A
telescope is an instrument designed for the observation of remote objects by the collection of
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
. The first known practically functioning telescopes were invented in the
NetherlandsThe Netherlands is a country in Northwestern Europe, constituting the major portion of the Kingdom of the Netherlands. It is a parliamentary democratic constitutional monarchy. The Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east...
at the beginning of the 17th century. "Telescopes" can refer to a whole range of instruments operating in most regions of the
electromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
.
The word "
telescope" (from the
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
tele = 'far' and
skopein = 'to look or see';
teleskopos = 'far-seeing') was coined in 1611 by the Greek mathematician
Giovanni DemisianiGiovanni Demisiani , a Greek from Zakynthos, was a theologian, chemist, mathematician to Cardinal Gonzaga, and member of the Accademia dei Lincei...
for one of
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
's instruments presented at a banquet at the
Accademia dei LinceiThe Accademia dei Lincei, , is an Italian science academy, located at the Palazzo Corsini on the Via della Lungara in Rome, Italy....
. In the
Starry Messenger Galileo had used the term "perspicillum".
History
The earliest evidence of working telescopes were the
refracting telescopeA refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image...
s that appeared in the
NetherlandsThe Netherlands is a country in Northwestern Europe, constituting the major portion of the Kingdom of the Netherlands. It is a parliamentary democratic constitutional monarchy. The Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east...
in 1608. Their development is credited to three individuals:
Hans LippersheyHans Lippershey , also known as Johann Lippershey or Lipperhey, was a German-Dutch lensmaker, generally credited as being the inventor of the telescope....
and
Zacharias JanssenSacharias Jansen was a Dutch spectacle-maker from Middelburg credited with inventing, or contributing advances towards the invention of the first telescope. Jansen is sometimes credited for inventing the first truly compound microscope...
, who were spectacle makers in Middelburg, and
Jacob MetiusJacob Metius , was a Dutch instrument-maker and optician. He was born in Alkmaar and was the brother of Adriaan Adriaanszoon . A specialist in grinding lenses, he claimed to have invented the telescope...
of
AlkmaarAlkmaar is a municipality and a city in the Netherlands, in the province of Noord Holland. Alkmaar is well-known for its traditional cheese market. For tourists, it is a popular cultural destination.-History:...
.
GalileoGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
greatly improved upon these designs the following year.
The idea that a mirror could be used as an
objectiveIn optics, an objective is the lens or mirror in a microscope, telescope, camera or other optical instrument that gathers the light coming from the object being observed, and focuses the rays to produce a real image...
instead of a lens was being investigated soon after the invention of the refracting telescope. The potential advantages of using
parabolic mirrorsA parabolic reflector is a reflective device used to collect or project energy such as light, sound, or radio waves. Its shape is that of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis...
, primarily reduction of
spherical aberrationthumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the optic axis. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the axis. It therefore...
with no
chromatic aberrationIn optics, chromatic aberration is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light...
, led to many proposed designs and several attempts to build
reflecting telescopeA reflecting telescope 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...
s. In 1668,
Isaac NewtonSir Isaac Newton FRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian who is perceived and considered by a substantial number of scholars and the general public as one of the most influential men in history...
built the first practical reflecting telescope that bears his name, the
Newtonian reflectorThe Newtonian telescope is a type of reflecting telescope invented by the British scientist Sir Isaac Newton , using a concave primary mirror and a flat diagonal secondary mirror. Newton’s first reflecting telescope was completed in 1668 and is the earliest known functional reflecting telescope...
.
The invention of the
achromatic lensAn 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 into focus in the same plane....
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 metalSpeculum metal is a very hard white alloy of roughly 75-80% copper and 20-25% tin; some compositions contained 1–2% of arsenic. Composition with 45% tin has more resistance to tarnishing....
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 then 10 m (33 feet).
The 20th century also saw the development of telescopes that worked in a wide range of wavelengths from
radioA radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
to gamma-rays. The first purpose built radio telescope went into operation in 1937. Since then, a tremendous variety of complex astronomical instruments have been developed.
Types of telescopes
The name "telescope" covers a wide range of instruments and is difficult to define. They all have the attribute of collecting electromagnetic radiation so it can be studied or analyzed in some manner. The most common type is the optical telescope; other types also exist and are listed below.
Optical telescopes
An optical telescope gathers and
focusesIn geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
light mainly from the visible part of the
electromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
(although some work in the
infraredInfrared radiation is electromagnetic radiation whose wavelength is longer than that of visible light , but shorter than that of terahertz radiation and microwaves...
and
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
). Optical telescopes increase the apparent angular size of distant objects as well as their apparent
brightnessBrightness 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...
. In order 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
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
—
lensesA lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens is a lens consisting of a single optical element...
, or
mirrorA mirror is an object with at least one polished and therefore specularly reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface...
s to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used for
astronomyAstronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
and in many non-astronomical instruments, including:
theodoliteA theodolite is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a key tool in surveying and engineering work, particularly on inaccessible ground, but theodolites have been adapted for other specialized purposes in fields like meteorology...
s (including
transits),
spotting scopeA spotting scope is a portable telescope, optimized for the observation of terrestrial objects. The magnification of a spotting scope is typically on the order of 20X to 60X...
s,
monocularA monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and prisms; the use of prisms results in a lightweight telescope...
s,
binocularsBinoculars, field glasses or binocular telescopes are a pair of identical or mirror-symmetrical telescopes mounted side-by-side and aligned to point accurately in the same direction, allowing the viewer to use both eyes with binocular vision when viewing distant objects. Most are sized to be held...
, camera lenses, and
spyglasses. There are three main types:
- The refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image...
which uses lenses to form an image.
- The reflecting telescope
A reflecting telescope 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...
which uses an arrangement of mirrors to form an image.
- The catadioptric telescope which uses mirrors combined with lenses to form an image.
Other optical telescopes:
- Infrared telescope
-Existing infrared telescopes:* Infrared Telescope Facility* Gornergrat Infrared Telescope* Infrared Optical Telescope Array* United Kingdom Infrared Telescope* Spitzer Space Telescope-See also:* Infrared astronomy...
s
- Submillimetre telescopes
Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a...
- Ultraviolet telescopes
- 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 light on a certain point by using the phenomenon of...
Radio telescopes
Radio telescopes are
directionalA directional antenna or beam antenna is an antenna which radiates greater power in one or more directions allowing for increased performance on transmit and receive and reduced interference from unwanted sources....
radio antennas that often have a parabolic shape. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than the
wavelengthIn physics, the wavelength of a sinusoidal wave is the spatial period of the wave – the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
being observed. Multi-element
Radio telescopeA radio telescope is a form of directional radio antenna used in radio astronomy. The same types of antennas are also used in tracking and collecting data from satellites and space probes...
s are constructed from pairs or larger groups of these dishes to synthesize large 'virtual' apertures that are similar in size to the separation between the telescopes; this process is known as
aperture synthesisAperture 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...
. As of 2005, the current record array size is many times the width of the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
—utilizing space-based
Very Long Baseline InterferometryVery Long Baseline Interferometry is a type of astronomical interferometry used in radio astronomy. It allows observations of an object that are made simultaneously by many telescopes to be combined, emulating a telescope with a size equal to the maximum separation between the telescopes.Data...
(VLBI) telescopes such as the
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
ese
HALCAThe HALCA , also known as MUSES-B before launch and Haruka after launch, is a Japanese owned 8 meter diameter radio telescope satellite which was used for Very Long Baseline Interferometry. It is in an orbit with an apogee altitude of 21,400 km and a perigee altitude of 560 km...
(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 interferometryAperture Masking Interferometry is a form of speckle interferometry, allowing diffraction limited imaging from ground-based telescopes. This technique allows ground based telescopes to reach the maximum possible resolution, allowing ground-based telescopes with large diameters to produce far...
at single reflecting telescopes. Radio telescopes are also used to collect microwave radiation, which is used to collect radiation when any visible light is obstructed or faint, such as from
quasarA quasi-stellar radio source is a powerfully energetic and distant galaxy with an active galactic nucleus. Quasars were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than extended...
s. Some radio telescopes are used by programs such as SETI and the
Arecibo ObservatoryThe Arecibo Observatory is a radio telescope located close to the city of Arecibo in Puerto Rico. It is operated by Cornell University under cooperative agreement with the National Science Foundation. The observatory works as the National Astronomy and Ionosphere Center although both names are...
to search for exterrestrial life. One particularly exciting example is the
Wow! signalThe Wow! signal was a strong narrowband radio signal detected by Dr. Jerry R. Ehman on August 15, 1977, while working on a SETI project at The Big Ear radio telescope of Ohio State University. The signal bore expected hallmarks of potential non-terrestrial and non-solar system origin. It lasted for...
, recorded in 1977.
High energy particle telescopes
High-energy astronomyHigh energy astronomy is the study of astronomical objects that release EM radiation of highly energetic wavelengths. It includes X-ray astronomy, gamma-ray astronomy, and extreme UV astronomy, as well as studies of neutrinos and cosmic rays...
requires specialized telescopes to make observations since most of these particles go through most metals and glasses.
X-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays...
telescopes use
Wolter telescopeA Wolter telescope is a telescope for X-rays using only grazing incidence optics. Visible light telescopes are built with lenses or parabolic mirrors at nearly normal incidence. Neither works well for X-rays. Lenses for visible light are made of a transparent material with an index of refraction...
s composed of ring-shaped 'glancing'
mirrorA mirror is an object with at least one polished and therefore specularly reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface...
s made of
heavy metalsA heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanoids, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic...
that are able to reflect the rays just a few
degreesA degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
. The mirrors are usually a section of a rotated
parabolaIn mathematics, the parabola is a conic section, the intersection of a right circular conical surface and a plane parallel to a generating straight line of that surface...
and a
hyperbolaIn mathematics a hyperbola is a smooth planar curve having two connected components or branches, each a mirror image of the other and resembling two infinite bows aimed at each other as seen in the figure below...
, or
ellipseIn mathematics, an ellipse is the bounded case of a conic section, the geometric shape that results from cutting a circular conical or cylindrical surface with an oblique plane...
. In 1952,
Hans WolterHans Wolter was a German physicist who designed an aplanatic system of grazing incidence mirrors that satisfied the Abbe sine condition ....
outlined 3 ways a telescope could be built using only this kind of mirror.
Gamma-ray telescopes refrain from focusing completely and use coded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.
X-ray and Gamma-ray telescopes are usually on Earth-orbiting
satelliteIn the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavor. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s or high-flying balloons since the
Earth's atmosphereThe Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
is opaque to this part of the electromagnetic spectrum.
In other types of high energy particle telescopes there is no
image-forming optical systemIn optics, an image-forming optical system is a system capable of being used for imaging. The diameter of the aperture of the main objective is a common criteria for comparison among optical systems, such as large telescopes....
.
Cosmic-ray telescopesA Cosmic-ray observatory is a scientific installation built to detect very high energy electromagnetic radiation coming from space called Cosmic rays. It is not yet possible to build a Cosmic ray image forming optics, like a Wolter telescope for lower energy X-rays, although some Cosmic-ray...
usually consist of an array of different detector types spread out over a large area. A
Neutrino telescopeIn astronomy, a neutrino telescope is usually a detector consisting of a large mass of water or ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. This allows the observation of astronomical objects by detecting the neutrinos they emit, rather than their light ...
consists of a large mass of
waterWater is an ubiquitous chemical substance that is composed of hydrogen and oxygen and is essential for all known forms of life.In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam. Water covers 71%...
or
iceIce is a solid phase, usually crystalline, of a non-metallic substance that is liquid or gas at room temperature, such as carbon dioxide ice , ammonia ice, or methane ice. However, the predominant use of the term ice is for water ice, technically restricted to one of the 15 known crystalline phases...
, surrounded by an array of sensitive light detectors known as
photomultiplierPhotomultiplier tubes , members of the class of vacuum tubes, and more specifically phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum...
tubes.
Other types of telescopes
- Gravitational wave telescopes
A gravitational wave detector is any experiment designed to measure gravitational waves, minute distortions of spacetime that are predicted by Einstein's theory of general relativity.-Complications:...
Lists of telescopes
:Category:Telescopes
:Category:Cosmic-ray telescopes
:Category:Gamma-ray telescopes
:Category:Gravitational wave telescopes
:Category:High energy particle telescopes
:Category:Infrared telescopes
:Category:Submillimetre telescopes
:Category:Ultraviolet telescopes
:Category:X-ray telescopes
See also
- Amateur telescope making
The field of amateur telescope making is considered an offshoot of the amateur astronomy community. Amateur telescope makers , as their name implies, are not paid professionals...
- Angular resolution
Angular resolution or 'spatial resolution' describes the resolving power of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye.- Definition of terms :...
- 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...
- ASCOM
ASCOM is an open initiative to provide a standard interface to a range of astronomy equipment including mounts, focussers and imaging devices in a Microsoft Windows environment....
open standards for computer control of telescopes
- Depth of field
In optics, particularly as relates to film and photography, the depth of field is the portion of a scene that appears acceptably sharp in the image...
- Dynameter
A dynameter is an instrument that measures the magnification of a telescope. It is usually a double-image micrometer used to measure the diameter of the image of the object glass. The magnifying power is found by comparing the actual diameter of the image on the glass with the measured image...
- f-number
In optics, the f-number of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-number is the focal length divided by the "effective" aperture diameter...
- First light
In astronomy, first light is when a telescope is first used to take an astronomical image after it has been constructed. This is not often the first viewing using the telescope; optical tests have likely already been performed during daylight to adjust the components...
- Hartmann mask
Hartmann mask is a tool to help focusing telescopes, mainly used by amateur astronomers.- Theory and practice :Even a part of a mirror or lens produces the same image as the whole optical element. The light is focused in the focal point. The light rays, however, go through different points of a...
- Keyhole problem
The keyhole problem in the context of astronomy refers to the difficulty that azimuth-elevation type telescopes or antenna gimbal systems encounter in crossing the zenith. -Area of Visible Sky:...
- Microscope
A microscope is an instrument to see objects too tiny for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope.-History:An early microscope was made in 1590 in Middelburg, The...
- Nimrud lens
The Nimrud lens is a 3000 year old piece of rock crystal, which was unearthed by Austen Henry Layard at the Assyrian palace of Nimrud. It may have been used as a magnifying glass, or as a burning-glass to start fires by concentrating sunlight...
- Remote Telescope Markup Language
The Remote Telescope Markup Language is an XML dialect for controlling remote and/or robotic telescopes.It was created by UC Berkeley's Hands-On Universe project in 1999.-External links:* http://hou.lbl.gov/rtml/...
- Robotic telescope
A robotic telescope is an astronomical telescope and detector system that makes observations without the intervention of a human. In astronomical disciplines, a telescope qualifies as robotic if it makes those observations without being operated by a human, even if a human has to initiate the...
- Timeline of telescope technology
Timeline of telescope technology* c.2560 BC - c.860 BC - Egyptian artisans polish rock crystal, semi-precious stones, and latterly glass to produce facsimile eyes for statuary and mummy cases. The intent appears to be to produce an optical illusion....
- Timeline of telescopes, observatories, and observing technology
-2100:*Xiangfen Astronomical Observatory, Xiangfen County, Linfen City, Shanxi Province, China-1100:* 11th-7th century B.C., Zhou dynasty astronomical observatory in today's Xian, China-200:...
External links