Optical telescope
Overview
 
An optical telescope is a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 which is used to gather and focus
Focus (optics)
In 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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 part of the electromagnetic spectrum
Electromagnetic spectrum
The 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....

 for directly viewing a magnified
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 image for making a photograph
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...

, or collecting data through electronic image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

s.

There are three primary types of optical telescope: Refractors which use lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 (dioptrics
Dioptrics
Dioptrics is the study of the refraction of light, especially by lenses. Telescopes that create their image with an objective that is a convex lens are said to be "dioptric" telescopes....

), reflectors which use mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s (catoptrics
Catoptrics
Catoptrics deals with the phenomena of reflected light and image-forming optical systems using mirrors. From the Greek κατοπτρικός ....

), and catadioptric telescopes which use both lenses and mirrors in combination.

A telescope's light gathering power and ability to resolve small detail is directly related to the diameter (or aperture) of its objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 (the primary lens or mirror that collects and focuses the light).
Discussions
Encyclopedia
An optical telescope is a telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

 which is used to gather and focus
Focus (optics)
In 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
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...

 part of the electromagnetic spectrum
Electromagnetic spectrum
The 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....

 for directly viewing a magnified
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 image for making a photograph
Photograph
A photograph is an image created by light falling on a light-sensitive surface, usually photographic film or an electronic imager such as a CCD or a CMOS chip. Most photographs are created using a camera, which uses a lens to focus the scene's visible wavelengths of light into a reproduction of...

, or collecting data through electronic image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...

s.

There are three primary types of optical telescope: Refractors which use lenses
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 (dioptrics
Dioptrics
Dioptrics is the study of the refraction of light, especially by lenses. Telescopes that create their image with an objective that is a convex lens are said to be "dioptric" telescopes....

), reflectors which use mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

s (catoptrics
Catoptrics
Catoptrics deals with the phenomena of reflected light and image-forming optical systems using mirrors. From the Greek κατοπτρικός ....

), and catadioptric telescopes which use both lenses and mirrors in combination.

A telescope's light gathering power and ability to resolve small detail is directly related to the diameter (or aperture) of its objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 (the primary lens or mirror that collects and focuses the light). The larger the objective, the more light the telescope can collect and the finer detail it can resolve.

History

The telescope is more a discovery of optical craftsmen than an invention of scientist. The lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

 and the properties of refracting and reflecting light had been known since antiquity
Ancient history
Ancient history is the study of the written past from the beginning of recorded human history to the Early Middle Ages. The span of recorded history is roughly 5,000 years, with Cuneiform script, the oldest discovered form of coherent writing, from the protoliterate period around the 30th century BC...

 and theory on how they worked were developed by ancient Greek
Greek philosophy
Ancient Greek philosophy arose in the 6th century BCE and continued through the Hellenistic period, at which point Ancient Greece was incorporated in the Roman Empire...

 philosophers, preserved and expanded on in the medieval Islamic world
Islamic Golden Age
During the Islamic Golden Age philosophers, scientists and engineers of the Islamic world contributed enormously to technology and culture, both by preserving earlier traditions and by adding their own inventions and innovations...

, and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe
Early modern Europe
Early modern Europe is the term used by historians to refer to a period in the history of Europe which spanned the centuries between the end of the Middle Ages and the beginning of the Industrial Revolution, roughly the late 15th century to the late 18th century...

. But the most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

, first in Venice and Florence in the thirteenth century, and later in the spectacle making centers in both the Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 and Germany. It is in the Netherlands in 1608 where the first recorded optical telescopes (refracting telescope
Refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

s) appeared. The invention is credited to the spectacle makers Hans Lippershey
Hans Lippershey
Hans Lippershey , also known as Johann Lippershey or Lipperhey, was a German-Dutch lensmaker commonly associated with the invention of the telescope, although it is unclear if he was the first to build one.-Biography:...

 and Zacharias Janssen
Zacharias Janssen
Zacharias Jansen was a Dutch spectacle-maker from Middelburg associated with the invention of the first optical telescope. Jansen is sometimes also credited for inventing the first truly compound microscope...

 in Middelburg, and the instrument-maker and optician Jacob Metius
Jacob Metius
Jacob Metius was a Dutch instrument-maker and a specialist in grinding lenses. He was born in Alkmaar and was the brother of Adriaan Adriaanszoon...

 of Alkmaar
Alkmaar
Alkmaar 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:...

.

Galileo
Galileo Galilei
Galileo 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 and is generally credited with being the first to use a telescope for astronomical purposes. Galileo's telescope used Hans Lippershey's design of a convex objective lens
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 and a concave eye lens
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 and this design has come to be called a Galilean telescope. Johannes Kepler
Johannes Kepler
Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

 proposed an improvement on the design that used a convex eyepiece
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

, often called the Keplerian Telescope.

The next big step in the development of refractors was the advent of the Achromatic lens
Achromatic lens
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 into focus in the same plane....

 in the early 18th century that corrected chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

 seen in Keplerian telescopes up to that time, allowing for much shorter instruments with much larger objectives.

For reflecting telescope
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...

s, which use a curved mirror in place of the objective lens, theory preceded practice. The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen, whose theories had been widely disseminated in Latin translations of his work. Soon after the invention of the refracting telescope Galileo, Giovanni Francesco Sagredo
Giovanni Francesco Sagredo
Giovanni Francesco Sagredo was a Venetian mathematician and close friend of Galileo, who wrote:Many years ago I was often to be found in the marvelous city of Venice, in discussions with Signore Giovanni Francesco Sagredo, a man of noble extraction and trenchant wit. He was also a friend and...

, and others, spurred on by their knowledge that curved mirrors had similar properties as lenses, discussed the idea of building a telescope using a mirror as the image forming objective. The potential advantages of using parabolic mirrors
Parabolic reflector
A 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 a reduction of spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|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...

 with elimination of chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

) led to several proposed designs for reflecting telescopes, the most notable of which was published in 1663 by James Gregory
James Gregory (astronomer and mathematician)
James Gregory FRS was a Scottish mathematician and astronomer. He described an early practical design for the reflecting telescope – the Gregorian telescope – and made advances in trigonometry, discovering infinite series representations for several trigonometric functions.- Biography :The...

 and came to be called the Gregorian telescope
Gregorian telescope
The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke...

, but no working models were built. Isaac Newton
Isaac Newton
Sir Isaac Newton PRS was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian, who has been "considered by many to be the greatest and most influential scientist who ever lived."...

 has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope
Newtonian telescope
The 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...

, in 1668 although due to their difficulty of construction and the poor performance of the speculum metal
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 is used primarily to make different kinds of mirrors including early reflecting telescope optical mirrors...

 mirrors used it took over 100 years for reflectors to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror
Parabolic reflector
A 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...

 fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century, segmented mirror
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 . They are used as objectives for large reflecting telescopes...

s to allow larger diameters, and active optics
Active optics
Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...

 to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera
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. Other similar designs are the Wright Camera and Lurie-Houghton telescope....

, which uses both a lens (corrector plate) and mirror 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
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 and space telescopes to overcome the problems of astronomical seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

.

Principles

For detailed information on specific designs of reflecting, refracting, and catadioptric telescopes: see the main articles on Reflecting telescope
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...

s, Refracting telescope
Refracting telescope
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image . The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses...

s, and Catadioptric
Catadioptric
A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses and curved mirrors . Catadioptric combinations are used in focusing systems such as search lights, headlamps, early lighthouse focusing systems, optical telescopes,...

s.


The basic scheme is that the primary light-gathering element the objective
Objective (optics)
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes,...

 (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image
Real image
In optics, a real image is a representation of an object in which the perceived location is actually a point of convergence of the rays of light that make up the image. If a screen is placed in the plane of a real image the image will generally become visible on the screen...

 (5). This image may be recorded or viewed through an eyepiece
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 (2) which acts like a magnifying glass
Magnifying glass
A magnifying glass is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle ....

. The eye (3) then sees an inverted magnified
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 virtual image
Virtual image
In optics, a virtual image is an image in which the outgoing rays from a point on the object always diverge. It will appear to converge in or behind the optical device . A simple example is a flat mirror where the image of oneself is perceived at twice the distance from oneself to the mirror...

 (6) of the object.

Inverted images

All refracting and most reflecting telescope designs are said to produce an inverted image at the focal plane; these are referred to as inverting telescopes. In fact, the image is both inverted and reverted, or rotated 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect but everted (reversed left to right). In terrestrial telescopes such as Spotting scope
Spotting scope
A spotting scope is a small portable telescope with added optics to present an erect image, optimized for the observation of terrestrial objects...

s, monocular
Monocular
A monocular is a modified refracting telescope used to magnify the images of distant objects by passing light through a series of lenses and sometimes prisms; the use of prisms results in a lightweight telescope. Volume and weight are less than half those of binoculars of similar optical...

s and binoculars
Binoculars
Binoculars, 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 when viewing distant objects...

, prisms (e.g., Porro prism
Porro prism
In optics, a Porro prism, named for its inventor Ignazio Porro, is a type of reflection prism used in optical instruments to alter the orientation of an image....

s), or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector
Gregorian telescope
The Gregorian telescope is a type of reflecting telescope designed by Scottish mathematician and astronomer James Gregory in the 17th century, and first built in 1673 by Robert Hooke...

. These are referred to as erecting telescopes.

Design variants

Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design (Newtonian telescope
Newtonian telescope
The 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...

, Cassegrain reflector
Cassegrain reflector
The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas....

 or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improved image quality over a larger field of view.

Angular resolution

Ignoring blurring of the image by turbulence in the atmosphere (atmospheric seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

) and optical imperfections of the telescope, the angular resolution
Angular resolution
Angular resolution, or spatial resolution, describes the ability of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye, to distinguish small details of an object...

 of an optical telescope is determined by the diameter of the objective, termed its "aperture
Aperture
In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...

" (the primary mirror, or lens.)
The Rayleigh criterion for the resolution limit (in radian
Radian
Radian is the ratio between the length of an arc and its radius. The radian is the standard unit of angular measure, used in many areas of mathematics. The unit was formerly a SI supplementary unit, but this category was abolished in 1995 and the radian is now considered a SI derived unit...

s) is given by
where is the wavelength
Wavelength
In 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...

 and is the aperture. For visible light ( = 550 nm) in the small-angle approximation, this equation can be rewritten:
Here, denotes the resolution limit in arcseconds and is in millimeters.
In the ideal case, the two components of a double star
Double star
In observational astronomy, a double star is a pair of stars that appear close to each other in the sky as seen from Earth when viewed through an optical telescope. This can happen either because the pair forms a binary star, i.e...

 system can be discerned even if separated by slightly less than . This is taken into account by the Dawes limit
The equation shows that, all else being equal, the larger the aperture, the better the angular resolution. The resolution is not given by the maximum magnification
Magnification
Magnification is the process of enlarging something only in appearance, not in physical size. This enlargement is quantified by a calculated number also called "magnification"...

 (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

For large ground-based telescopes, the resolution is limited by atmospheric seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

. This limit can be overcome by placing the telescopes above the atmosphere, e.g., on the summits of high mountains, on balloon and high-flying airplanes, or in space. Resolution limits can also be overcome by adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

, speckle imaging
Speckle imaging
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add method or on speckle interferometry methods...

 or lucky imaging
Lucky imaging
Lucky imaging is one form of speckle imaging used for astronomical photography. Speckle imaging techniques use a high-speed camera with exposure times short enough so that the changes in the Earth's atmosphere during the exposure are minimal.With lucky imaging, those optimum exposures least...

 for ground-based telescopes.

Recently, it has become practical to perform aperture synthesis
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...

 with arrays of optical telescopes. Very high resolution images can be obtained with groups of widely-spaced smaller telescopes, linked together by carefully controlled optical paths, but these interferometers can only be used for imaging bright objects such as stars or measuring the bright cores of active galaxies. Example images of starspots on Betelgeuse
Betelgeuse
Betelgeuse, also known by its Bayer designation Alpha Orionis , is the eighth brightest star in the night sky and second brightest star in the constellation of Orion, outshining its neighbour Rigel only rarely...

 can be seen here.

Focal length and f-ratio

The focal length determines how wide an angle the telescope can view with a given eyepiece
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 so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings...

 or size of a CCD detector
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 or photographic plate
Photographic plate
Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...

.
The f-ratio
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...

 (or focal ratio, or f-number) of a telescope is the ratio between the focal length
Focal length
The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus...

 and the aperture (i.e., diameter) of the objective. Thus, for a given aperture (light-gathering power), low f-ratios indicate wide fields of view.
Wide-field telescopes (such as astrograph
Astrograph
An astrograph is a telescope designed for the sole purpose of astrophotography. Astrographs are usually used in wide field surveys of the night sky as well as detection of objects such as asteroids, meteors, and comets.-Design:...

s) are used to track satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

s and asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s, for cosmic-ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...

 research, and for astronomical surveys of the sky. It is more difficult to reduce optical aberrations
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...

 in telescopes with low f-ratio than in telescopes with larger f-ratio.

Light-gathering power

The light-gathering power (or light grasp) of an optical telescope is directly related to the square of the diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 (or aperture) of the objective lens or mirror. Note that the area of a circle is proportional to the square of the radius. A telescope with a lens which has a diameter three times that of another will have nine times the light-gathering power. Larger objectives gather more light, and more sensitive imaging equipment can produce better images from less light.

For a survey of a given area, the field of view is just as important as raw light gathering power. Survey telescopes such as Large Synoptic Survey Telescope
Large Synoptic Survey Telescope
The Large Synoptic Survey Telescope is a planned wide-field "survey" reflecting telescope that will photograph the available sky every three nights. The LSST is currently in its design and development phase and will achieve engineering first light four years after construction starts...

 therefore try to maximize the product of mirror area and field of view (or etendue
Etendue
Etendue or étendue is a property of pencils of rays in an optical system, which characterizes how "spread out" light is in area and angle. It may also be seen as a volume in phase space....

) rather than raw light gathering ability alone.

Imperfect images

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens, the effects of aperture diffraction are unavoidable. In reality, perfect mirrors and perfect lenses do not exist, so image aberrations
Aberration in optical systems
Aberrations are departures of the performance of an optical system from the predictions of paraxial optics. Aberration leads to blurring of the image produced by an image-forming optical system. It occurs when light from one point of an object after transmission through the system does not converge...

 in addition to aperture diffraction must be taken into account. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von Seidel
Philipp Ludwig von Seidel
Philipp Ludwig von Seidel was a German mathematician. His mother was Julie Reinhold and his father was Justus Christian Felix Seidel.Lakatos credits von Seidel with discovering, in 1847, the crucial analytic concept of uniform convergence, while analyzing an incorrect proof of Cauchy's.In 1857,...

 (1821–1896) decomposed the first order monochromatic aberrations into five constituent aberrations. They are now commonly referred to as the five Seidel Aberrations.

The five Seidel aberrations

Spherical aberration
Spherical aberration
thumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the [[Optical axis|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...

 : The difference in focal length between paraxial rays and marginal rays, proportional to the square of the aperture.
Coma
Coma (optics)
In optics , the coma in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components which results in off-axis point sources such as stars appearing distorted, appearing to have a tail like a comet...

 : A most objectionable defect by which points are imaged as comet-like asymmetrical patches of light with tails, which makes measurement very imprecise. Its magnitude is usually deduced from the optical sine theorem
Optical sine theorem
In optics, the optical sine theorem states that the products of the index, height, and sine of the slope angle of a ray in object space and its corresponding ray in image space are equal. That is:...

.
Astigmatism
Astigmatism
An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances...

 : The image of a point forms focal lines at the sagittal and tangental foci and in between (in the absence of coma) an elliptical shape.
Curvature of Field: The Petzval field curvature
Petzval field curvature
Petzval field curvature, named for Joseph Petzval, describes the optical aberration in which a flat object normal to the optical axis cannot be brought into focus on a flat image plane....

 means that the image instead of lying in a plane actually lies on a curved surface which is described as hollow or round. This causes problems when a flat imaging device is used e.g. a photographic plate or CCD image sensor.
Distortion: Either barrel or pincushion, a radial distortion which must be corrected for if multiple images are to be combined (similar to stitching multiple photos into a panoramic photo
Panoramic photography
Panoramic photography is a technique of photography, using specialized equipment or software, that captures images with elongated fields of view. It is sometimes known as wide format photography. The term has also been applied to a photograph that is cropped to a relatively wide aspect ratio...

).

They are always listed in the above order since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration, Spherical Aberration, is independent of the position of the exit pupil (as it is the same for axial and extra-axial pencils). The second, coma, changes as a function of pupil distance and spherical aberration, hence the well-known result that it is impossible to correct the coma in a lens free of spherical aberration by simply moving the pupil. Similar dependencies affect the remaining aberrations in the list.

The chromatic aberrations

Longitudinal chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

: As with spherical aberration this is the same for axial and oblique pencils.
Transverse chromatic aberration (chromatic aberration of magnification)

Astronomical research telescopes

Optical telescopes have been used in astronomical research since the time of their invention in the early 17th century. Many types have be constructed over the years depending on the optical technology, such as refracting and reflecting, the nature of the light or object being imaged, and even where they are placed, such as space telescopes. Some are classified by the task they perform such as Solar telescope
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.-Professional solar telescopes:...

s,

Large reflectors

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:
  • In a lens the entire volume of material has to be free of imperfection and inhomogeneities, whereas in a mirror, only one surface has to be perfectly polished.
  • Light of different colors travels through a medium other than vacuum at different speeds. This causes chromatic aberration
    Chromatic aberration
    In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...

    .
  • Reflectors work in a wider spectrum
    Spectrum
    A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...

     of light since certain wavelengths are absorbed when passing through glass elements like those found in a refractor or catadioptric.
  • There are technical difficulties involved in manufacturing and manipulating large-aperture lenses. One of them is that all real materials sag in gravity. A lens can only be held by its perimeter. A mirror, on the other hand, can be supported by the whole side opposite to its reflecting face.


Most large research telescopes can operate as either a Cassegrain telescope (longer focal length, and a narrower field with higher magnification) or a Newtonian telescope
Newtonian telescope
The 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...

 (brighter field). They have a pierced primary mirror, a Newtonian focus, and a spider to mount a variety of replaceable secondary mirrors.
A new era of telescope making was inaugurated by the Multiple Mirror Telescope (MMT), with a mirror composed of six segments synthesizing a mirror of 4.5 meter
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

s diameter. This has now been replaced by a single 6.5 m mirror. Its example was followed by the Keck telescopes with 10 m segmented mirrors.

The largest current ground-based telescopes have a primary mirror
Primary mirror
A primary mirror is the principal light-gathering surface of a reflecting telescope.-Description:The primary mirror of a reflecting telescope is a spherical or parabolic shaped disks of polished reflective metal , or in later telescopes, glass or other material coated with a reflective layer...

 of between 6 and 11 meters in diameter. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics
Active optics
Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress...

). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.

Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are robotic telescope
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...

s, computer controlled over the internet (see e.g. the Liverpool Telescope
Liverpool Telescope
The Liverpool Telescope is a fully robotic telescope that observes autonomously, i.e. it operates without human intervention. Professional astronomers and other registered users submit observation specifications to be considered by the telescope's robotic control system at any time of the day or...

 and the Faulkes Telescope North
Faulkes Telescope North
The Faulkes Telescope North is a clone of the Liverpool Telescope, and is located at Haleakala Observatory in the U.S. state of Hawaii.The telescope is owned and operated by LCOGT. This telescope and its sister telescope Faulkes Telescope South are used by research and education groups across the...

 and South
Faulkes Telescope South
The Faulkes Telescope South is a clone of the Liverpool Telescope and is located at Siding Spring Observatory. It is a Ritchey-Chrétien telescope....

), allowing automated follow-up of astronomical events.

Initially the detector used in telescopes was the human eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

. Later, the sensitized photographic plate
Photographic plate
Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...

 took its place, and the spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...

 was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors, such as the charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 (CCDs), have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage.

Current research telescopes have several instruments to choose from such as:
  • imagers, of different spectral responses
  • spectrographs, useful in different regions of the spectrum
  • polarimeters, that detect light polarization.


The phenomenon of optical diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

 sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc
Airy disc
In optics, the Airy disk and Airy pattern are descriptions of the best focused spot of light that a perfect lens with a circular aperture can make, limited by the diffraction of light....

, which limits how close two such discs can be placed. This absolute limit is called the diffraction limit (and may be approximated by the Rayleigh criterion, Dawes limit
Dawes limit
Dawes' limit is a formula to express the maximum resolving power of a microscope or telescope. It is so named for its discoverer, W. R. Dawes, although it is also credited to Lord Rayleigh.The formula takes different forms depending on the units....

 or Sparrow's resolution limit
Sparrow's resolution limit
Sparrow's Resolution Limit is an estimate of the angular resolution limit of an optical telescope or microscope.When a star is observed with a telescope, the light is spread into an Airy disk...

). This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter
Diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

 of the telescope mirror. This means that a telescope with a certain mirror diameter can theoretically resolve up to a certain limit at a certain wavelength. For conventional telescopes on Earth, the diffraction limit is not relevant for telescopes bigger than about 10 cm. Instead, the seeing
Astronomical seeing
Astronomical seeing refers to the blurring and twinkling of astronomical objects such as stars caused by turbulent mixing in the Earth's atmosphere varying the optical refractive index...

, or blur caused by the atmosphere, sets the resolution limit. But in space, or if adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

 are used, then reaching the diffraction limit is sometimes possible. At this point, if greater resolution is needed at that wavelength, a wider mirror has to be built or aperture synthesis
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...

 performed using an array of nearby telescopes.

In recent years, a number of technologies to overcome the distortions caused by atmosphere
Earth's atmosphere
The atmosphere of Earth 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...

 on ground-based telescopes have been developed, with good results. See adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...

, speckle imaging
Speckle imaging
Speckle imaging describes a range of high-resolution astronomical imaging techniques based either on the shift-and-add method or on speckle interferometry methods...

 and optical interferometry.

Famous optical telescopes

  • The Hubble Space Telescope
    Hubble Space Telescope
    The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

     is in orbit beyond Earth's atmosphere to allow for observations not distorted by atmospheric seeing. In this way the images can be diffraction
    Diffraction
    Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...

     limited, and used for coverage in the ultraviolet
    Ultraviolet
    Ultraviolet 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...

     (UV) and infrared. Also there is no background
    Background (astronomy)
    In astronomy, background commonly refers to the incoming light from an apparently empty part of the night sky.Even if no visible astronomical objects are present in given part of the sky, there always is some low luminosity present, due mostly to light diffusion from the atmosphere...

     from light scattered by the air so very deep images are possible, despite the relatively small mirror size.
  • The Keck telescopes are no longer the largest, and were superseded by the Gran Telescopio Canarias
    Gran Telescopio Canarias
    The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...

    .
  • The Hobby-Eberly Telescope
    Hobby-Eberly Telescope
    The Hobby-Eberly Telescope is a 9.2-meter aperture telescope located at the McDonald Observatory. It combines a number of features that differentiate it from most telescope designs, resulting in greatly lowered construction costs...

     and Southern African Large Telescope
    Southern African Large Telescope
    The Southern African Large Telescope is a 66m2 area optical telescope with a nominally 9.2 meter aperture but up to about 11.1m x ~9.8 m diameter aperture, and designed mainly for spectroscopy. It is located close to the town of Sutherland in the semi-desert region of the Karoo, South Africa...

     are large 9.2 meter telescopes of a very different design. The mirror is held stationary and objects tracked by moving the instruments. This has significant operational restrictions, but gives a large aperture for a fraction of the cost of a fully steerable telescope.
  • The Very Large Telescope
    Very Large Telescope
    The Very Large Telescope is a telescope operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. The VLT consists of four individual telescopes, each with a primary mirror 8.2m across, which are generally used separately but can be used together to...

     array (VLT) at Paranal Observatory
    Paranal Observatory
    Paranal Observatory is an astronomical observatory located on Cerro Paranal at 2,635 m altitude and operated by the European Southern Observatory. The Very Large Telescope is the largest telescope on Paranal, actually composed of four separate 8.2 m telescopes...

     is currently the record holder for total collecting area in an array of telescopes, with four telescopes each 8 meter
    Metre
    The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...

    s in diameter. The four telescopes, belonging to the European Southern Observatory
    European Southern Observatory
    The European Southern Observatory is an intergovernmental research organisation for astronomy, supported by fifteen countries...

     (ESO) and located in the Atacama desert
    Atacama Desert
    The Atacama Desert is a plateau in South America, covering a strip of land on the Pacific coast, west of the Andes mountains. It is, according to NASA, National Geographic and many other publications, the driest desert in the world...

     in Chile
    Chile
    Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...

    , are usually operated independently for faint astronomical observations, but up to three telescopes can be operated together for aperture synthesis
    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...

     observations of bright objects.
  • The Navy Prototype Optical Interferometer
    Navy Prototype Optical Interferometer
    The Navy Prototype Optical Interferometer , is an astronomical interferometer operated by the United States Naval Observatory Flagstaff Station, in collaboration with the Naval Research Laboratory and The Lowell Observatory...

     is the optical telescope (array) that can currently produce the highest resolution images at visible wavelengths.
  • The CHARA (Center for High Angular Resolution Astronomy) array
    CHARA array
    The CHARA Array is an optical astronomical interferometer operated by The Center for High Angular Resolution Astronomy of the Georgia State University . CHARA is the World's highest angular resolution telescope at near-infrared wavelengths...

     is the telescope array that can currently produce the highest resolution images at near-infrared wavelengths.
  • There are many plans for even larger telescopes. One of them is the Overwhelmingly Large Telescope
    Overwhelmingly Large Telescope
    The Overwhelmingly Large Telescope is a conceptual design by the European Southern Observatory organization for an extremely large telescope, which was intended to have a single aperture of 100 meters in diameter...

     (OWL), which is intended to have a single aperture of 100 meters in diameter.
  • The 200-inch (5.08 m) Hale telescope
    Hale telescope
    The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...

     on Palomar Mountain
    Palomar Mountain
    Palomar Mountain is a mountain in the Peninsular Ranges in northern San Diego County. It is famous as the location of the Palomar Observatory and Hale Telescope, and known for the Palomar Mountain State Park.-History:...

     was the largest conventional research telescope for many years. It has a single borosilicate
    Borosilicate glass
    Borosilicate glass is a type of glass with the main glass-forming constituents silica and boron oxide. Borosilicate glasses are known for having very low coefficients of thermal expansion , making them resistant to thermal shock, more so than any other common glass...

     (Pyrex
    Pyrex
    Pyrex is a brand name for glassware, introduced by Corning Incorporated in 1915.Originally, Pyrex was made from borosilicate glass. In the 1940s the composition was changed for some products to tempered soda-lime glass, which is the most common form of glass used in glass bakeware in the US and has...

    ) mirror that was famously difficult to construct. The mounting is a special design of equatorial mount called a yoke mount, which permits the telescope to be pointed at and near the north celestial pole.
  • The 100-inch (2.54 m) Hooker Telescope at the Mount Wilson Observatory
    Mount Wilson Observatory
    The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...

     was used by Edwin Hubble
    Edwin Hubble
    Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...

     to discover galaxies
    Galaxy
    A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...

     and the redshift
    Redshift
    In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...

    . The mirror was made of green glass by Saint-Gobain
    Saint-Gobain
    Saint-Gobain S.A. is a French multinational corporation, founded in 1665 in Paris and headquartered on the outskirts of Paris at La Défense and in Courbevoie. Originally a mirror manufacturer, it now also produces a variety of construction and high-performance materials.The company has its head...

    . In 1919, the telescope was used for the first stellar diameter measurements using interferometry. The telescope now has an adaptive optics system, and is still useful for advanced research.
  • The 72-inch Leviathan
    Leviathan of Parsonstown
    Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in aperture, which was the largest telescope in the world from 1845 until the construction of the 100 in Hooker Telescope in 1917...

     at Birr Castle
    Birr Castle
    Birr Castle is a large castle in the town of Birr in County Offaly, Ireland. It is the home of the seventh Earl of Rosse, and as such the residential areas of the castle are not open to the public, though the grounds and gardens of the demesne are publicly accessible.-Ireland's Historic Science...

     (in Ireland
    Ireland
    Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

    ) was the largest telescope in the world from 1845 until it was dismantled in 1908. It was not exceeded in size until the construction of the Hooker Telescope.
  • The 1.02-meter Yerkes Telescope
    Yerkes Observatory
    Yerkes Observatory is an astronomical observatory operated by the University of Chicago in Williams Bay, Wisconsin. The observatory, which calls itself "the birthplace of modern astrophysics," was founded in 1897 by George Ellery Hale and financed by Charles T. Yerkes...

     (in Wisconsin
    Wisconsin
    Wisconsin is a U.S. state located in the north-central United States and is part of the Midwest. It is bordered by Minnesota to the west, Iowa to the southwest, Illinois to the south, Lake Michigan to the east, Michigan to the northeast, and Lake Superior to the north. Wisconsin's capital is...

    ) is the largest aimable refracting telescope in use.
  • The Great Lick 36-inch (0.91 m) refractor built in 1889 at the Lick Observatory
    Lick Observatory
    The Lick Observatory is an astronomical observatory, owned and operated by the University of California. It is situated on the summit of Mount Hamilton, in the Diablo Range just east of San Jose, California, USA...

     on Mt. Hamilton near San Jose, California
    San Jose, California
    San Jose is the third-largest city in California, the tenth-largest in the U.S., and the county seat of Santa Clara County which is located at the southern end of San Francisco Bay...

    .
  • The 0.76-meter Nice refractor
    Nice Observatory
    The Observatoire de Nice is an astronomical observatory located in Nice, France on the summit of Mont Gros. The observatory was initiated in 1879 by the banker Raphaël Bischoffsheim...

     (in France
    France
    The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...

    ) that became operational in 1888 was at that time the world's largest refractor. It was exceeded in size one year later; this was the last time the most powerful operational telescope in the world was located in Europe.
  • The Great Paris Exhibition Telescope of 1900
    Great Paris Exhibition Telescope of 1900
    The Great Paris Exhibition Telescope of 1900, with an objective lens of 1.25 m in diameter, was the largest refracting telescope ever constructed. It was built as the centerpiece of the Paris Universal Exhibition of 1900. Its construction was instigated in 1892 by François Deloncle , a member of...

    , the largest refractor ever constructed. It was on display at the 1900 Paris Exposition. Its lens was stationary, prefigured so as to sag into the correct shape. The telescope was aimed by the aid of a Foucault sidérostat, which is a movable plane mirror with a 2-meter diameter, mounted in a large cast-iron frame. The horizontal tube was 60 m long and the objective was 1.25 m in diameter. It was a never successful as a research telescope and was dismantled and scrapped after the Exhibition.
  • The Gran Telescopio Canarias
    Gran Telescopio Canarias
    The Gran Telescopio Canarias , also known as GranTeCan or GTC, is a reflecting telescope undertaking commissioning observations at the Roque de los Muchachos Observatory on the island of La Palma, in the Canary Islands of Spain, as of July 2009.Construction of the telescope, sited on a volcanic...

     (Grantecan, also GTC), is a high performance segmented 10.4 meter telescope, located in one of the best sites of the Northern Hemisphere: the Roque de los Muchachos Observatory (La Palma, Canary Islands
    Canary Islands
    The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...

    , Spain
    Spain
    Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

    ). After completion in 2009, it became the world's largest single-aperture optical telescope.
  • The refracting Swedish 1-m Solar Telescope
    Swedish Solar Telescope
    The Swedish 1-m Solar Telescope is a refracting solar telescope at Roque de los Muchachos Observatory, La Palma in the Canary Islands. It is run by the Institute for Solar Physics of the Royal Swedish Academy of Sciences. The primary element is a single fused silica lens, making it the second...

     (SST) on La Palma (Spain
    Spain
    Spain , officially the Kingdom of Spain languages]] under the European Charter for Regional or Minority Languages. In each of these, Spain's official name is as follows:;;;;;;), is a country and member state of the European Union located in southwestern Europe on the Iberian Peninsula...

    ), is currently the highest-resolution solar telescope in the world.
  • The 26 inch refracting US Naval Observatory Telescope
    United States Naval Observatory
    The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...

     in Washington D.C. was used in the discovery of the two moons of Mars
    Mars
    Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...

    , Phobos
    Phobos (moon)
    Phobos is the larger and closer of the two natural satellites of Mars. Both moons were discovered in 1877. With a mean radius of , Phobos is 7.24 times as massive as Deimos...

     and Deimos
    Deimos (moon)
    Deimos is the smaller and outer of Mars's two moons . It is named after Deimos, a figure representing dread in Greek Mythology. Its systematic designation is '.-Discovery:Deimos was discovered by Asaph Hall, Sr...

    .

See also

  • Astronomy
    Astronomy
    Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

  • Astrophotography
    Astrophotography
    Astrophotography is a specialized type of photography that entails recording images of astronomical objects and large areas of the night sky. The first photographs of an astronomical object were taken in the 1840s, but it was not until the late 19th century that advances in technology allowed for...

  • Amateur telescope making
    Amateur telescope making
    Amateur telescope making is the activity of building telescopes as a hobby, as opposed to being a paid professional. Amateur telescope makers build their instruments for personal enjoyment of a technical challenge, as a way to obtain an inexpensive or personally customized telescope, or as a...

  • Depth of field
    Depth of field
    In optics, particularly as it relates to film and photography, depth of field is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image...

  • Dipleidoscope
    Dipleidoscope
    A dipleidoscope is an instrument used to determine true noon. It consists of a small telescope and a prism that creates a double image of the sun. When the two images overlap, it is local true noon...

  • Globe effect
    Globe effect
    The globe effect, sometimes called the rolling ball effect or the spinning globe effect, is an optical phenomenon that occurs with visual optical instruments, in particular binoculars and telescopes, that are designed to be free of distortion. When these instruments are panned, the moving image...

  • Bahtinov mask
    Bahtinov mask
    The Bahtinov mask is a device used to accurately focus astronomical telescopes.It is named after its inventor Pavel Bahtinov. Accurate focusing of telescopes and astrographs is particularly of concern to those involved in astrophotography....

  • Carey mask
    Carey mask
    A Carey mask is a focusing aid for astronomical telescopes. The mask is in the form of a thin card or sheet that is placed over the front aperture of the telescope. There are four series of slits in the mask which form a diffraction pattern in the image plane.In this example the two sets of slits...

  • Hartmann mask
    Hartmann mask
    Hartmann mask is a tool to help focusing telescopes, mainly used by amateur astronomers.- Theory and practice :...

  • History of optics
    History of optics
    Optics began with the development of lenses by the ancient Egyptians and Mesopotamians, followed by theories on light and vision developed by ancient Greek and Indian philosophers, and the development of geometrical optics in the Greco-Roman world. The word optics is derived from the Greek term τα...

  • List of optical telescopes
  • List of largest optical reflecting telescopes
  • List of largest optical refracting telescopes
  • List of largest optical telescopes historically
  • List of solar telescopes
  • List of space telescopes
  • List of telescope types

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK