoun that is used as a grammatical object * Objective Productions, a Briti ...
, or light-gathering element, could be a mirror instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using parabolic mirrors—reduction of
spherical aberration Spherical aberration (SA) is a type of aberration found in optical systems that use elements with spherical surfaces. Lenses and curved mirrors are most often made with surfaces that are spherical, because this shape is easier to form than non ...

spherical aberration
and no
chromatic aberration In optics Optics is the branch of that studies the behaviour and properties of , including its interactions with and the construction of that use or it. Optics usually describes the behaviour of , , and light. Because light is an , oth ...

chromatic aberration
—led to many proposed designs and several attempts to build reflecting telescopes. In 1668,
Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Greek: ) includes the study of such topics a ...

Isaac Newton
built the first practical reflecting telescope, of a design which now bears his name, the
Newtonian reflector The Newtonian telescope, also called the Newtonian reflector or just the Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was ...

Newtonian reflector
. The invention of the
achromatic lens An achromatic lens or achromat is a lens (optics), lens that is designed to limit the effects of chromatic aberration, chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into foc ...

achromatic lens
in 1733 partially corrected color aberrations present in the simple lens and enabled the construction of shorter, more functional refracting telescopes. Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishing
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 ai ...
mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932. The maximum physical size limit for refracting telescopes is about 1 meter (40 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger than 10 m (33 feet), and work is underway on several 30-40m designs. The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...

radio
to
gamma-rays A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, i ...
. The first purpose built radio telescope went into operation in 1937. Since then, a large variety of complex astronomical instruments have been developed.


Types

The name "telescope" covers a wide range of instruments. Most detect
electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. ...

electromagnetic radiation
, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands. Telescopes may be classified by the wavelengths of light they detect: *
X-ray telescope An X-ray telescope (XRT) is a telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photon The photon (Greek: φῶς, phōs, light) is a type of elementa ...
s, using shorter wavelengths than ultraviolet light * Ultraviolet telescopes, using shorter wavelengths than visible light *
Optical telescope An optical telescope is a telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photon The photon (Greek: φῶς, phōs, light) is a type of elementary ...
s, using
visible light Light or visible light is electromagnetic radiation within the portion of the electromagnetic spectrum that is visual perception, perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nano ...
*
Infrared telescope An infrared telescope is a telescope A telescope is an optical instrument using lenses, curved mirrors, or a combination of both to observe distant objects, or various devices used to observe distant objects by their emission, absorption, or ...
s, using longer wavelengths than visible light *
Submillimetre telescopes Submillimetre astronomy or submillimeter astronomy (see spelling differences Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variation ...
, using
microwave Microwave is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space a ...

microwave
wavelengths that are longer than those of infrared light *
Radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

Radio telescope
s that use even longer wavelengths As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be collected much like visible light, however in the far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example, the
James Clerk Maxwell Telescope The James Clerk Maxwell Telescope (JCMT) is a submillimetre-wavelength radio telescope at Mauna Kea Observatory in Hawaii, US. The telescope is near the summit of Mauna Kea at . Its primary mirror is 15 metres (16.4 yards) across: it is the larg ...
observes from wavelengths from 3 μm (0.003 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna. On the other hand, the
Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), is a retired infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible lig ...
, observing from about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflecting optics, the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a 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 ...

Hubble Space Telescope
with
Wide Field Camera 3 The Wide Field Camera 3 (WFC3) is 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 Orbit ...

Wide Field Camera 3
can observe in the frequency range from about 0.2 μm (0.0002 mm) to 1.7 μm (0.0017 mm) (from ultra-violet to infrared light). With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than fully reflecting optics are used. Telescopes such as
TRACE Trace may refer to: Arts and entertainment Music * ''Trace'' (Son Volt album), 1995 * ''Trace'' (Died Pretty album), 1993 * Trace (band) Trace was a Netherlands, Dutch progressive rock trio founded by Rick van der Linden in 1974 after leavin ...

TRACE
and
SOHO Soho is an area of the City of Westminster The City of Westminster is a City status in the United Kingdom, city and London boroughs, borough in Inner London which forms a core part of Central London. It is the site of the United Kingdom's ...

SOHO
use special mirrors to reflect
Extreme ultraviolet 250px, 13.5 nm extreme ultraviolet light is used commercially for photolithography as part of the semiconductor fabrication process. This image shows an early, experimental tool. Extreme ultraviolet radiation (EUV or XUV) or high-energy ...
, producing higher resolution and brighter images than are otherwise possible. A larger aperture does not just mean that more light is collected, it also enables a finer angular resolution. Telescopes may also be classified by location: ground telescope,
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 launched ...
, or flying telescope. They may also be classified by whether they are operated by
professional astronomers
professional astronomers
or
amateur astronomer 250px, Amateur astronomers watch the night sky during the Perseid meteor shower. Amateur astronomy is a hobby A hobby is considered to be a regular activity that is done for enjoyment, typically during one's leisure time, not professiona ...
s. A vehicle or permanent campus containing one or more telescopes or other instruments is called an
observatory An observatory is a location used for observing terrestrial, marine, or celestial events. Astronomy, climatology/meteorology, geophysics, geophysical, oceanography and volcanology are examples of disciplines for which observatories have been cons ...

observatory
.


Optical telescopes

An optical telescope gathers and focuses light mainly from the visible part of the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

electromagnetic spectrum
(although some work in the
infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior ...

infrared
and
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
). Optical telescopes increase the apparent
angular size The angular diameter, angular size, apparent diameter, or apparent size is an angular distance describing how large a sphere of a sphere A sphere (from Greek language, Greek —, "globe, ball") is a Geometry, geometrical object in solid geometr ...

angular size
of distant objects as well as their apparent
brightness Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. It is not necessarily proportional to lumina ...

brightness
. For the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from glass
lenses A lens is a transmissive optics, optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simple ...

lenses
and/or
mirror A mirror is an object that reflects an image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment Environment most often refers t ...

mirror
s, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for
astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses mathematics, phys ...
and in many non-astronomical instruments, including: ''
theodolite A theodolite is a precision optical instrument for measuring angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), ...

theodolite
s'' (including ''transits''), ''
spotting scope A spotting scope is a compact high-power telescope optimized for detailed observation of distant objects. They are used as portable optical instrument, optical enhancement devices for various outdoor activities such as birdwatchers, birdwatchin ...
s'', ''
monocular A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocular ...

monocular
s'', ''
binoculars Binoculars or field glasses are two telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and ...
,'' ''
camera lens A camera lens (also known as photographic lens or photographic objective) is an optics, optical lens (optics), lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film o ...
es'', and ''spyglasses''. There are three main optical types: * The
refracting telescope A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens (optics), lens as its objective (optics), objective to form an image (also referred to a dioptrics, dioptric telescope). The refracting telescope de ...
which uses lenses to form an image. * The reflecting telescope which uses an arrangement of mirrors to form an image. * The catadioptric telescope which uses mirrors combined with lenses to form an image. A
Fresnel Imager A Fresnel imager is a proposed ultra-lightweight design for a space telescope that uses a Fresnel array as primary optics instead of a typical lens. It focuses light with a thin opaque foil sheet punched with specially shaped holes, thus focusing li ...
is a proposed ultra-lightweight design for a space telescope that uses a
Fresnel lens A Fresnel lens is a type of composite compact lens A lens is a transmissive optical Optics is the branch of physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýs ...

Fresnel lens
to focus light. Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as
astrograph observatory. File:Astrograph in Heidelberg-Königstuhl-2.jpg, The ''Bruce double astrograph'' at the Landessternwarte Heidelberg-Königstuhl observatory. An astrograph (or astrographic camera) is a telescope designed for the sole purpose of as ...
s,
comet seeker A comet seeker is a type of small telescope adapted especially to searching for comets: commonly of short focal length and large aperture, in order to secure the greatest brilliancy of light. This style of telescope was used to discover the astero ...
s and
solar telescope A solar telescope is a special purpose telescope used to observe the Sun. Solar telescopes usually detect light with wavelengths in, or not far outside, the visible spectrum. Obsolete names for Sun telescopes include heliograph and photoheliograph. ...

solar telescope
s.


Radio telescopes

Radio telescopes are directional
radio antennas
radio antennas
that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the
wavelength In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular su ...

wavelength
being observed. Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs, a single dish contains an array of several receivers; this is known as a focal-plane array. By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed. Such multi-dish arrays are known as
astronomical interferometer An astronomical interferometer is an array of separate telescopes, mirror segments, or radio telescope antenna (radio), antennas that work together as a single telescope to provide higher resolution images of astronomical objects such as stars, neb ...
s and the technique is called
aperture synthesis Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation and or ...
. The 'virtual' apertures of these arrays are similar in size to the distance between the telescopes. As of 2005, the record array size is many times the diameter of the Earth – using space-based
Very Long Baseline Interferometry Very-long-baseline interferometry (VLBI) is a type of astronomical interferometer, astronomical interferometry used in radio astronomy. In VLBI a signal from an astronomical radio source, such as a quasar, is collected at multiple radio telescop ...
(VLBI) telescopes such as the Japanese
HALCA HALCA (Highly Advanced Laboratory for Communications and Astronomy), also known for its project name VSOP (VLBI Space Observatory Programme), the code name MUSES-B (for the second of the Mu (rocket family), Mu Space Engineering Spacecraft series), ...
(Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite. Aperture synthesis is now also being applied to optical telescopes using optical interferometers (arrays of optical telescopes) and
aperture masking interferometry 400px, a) shows a simple experiment using an aperture mask in a re-imaged aperture plane. b) and c) show diagrams of aperture masks which were placed in front of the Peter_Tuthill_and_collaborators._The_solid_black_shapes_represent_the_subaperture ...
at single reflecting telescopes. Radio telescopes are also used to collect
microwave radiation Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matte ...
, which has the advantage of being able to pass through the atmosphere and interstellar gas and dust clouds. Some radio telescopes are used by programs such as
SETI The search for extraterrestrial intelligence (SETI) is a collective term for scientific searches for intelligent extraterrestrial life Extraterrestrial lifeWhere "extraterrestrial" is derived from the Latin Latin (, or , ) is a classica ...
and the
Arecibo Observatory The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Esperanza, Arecibo, Puerto Rico, Barrio Esperanza, Arecibo, Puerto Rico own ...

Arecibo Observatory
to search for extraterrestrial life.


X-ray telescopes

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Moti ...

X-ray
s are much harder to collect and focus than electromagnetic radiation of longer wavelengths. X-ray telescopes can use
X-ray opticsX-ray optics is the branch of optics that manipulates X-rays instead of visible light. It deals with focusing and other ways of manipulating the X-ray beams for research techniques such as X-ray crystallography, X-ray fluorescence, small-angle X ...
, such as
Wolter telescope A Wolter telescope is a telescope for X-rays that only uses grazing incidence optics – mirrors that reflect X-rays at very shallow angles. Problems with conventional telescope designs Conventional telescope designs require reflection or refracti ...
s composed of ring-shaped 'glancing' mirrors made of
heavy metals upright=1.2, Crystals of osmium, a heavy metal nearly twice as dense as lead">lead.html" ;"title="osmium, a heavy metal nearly twice as dense as lead">osmium, a heavy metal nearly twice as dense as lead Heavy metals are generally defined as m ...
that are able to reflect the rays just a few
degrees Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
. The mirrors are usually a section of a rotated
parabola In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no ...

parabola
and a
hyperbola In mathematics, a hyperbola () (adjective form hyperbolic, ) (plural ''hyperbolas'', or ''hyperbolae'' ()) is a type of smooth function, smooth plane curve, curve lying in a plane, defined by its geometric properties or by equations for which it ...

hyperbola
, or
ellipse In , an ellipse is a surrounding two , such that for all points on the curve, the sum of the two distances to the focal points is a constant. As such, it generalizes a , which is the special type of ellipse in which the two focal points are t ...

ellipse
. In 1952,
Hans WolterHans Wolter (11 May 1911 in Dramburg – 17 August 1978 in Marburg) was a German physicist who designed an aplanatic system of angle of incidence (optics), grazing incidence mirrors that satisfied the Abbe sine condition (i.e. free of both spherical ...
outlined 3 ways a telescope could be built using only this kind of mirror. Examples of observatories using this type of telescope are the
Einstein Observatory Einstein Observatory (HEAO-2) was the first fully imaging X-ray telescope put into space and the second of NASA's three High Energy Astrophysical Observatories. Named HEAO B before launch, the observatory's name was changed to honor Albert ...
,
ROSAT ROSAT (short for Röntgensatellit; in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen) was a German Aerospace Center-led satellite X-ray telescope An X-ray telescope (XRT) is a telescope that is designed to observe r ...

ROSAT
, and the
Chandra X-Ray Observatory The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope A space telescope or space observatory is a telescope located in outer space Outer space i ...

Chandra X-Ray Observatory
. By 2010, Wolter focusing X-ray telescopes are possible up to photon energies of 79 keV.


Gamma-ray telescopes

Higher energy X-ray and
Gamma-ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, i ...
telescopes refrain from focusing completely and use
coded aperture Coded apertures or coded-aperture masks are grids, gratings, or other patterns of materials opaque to various wavelengths of electromagnetic radiation. The wavelengths are usually high-energy radiation such as X-rays and gamma rays. By blocking r ...
masks: the patterns of the shadow the mask creates can be reconstructed to form an image. X-ray and Gamma-ray telescopes are usually installed on Earth-orbiting
satellite In the context of spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either human spaceflight, with or uncrewed spaceflight, without humans on board. Most spaceflight ...

satellite
s or high-flying balloons since the
Earth's atmosphere The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter (the others being solid Solid is one of the four fundamental states of matter (the others being liquid, gas and plasma). The mo ...

Earth's atmosphere
is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is the
Fermi Gamma-ray Space Telescope The Fermi Gamma-ray Space Telescope (FGST), formerly called the Gamma-ray Large Area Space Telescope (GLAST), is a space observatory being used to perform gamma-ray astronomy observations from low Earth orbit. Its main instrument is the Large Area ...

Fermi Gamma-ray Space Telescope
. The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. An example of this type of observatory is
VERITAS In Roman mythology Roman mythology is the body of myths of ancient Rome as represented in the Latin literature, literature and Roman art, visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may ...

VERITAS
. A discovery in 2012 may allow focusing gamma-ray telescopes. At photon energies greater than 700 keV, the index of refraction starts to increase again.


Other types of telescopes

Astronomy is not limited to using electromagnetic radiation. Additional information can be obtained by detecting other signals, with detectors analogous to telescopes. These are: * Cosmic-ray telescopes detect
cosmic ray Cosmic rays are high-energy proton A proton is a subatomic particle, symbol or , with a positive electric charge of +1''e'' elementary charge and a mass slightly less than that of a neutron. Protons and neutrons, each with masses of approx ...
s and usually consist of an array of different detector types spread out over a large area. * Energetic neutral atom instruments study the
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

magnetosphere
of various bodies by detecting fast moving electrically neutral atoms created by the
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

solar wind
. *
Neutrino detector A neutrino detector is a physics apparatus which is designed to study neutrinos. Because neutrinos only Weak interaction, weakly interact with other particles of matter, neutrino detectors must be very large to detect a significant number of neutr ...
s, the equivalent of
neutrino A neutrino ( or ) (denoted by the Greek letter ) is a fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics and generally has half odd integer spin: spin 1/2, Spin (physics)#Higher spins, spin 3/2, etc. T ...

neutrino
telescopes, used for
neutrino astronomy Neutrino astronomy is the branch of astronomy that observes astronomical objects with Neutrino detector, neutrino detectors in special observatories. Neutrinos are created as a result of certain types of radioactive decay, Nuclear reaction, nuclear ...
. They consist of a large mass of water and ice, surrounded by an array of sensitive light detectors known as
photomultiplierA photomultiplier is a device that converts incident photons into an electrical signal. Kinds of photomultiplier include: * Photomultiplier tube, a vacuum tube A vacuum tube, an electron tube, valve (British usage) or tube (North America), i ...

photomultiplier
tubes. Originating direction of the neutrinos is determined by reconstructing the path of secondary particles scattered by neutrino impacts, from their interaction with multiple detectors. *
Gravitational-wave detector A gravitational-wave detector (used in a gravitational-wave observatory) is any device designed to measure tiny distortions of spacetime In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledg ...
s, the equivalent of gravitational wave telescopes, are used for gravitational-wave astronomy. Gravitational waves, caused by violent collisions in space, are detected by extremely precise measurements of the change in length of large earth-bound structures.


Types of mount

A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument. Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the stars as the Earth rotates. The two main types of tracking mount are: * Altazimuth mount * Equatorial mount * Zenith * Transit By the 21 century, although not a structure a type of control system called a GoTo (telescopes), GoTo telescope was more popular. In this case a computer software system can in part or whole direct the telescope to a certain coordinate in the sky.


Atmospheric electromagnetic opacity

Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth's surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum. For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to astronomical seeing.


Telescopic image from different telescope types

Different types of telescope, operating in different wavelength bands, provide different information about the same object. Together they provide a more comprehensive understanding.


By spectrum

Telescopes that operate in the
electromagnetic spectrum The electromagnetic spectrum is the range of frequencies Frequency is the number of occurrences of a repeating event per unit of time A unit of time is any particular time Time is the indefinite continued sequence, progress of existe ...

electromagnetic spectrum
: *Links to categories.


Lists of telescopes

* List of optical telescopes * List of largest optical reflecting telescopes * List of largest optical refracting telescopes * List of largest optical telescopes historically * List of radio telescopes * List of solar telescopes * List of space observatories * List of telescope parts and construction * List of telescope types * :Telescopes * :Cosmic-ray telescopes * :Gamma-ray telescopes * :Gravitational wave telescopes * :High energy particle telescopes * :Infrared telescopes * :Submillimetre telescopes * :Ultraviolet telescopes * :X-ray telescopes


See also

* Air mass (astronomy), Airmass * Amateur telescope making * Angular resolution * ASCOM (standard), ASCOM open standards for computer control of telescopes * Bahtinov mask * Bioptics (device), Bioptic telescope * Carey mask * Dew shield * Dynameter * f-number * First light (astronomy), First light * Hartmann mask * Keyhole problem * Microscope * List of planetariums, Planetariums * Remote Telescope Markup Language * Robotic telescope * Timeline of telescope technology * Timeline of telescopes, observatories, and observing technology


References


Further reading

* ''Contemporary Astronomy – Second Edition'', Jay Pasachoff, Jay M. Pasachoff, Saunders Colleges Publishing – 1981, * * * *


External links


''Galileo to Gamma Cephei – The History of the Telescope''




by the American Institute of Physics *

* {{Authority control Telescopes, Astronomical imaging Astronomical instruments Science and technology in the Dutch Republic 1600s in the Dutch Republic 17th-century inventions Dutch inventions