History of telescopes - AbsoluteAstronomy.com

The earliest known working 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...

s appeared in 1608 and are credited to 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:...

. Among many others who claimed to have made the discovery were 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...

, a spectacle-maker in Middelburg

Middelburg

Middelburg is a municipality and a city in the south-western Netherlands and the capital of the province of Zeeland. It is situated in the Midden-Zeeland region. It has a population of about 48,000.- History of Middelburg :...

, and 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:...

. The design of these early 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 consisted of a convex 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,...

lens and a concave 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...

. 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...

used this design the following year. In 1611, 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...

described how a telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces. Hans Lippershey is the earliest person documented to have applied for a patent for the device.

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."...

is credited with building the first "practical" reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain

Laurent Cassegrain

Laurent Cassegrain was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector, a folded two mirror reflecting telescope design.-Biography:...

in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.

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....

, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall

Chester Moore Hall

Chester Moore Hall was a British lawyer and inventor who produced the first achromatic lenses in 1729 or 1733 ....

, who did not publicize it. John Dollond

John Dollond

John Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...

independently developed achromatic lenses and produced telescopes using them in commercial quantities, starting in 1758.

Important developments in reflecting telescopes were John Hadley

John Hadley

John Hadley was an English mathematician, inventor of the octant, a precursor to the sextant, around 1730.He was born in Bloomsbury, London, to Katherine FitzJames and George Hadley....

's production of larger paraboloid

Paraboloid

In mathematics, a paraboloid is a quadric surface of special kind. There are two kinds of paraboloids: elliptic and hyperbolic. The elliptic paraboloid is shaped like an oval cup and can have a maximum or minimum point....

al mirrors in 1721; the process of silvering

Silvering

Silvering is the chemical process of coating glass with a reflective substance. When glass mirrors first gained widespread usage in Europe during the 16th century, most were made of an amalgam of tin and mercury, but by the 19th century mirrors were commonly made through a process by which silver...

glass mirrors introduced by Léon Foucault

Léon Foucault

Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...

in 1857; and the adoption of long lasting aluminized coatings on reflector mirrors in 1932. Almost all of the large optical research telescopes used today are reflectors.

The era of radio telescope

Radio telescope

A 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 (along with radio astronomy

Radio astronomy

Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The initial detection of radio waves from an astronomical object was made in the 1930s, when Karl Jansky observed radiation coming from the Milky Way. Subsequent observations have identified a number of...

) was born with Karl Guthe Jansky

Karl Guthe Jansky

Karl Guthe Jansky was an American physicist and radio engineer who in August 1931 first discovered radio waves emanating from the Milky Way. He is considered one of the founding figures of radio astronomy.- Early life :...

's serendipitous

Serendipity

Serendipity means a "happy accident" or "pleasant surprise"; specifically, the accident of finding something good or useful without looking for it. The word has been voted as one of the ten English words hardest to translate in June 2004 by a British translation company. However, due to its...

discovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays.

Optical foundations

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...

es and their properties were known well before the invention of the optical telescope; simple 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...

made from rock crystal have been known from before recorded history. Ptolemy

Ptolemy

Claudius Ptolemy , was a Roman citizen of Egypt who wrote in Greek. He was a mathematician, astronomer, geographer, astrologer, and poet of a single epigram in the Greek Anthology. He lived in Egypt under Roman rule, and is believed to have been born in the town of Ptolemais Hermiou in the...

(in his work Optics written in the 2nd century AD) wrote about the properties of light including reflection

Reflection (physics)

Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...

, refraction

Refraction

Refraction is the change in direction of a wave due to a change in its speed. It is essentially a surface phenomenon . The phenomenon is mainly in governance to the law of conservation of energy. The proper explanation would be that due to change of medium, the phase velocity of the wave is changed...

, and color

Color

Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

. During the 10th century, the Persian scholar Ibn Sahl

Ibn Sahl

This article is about the physicist. For the physician, see Ali ibn Sahl Rabban al-Tabari. For the poet, see Ibn Sahl of Sevilla.Ibn Sahl was a Muslim Persian mathematician, physicist and optics engineer of the Islamic Golden Age associated with the Abbasid court of Baghdad...

, was to make some of the most refined descriptions in respect to optics at the time.

It was approximately from the 12th century in Europe that 'reading stone

Reading stone

A reading stone was an approximately hemispherical lens that was placed on top of text to magnify the letters so that people with presbyopia could read it more easily. Reading stones were among the earliest common uses of lenses....

s' (magnifying lenses placed on the reading material) were well documented—as well as the use of lenses as burning glasses. It is generally considered that spectacles for correcting long sightedness

Presbyopia

Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

with convex lenses were invented in Northern Italy in the late 13th to early 14th century, and the invention of the use of concave lenses to correct near-sightedness

Myopia

Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

is ascribed to Nicholas of Cusa

Nicholas of Cusa

Nicholas of Kues , also referred to as Nicolaus Cusanus and Nicholas of Cusa, was a cardinal of the Catholic Church from Germany , a philosopher, theologian, jurist, mathematician, and an astronomer. He is widely considered one of the great geniuses and polymaths of the 15th century...

in 1451. Thus, early knowledge of lenses and the availability of lenses for spectacles from the 13th century onwards through the 16th century means that it was possible for many individuals to discover the principles of a telescope using a combination of concave or concave and convex lenses; in the 13th century, Robert Grosseteste

Robert Grosseteste

Robert Grosseteste or Grossetete was an English statesman, scholastic philosopher, theologian and Bishop of Lincoln. He was born of humble parents at Stradbroke in Suffolk. A.C...

wrote several scientific treatises between 1230 and 1235, including De Iride (Concerning the Rainbow), in which he said:
Roger Bacon

Roger Bacon

Roger Bacon, O.F.M. , also known as Doctor Mirabilis , was an English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empirical methods...

was a pupil of Grosseteste at Oxford, and is frequently stated as having described a magnifying device in the 13th century, however it is not certain if he built a working model.

Pre 17th century developments

There is some documentary evidence, but no surviving designs or physical evidence, that the principles of telescopes were known in the late 16th century. Writings by John Dee

John Dee (mathematician)

John Dee was an English mathematician, astronomer, astrologer, occultist, navigator, imperialist and consultant to Queen Elizabeth I. He devoted much of his life to the study of alchemy, divination and Hermetic philosophy....

and Thomas Digges

Thomas Digges

Sir Thomas Digges was an English mathematician and astronomer. He was the first to expound the Copernican system in English but discarded the notion of a fixed shell of immoveable stars to postulate infinitely many stars at varying distances; he was also first to postulate the "dark night sky...

in England in 1570 and 1571, respectively ascribe the use of both reflecting and refracting telescopes to Thomas' father Leonard Digges, and it is independently confirmed by a report by William Bourne

William Bourne (mathematician)

William Bourne was an English mathematician, innkeeper and former Royal Navy gunner who invented the first navigable submarine and wrote important navigational manuals...

in approximately 1580. They may have been experimental devices and were never widely reported or reproduced. Thomas Digges describes his father's device as follows:
Although Digges may have created a rudimentary instrument involving lenses and mirrors, the optical performance required to see the details of coins lying about in fields, or private activities seven miles away, was far beyond the technology of the time.

In Italy, Giambattista della Porta

Giambattista della Porta

Giambattista della Porta , also known as Giovanni Battista Della Porta and John Baptist Porta, was an Italian scholar, polymath and playwright who lived in Naples at the time of the Scientific Revolution and Reformation....

also described a possible telescope as early as 1586 when he wrote in a letter,"...to make glasses that can recognize a man several miles away." In his Natural Magic
Natural Magic
is a work of popular science by Giambattista della Porta first published in Naples in 1558. Its popularity ensured it was republished in five Latin editions within ten years, with translations into Italian , French, and Dutch printed.Natural Magic was revised and considerably expanded...

published in 1589 he wrote:
Della Porta was preoccupied with other things at the time and thought the idea of a "telescope" unimportant. Similar claims have been made about the Catalan Joan Roget (died before 1624) inventing early telescope-like devices.

The first known telescopes

The practical exploitation of the instrument was certainly achieved and came to public attention in the Netherlands at about 1608, but the credit of the original invention has been claimed on behalf of three individuals: Hans Lippershey

Hans Lippershey

and Sacharias Jansen—spectacle-makers in Middelburg

Middelburg

, and 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

(also known as Jacob Adriaanszoon). Hans Lippershey was credited with creating and disseminating designs for the first practical telescope—later applying to the States-General of the Netherlands

States-General of the Netherlands

The States-General of the Netherlands is the bicameral legislature of the Netherlands, consisting of the Senate and the House of Representatives. The parliament meets in at the Binnenhof in The Hague. The archaic Dutch word "staten" originally related to the feudal classes in which medieval...

on October 2, 1608, for a patent

Patent

A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

for an instrument "for seeing things far away as if they were nearby," (beating 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...

's patent by a few weeks). Lippershey failed to receive a patent since the same claim for invention had been made by other spectacle-makers. Lippershey was handsomely rewarded by the Dutch government

Government

Government refers to the legislators, administrators, and arbitrators in the administrative bureaucracy who control a state at a given time, and to the system of government by which they are organized...

for copies of his design

Design

Design as a noun informally refers to a plan or convention for the construction of an object or a system while “to design” refers to making this plan...

. Sacharias Jansen's design for a telescope may have pre-dated Lippershey and Metius, but the invention was never widely publicized.

The original Dutch telescopes were composed of a convex and a concave lens—telescopes that are constructed this way do not invert the image. Lippershey's original design had only 3x 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"...

. Telescopes seem to have been made in the Netherlands in considerable numbers soon after the date of their invention, and rapidly found their way all over Europe.

Galileo

Galileo Galilei

happened to be in Venice

Venice

Venice is a city in northern Italy which is renowned for the beauty of its setting, its architecture and its artworks. It is the capital of the Veneto region...

in June 1609 and there heard of the "Dutch perspective glass" by means of which distant objects appeared nearer and larger. Galileo states that he solved the problem of the construction of a telescope the first night after his return to Padua

Padua

Padua is a city and comune in the Veneto, northern Italy. It is the capital of the province of Padua and the economic and communications hub of the area. Padua's population is 212,500 . The city is sometimes included, with Venice and Treviso, in the Padua-Treviso-Venice Metropolitan Area, having...

from Venice and made his first telescope the next day by fitting a convex lens in one extremity of a leaden tube and a concave lens in the other one. A few days afterwards, having succeeded in making a better telescope than the first, he took it to Venice where he communicated the details of his invention to the public and presented the instrument itself to the doge

Doge of Venice

The Doge of Venice , often mistranslated Duke was the chief magistrate and leader of the Most Serene Republic of Venice for over a thousand years. Doges of Venice were elected for life by the city-state's aristocracy. Commonly the person selected as Doge was the shrewdest elder in the city...

Leonardo Donato

Leonardo Donà, or Donato was the 90th Doge of Venice, reigning from January 10, 1606 until his death...

, who was sitting in full council. The senate

Senate

A senate is a deliberative assembly, often the upper house or chamber of a legislature or parliament. There have been many such bodies in history, since senate means the assembly of the eldest and wiser members of the society and ruling class...

in return settled him for life in his lectureship at Padua and doubled his salary.

Galileo devoted his time to improving and perfecting the telescope and soon succeeded in producing telescopes of greatly increased power. His first telescope magnified three diameters, but he soon made instruments which magnified eight diameters and finally, one that magnified thirty-three diameters. With this last instrument, he discovered in 1610 the satellites of Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

and soon afterwards the spots on the sun

Sunspot

Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....

, the phases of Venus

Venus

Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...

, and the hills and valleys on the Moon

Moon

The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...

. In this last achievement he now appears to have been preceded by Thomas Harriot

Thomas Harriot

Thomas Harriot was an English astronomer, mathematician, ethnographer, and translator. Some sources give his surname as Harriott or Hariot or Heriot. He is sometimes credited with the introduction of the potato to Great Britain and Ireland...

who made the first drawings of the moon with the aid of a telescope in July 1609. Galileo demonstrated the revolution of the satellites of Jupiter around the planet and gave rough predictions of their configurations, proved the rotation of the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

on its axis, established the general truth of the Copernican system as compared with that of Ptolemy

Ptolemy

, and fairly routed the fanciful dogma

Dogma

Dogma is the established belief or doctrine held by a religion, or a particular group or organization. It is authoritative and not to be disputed, doubted, or diverged from, by the practitioners or believers...

s of the philosophers. Galileo's instrument was the first to be given the name "telescope". The name was invented by the Greek poet/theologian Giovanni Demisiani

Giovanni Demisiani

Giovanni Demisiani , a Greek from Zakynthos, was a theologian, chemist, mathematician to Cardinal Gonzaga, and member of the Accademia dei Lincei...

at a banquet held on April 14, 1611 by Prince Federico Cesi

Federico Cesi

Federico Angelo Cesi was an Italian scientist, naturalist, and founder of the Accademia dei Lincei. On his father's death in 1630, he became briefly lord of Acquasparta.- Biography :...

to make Galileo Galilei

Galileo Galilei

a member of the Accademia dei Lincei

Accademia dei Lincei

The Accademia dei Lincei, , is an Italian science academy, located at the Palazzo Corsini on the Via della Lungara in Rome, Italy....

. The word was created from the Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing'.

These brilliant achievements, together with Galileo's immense improvement of the instrument, overshadowed to a great degree the credit due to the original inventor, and led to the universal adoption of the name of the Galilean telescope for the form of the instrument invented by Lippershey.

Refracting telescopes

Johannes Kepler

first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611). The first person who actually constructed a telescope of this form was the Jesuit

Society of Jesus

The Society of Jesus is a Catholic male religious order that follows the teachings of the Catholic Church. The members are called Jesuits, and are also known colloquially as "God's Army" and as "The Company," these being references to founder Ignatius of Loyola's military background and a...

Christoph Scheiner

Christoph Scheiner SJ was a Jesuit priest, physicist and astronomer in Ingolstadt....

who gives a description of it in his Rosa Ursina (1630).

William Gascoigne

William Gascoigne (scientist)

William Gascoigne was an English astronomer, mathematician and maker of scientific instruments from Middleton near Leeds who invented the micrometer...

was the first who commanded a chief advantage of the form of telescope suggested by Kepler: that a small material object could be placed at the common focal plane of the objective and the eyepiece. This led to his invention of the micrometer

Micrometer

A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...

, and his application of telescopic sights to precision astronomical instruments. It was not till about the middle of the 17th century that Kepler's telescope came into general use: not so much because of the advantages pointed out by Gascoigne, but because its field of view

Field of view

The field of view is the extent of the observable world that is seen at any given moment....

was much larger than in the Galilean telescope.

The first powerful telescopes of Keplerian construction were made by Christiaan Huygens after much labor—in which his brother assisted him. With one of these: an objective diameter of 2.24 inches (57mm) and a 12 ft (3.7 m) focal length, he discovered the brightest of Saturn's satellites (Titan

Titan (moon)

Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....

) in 1655; in 1659, he published his "Systema Saturnium" which, for the first time, gave a true explanation of Saturn's ring

Planetary ring

A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...

—founded on observations made with the same instrument.

Long focal length refractors

The sharpness of the image in Kepler's telescope was limited by the 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...

introduced by the non-uniform refractive properties of the objective lens. The only way to overcome this limitation at high magnifying powers was to create objectives with very long focal lengths. Giovanni Cassini

Giovanni Domenico Cassini

This article is about the Italian-born astronomer. For his French-born great-grandson, see Jean-Dominique Cassini.Giovanni Domenico Cassini was an Italian/French mathematician, astronomer, engineer, and astrologer...

discovered Saturn's fifth satellite (Rhea

Rhea (moon)

Rhea is the second-largest moon of Saturn and the ninth largest moon in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini.-Name:Rhea is named after the Titan Rhea of Greek mythology, "mother of the gods"...

) in 1672 with a telescope 35 ft (10.7 m) long. Astronomers such as Johannes Hevelius

Johannes Hevelius

Johannes Hevelius Some sources refer to Hevelius as Polish:Some sources refer to Hevelius as German:*Encyplopedia Britannica * of the Royal Society was a councilor and mayor of Danzig , Pomeranian Voivodeship, in the Polish-Lithuanian Commonwealth...

were constructing telescopes with focal lengths as long as 150 feet (45 m). Besides having really long tubes these telescopes needed scaffolding or long masts and cranes to hold them up. Their value as research tools was minimal since the telescope's frame "tube" flexed and vibrated in the slightest breeze and sometimes collapsed altogether.

Aerial telescopes

In some of the very long refracting telescopes constructed after 1675, no tube was employed at all. The objective was mounted on a swiveling ball-joint on top of a pole, tree, or any available tall structure and aimed by means of string or connecting rod. The eyepiece was hand held or mounted on a stand at the focus, and the image was found by trial and error. These were consequently termed aerial telescope

Aerial telescope

An aerial telescope is a type of very-long-focal-length refracting telescope built in the second half of the 17th century that did not use a tube. Instead, the objective was mounted on a pole, tree, tower, building or other structure on a swivel ball-joint. The observer stood on the ground and held...

s. and have been attributed to Christiaan Huygens and his brother Constantijn Huygens, Jr.

Constantijn Huygens, Jr.

Constantijn Huygens Jr. was a Dutch statesman also known for his work on scientific instruments and as a chronicler of his times...

although it is not clear that they invented it. Christiaan Huygens and his brother made objectives up to 8.5 inch (220mm) diameter and 210 ft (64 m) focal length and others such as Adrien Auzout

Adrien Auzout

Adrien Auzout was a French astronomer.He was born in Rouen, France, the son of a clerk in the court of Rouen. His educational background is unknown. In 1664–1665 he made observations of comets, and argued in favor of their following elliptical or parabolic orbits...

made telescopes with focal lengths up to 600 ft (180 m). Telescopes of such great length were naturally difficult to use and must have taxed to the utmost the skill and patience of the observers. Aerial telescopes were employed by several other astronomers. Cassini discovered Saturn's third and fourth satellites in 1684 with aerial telescope objectives made by Giuseppe Campani

Giuseppe Campani

Giuseppe Campani was an Italian optician and astronomer who lived in Rome during the latter half of the 17th century.His brother, Matteo Campani-Alimenis, and he were experts in grinding and polishing lenses, especially for very long focal length aerial telescope objectives...

that were 100 and 136 ft (30.5 and 41.5 m) in focal length.

Reflecting telescopes

The ability of a curved mirror

Curved mirror

A curved mirror is a mirror with a curved reflective surface, which may be either convex or concave . Most curved mirrors have surfaces that are shaped like part of a sphere, but other shapes are sometimes used in optical devices...

to form an image may have been known since the time of Euclid

Euclid

Euclid , fl. 300 BC, also known as Euclid of Alexandria, was a Greek mathematician, often referred to as the "Father of Geometry". He was active in Alexandria during the reign of Ptolemy I...

and had been extensively studied by Alhazen in the 11th century. 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 to lenses, discussed the idea of building a telescope using a mirror as the image forming objective. Niccolò Zucchi

Niccolo Zucchi

Niccolò Zucchi was an Italian Jesuit, astronomer, and physicist.As an astronomer he may have been the first to see the belts on the planet Jupiter , and reported spots on Mars in 1640....

, an Italian Jesuit astronomer and physicist, wrote in his book Optica pilosophia of 1652 that he tried replacing the lens of a refracting telescope with a bronze concave mirror in 1616. Zucchi tried looking into the mirror with a hand held concave lens but did not get a satisfactory image, possibly due to the poor quality of the mirror, the angle it was tilted at, or the fact that his head partially obstructed the image.

In 1636 Marin Mersenne

Marin Mersenne

Marin Mersenne, Marin Mersennus or le Père Mersenne was a French theologian, philosopher, mathematician and music theorist, often referred to as the "father of acoustics"...

proposed a telescope consisting of a paraboloidal primary mirror and a paraboloidal secondary mirror bouncing the image through a hole in the primary, solving the problem of viewing the image. 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...

went into further detail in his book Optica Promota (1663), pointing out that a reflecting telescope with a mirror that was shaped like the part of a conic section

Conic section

In mathematics, a conic section is a curve obtained by intersecting a cone with a plane. In analytic geometry, a conic may be defined as a plane algebraic curve of degree 2...

, would correct 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...

as well as the chromatic aberration seen in refractors. The design he came up with bears his name: 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 according to his own confession, Gregory had no practical skill and he could find no optician capable of realizing his ideas and after some fruitless attempts, was obliged to abandon all hope of bringing his telescope into practical use.

In 1666 Isaac Newton

Isaac Newton

, based on his theories of refraction and color, perceived that the faults of the refracting telescope were due more to a lens's varying refraction of light of different colors than to a lens's imperfect shape. He over-hastily concluded from some rough experiments that all refracting substances diverged the prismatic colors in a constant proportion to their mean refraction; and he drew the natural conclusion that light could not be refracted through a lens without causing chromatic aberrations and therefore, that no improvement could be made in the refracting telescope. However, having ascertained by experiment that for all colors of light the angle of incidence

Angle of incidence

Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...

reflected in a mirror was equal to the angle of reflection, he turned his attention to the construction of reflecting telescopes. After much experiment, he chose an alloy

Alloy

An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

(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...

) of tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

and copper

Copper

Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

as the most suitable material for his objective

Objective (optics)

mirror. He later devised means for grinding and polishing them, but chose a spherical shape for his mirror instead of a parabola to simplify construction: he had satisfied himself that the chromatic—and not the spherical aberration—formed the chief faults of previous refracting telescopes. He added to his reflector what is the hallmark of the design of a "Newtonian telescope", a secondary "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle to an eyepiece

Eyepiece

mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made all the tube, mount

Telescope mount

A telescope mount is a mechanical structure which supports a telescope. Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument...

, and fittings. Newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5. With it he found that he could see the four Galilean moons

Galilean moons

The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...

of Jupiter

Jupiter

and the crescent phase of the planet Venus. Encouraged by this success, he made a second telescope with a magnifying power of 38 diameters which he presented to the Royal Society of London in December 1672. This type of telescope is still called 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...

A third form of reflecting telescope, the "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....

" was devised in 1672 by Laurent Cassegrain

Laurent Cassegrain

Laurent Cassegrain was a Catholic priest who is notable as the probable inventor of the Cassegrain reflector, a folded two mirror reflecting telescope design.-Biography:...

. The telescope had a small convex hyperboloidal secondary mirror placed near the prime focus to reflect light through a central hole in the main mirror.

No further practical advance appears to have been made in the design or construction of the reflecting telescopes until the year 1721 when John Hadley

John Hadley

John Hadley was an English mathematician, inventor of the octant, a precursor to the sextant, around 1730.He was born in Bloomsbury, London, to Katherine FitzJames and George Hadley....

(best known as the inventor of the octant

Octant (instrument)

The octant, also called reflecting quadrant, is a measuring instrument used primarily in navigation. It is a type of reflecting instrument.-Etymology:...

) presented to the Royal Society a Newtonian reflector with a metallic speculum objective mirror of a 6 inch (15 cm) aperture and 62¾ inch (159 cm) focal length. The instrument was examined by Pound

James Pound

-Life:He was the son of John Pound, of Bishop's Canning, Wiltshire, where he was born . He matriculated at St. Mary Hall, Oxford, on 16 March 1687; graduated B.A. from Hart Hall on 27 February 1694, and M.A...

and Bradley

James Bradley

James Bradley FRS was an English astronomer and served as Astronomer Royal from 1742, succeeding Edmund Halley. He is best known for two fundamental discoveries in astronomy, the aberration of light , and the nutation of the Earth's axis...

. After remarking that Newton's telescope had lain neglected for fifty years, they stated that Hadley had sufficiently shown that the invention did not consist in bare theory. They compared its performance with that of a 7.5 inch (190mm) diameter aerial telescope originally presented to the Royal Society by Constantijn Huygens, Jr. and found that Hadley's reflector, "will bear such a charge as to make it magnify the object as many times as the latter with its due charge", and that it represents objects as distinct, though not altogether so clear and bright.

Bradley and Samuel Molyneux

Samuel Molyneux

Samuel Molyneux FRS , son of William Molyneux, was an 18th-century member of the British parliament from Kew and an amateur astronomer whose work with James Bradley attempting to measure stellar parallax led to the discovery of the aberration of light...

, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 ft (2.4 m). Molyneux communicated these methods to two London opticians —Scarlet and Hearn— who started a business manufacturing telescopes.

It was however reserved for James Short of Edinburgh

Edinburgh

Edinburgh is the capital city of Scotland, the second largest city in Scotland, and the eighth most populous in the United Kingdom. The City of Edinburgh Council governs one of Scotland's 32 local government council areas. The council area includes urban Edinburgh and a rural area...

to give practical effect to Gregory's original idea. Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of the professor of mathematics at the local university, Colin Maclaurin

Colin Maclaurin

Colin Maclaurin was a Scottish mathematician who made important contributions to geometry and algebra. The Maclaurin series, a special case of the Taylor series, are named after him....

who permitted him in about 1732 to make use of his rooms in the college buildings for experiments in the construction of prototypes. In Short's first telescopes, the objective mirrors were made of glass as suggested by Gregory, but he later used speculum metal mirrors only and succeeded in giving to them true parabolic

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...

and elliptic figures. Short then adopted telescope-making as his profession which he practised first in Edinburgh, and afterward in London. All Short's telescopes were of the Gregorian form. Short died in London in 1768, having made a considerable fortune selling telescopes.

Since speculum metal mirror secondaries or diagonal mirrors greatly reduced the light that reached the eyepiece, several reflecting telescope designers tried to do away with them. In 1762 Mikhail Lomonosov

Mikhail Lomonosov

Mikhail Vasilyevich Lomonosov was a Russian polymath, scientist and writer, who made important contributions to literature, education, and science. Among his discoveries was the atmosphere of Venus. His spheres of science were natural science, chemistry, physics, mineralogy, history, art,...

presented a reflecting telescope before the Russian Academy of Sciences

Russian Academy of Sciences

The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....

forum. It had its primary mirror tilted at four degrees to telescope's axis so the image could be viewed via an eyepiece mounted at the front of the telescope tube without the observer's head blocking the incoming light. This innovation was not published until 1827, so this type came to be called the Herschelian telescope after a similar design by William Herschel

William Herschel

Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...

.

About the year 1774 Herschel (then a teacher of music in Bath) began to occupy his leisure hours with the construction of reflector telescope mirrors and finally devoted himself entirely to their construction and use. In 1778, he selected a 6¼ inch (16 cm) reflector mirror (the best of some 400 telescope mirrors which he had made) and with it, built a 7 foot (2.1 m) focal length telescope. Using this telescope, he made his early brilliant astronomical discoveries. In 1783, Herschel completed a reflector of approximately 18 inches (46 cm) in diameter and 20 ft (6 m) focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times.

Achromatic refracting telescopes

From the time of the invention of the first refracting telescopes it was generally supposed that chromatic errors seen in lenses simply arose from errors in the spherical figure of their surfaces. Opticians tried to construct lenses of varying forms of curvature to correct these errors. Isaac Newton discovered in 1666 that chromatic colors actually arose from the un-even refraction of light as it passed through the glass medium. This led opticians to experiment with lenses constructed of more than one type of glass in an attempt to canceling the errors produced by each type of glass. It was hoped that this would create an "achromatic lens

Achromatic lens

"; a lens that would focus all colors to a single point, and produce instruments of much shorter focal length.

The first person who succeeded in making a practical achromatic refracting telescope was Chester Moore Hall

Chester Moore Hall

Chester Moore Hall was a British lawyer and inventor who produced the first achromatic lenses in 1729 or 1733 ....

from Essex, England. He argued that the different humours of the human eye refract rays of light to produce an image on the retina

Retina

The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

which is free from color, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media. After devoting some time to the inquiry he found that by combining two lenses formed of different kinds of glass, he could make an achromatic lens where the effects of the unequal refractions of two colors of light (red and blue) was corrected. In 1733, he succeeded in constructing telescope lenses which exhibited much reduced chromatic aberration

Chromatic aberration

. One of his instruments had an objective measuring 2½ inches (6.4 cm) with a relatively short focal length of 20 inches (51 cm).

Hall was a man of independent means and seems to have been careless of fame; at least he took no trouble to communicate his invention to the world. At a trial in Westminster Hall about the patent rights granted to John Dollond

John Dollond

John Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...

(Watkin v. Dollond), Hall was admitted to be the first inventor of the achromatic telescope. However, it was ruled by Lord Mansfield that it was not the original inventor who ought to profit from such invention, but the one who brought it forth for the benefit of mankind.

In 1747, Leonhard Euler

Leonhard Euler

Leonhard Euler was a pioneering Swiss mathematician and physicist. He made important discoveries in fields as diverse as infinitesimal calculus and graph theory. He also introduced much of the modern mathematical terminology and notation, particularly for mathematical analysis, such as the notion...

sent to the Prussian Academy of Sciences

Prussian Academy of Sciences

The Prussian Academy of Sciences was an academy established in Berlin on 11 July 1700, four years after the Akademie der Künste or "Arts Academy", to which "Berlin Academy" may also refer.-Origins:...

a paper in which he tried to prove the possibility of correcting both the chromatic and the spherical aberration of a lens. Like Gregory and Hall, he argued that since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect telescope objective

Objective (optics)

. Adopting a hypothetical law of the dispersion of differently colored rays of light, he proved analytically the possibility of constructing an achromatic objective composed of lenses of glass and water.

All of Euler's efforts to produce an actual objective of this construction were fruitless—a failure which he attributed solely to the difficulty of procuring lenses that worked precisely to the requisite curves. John Dollond

John Dollond

John Dollond was an English optician, known for his successful optics business and his patenting and commercialization of achromatic doublets.-Biography:...

agreed with the accuracy of Euler's analysis, but disputed his hypothesis on the grounds that it was purely a theoretical assumption: that the theory was opposed to the results of Newton's experiments

Scientific method

Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...

on the refraction of light, and that it was impossible to determine a physical law

Physical law

A physical law or scientific law is "a theoretical principle deduced from particular facts, applicable to a defined group or class of phenomena, and expressible by the statement that a particular phenomenon always occurs if certain conditions be present." Physical laws are typically conclusions...

from analytical reasoning alone.

In 1754, Euler sent to the Berlin Academy a further paper in which starting from the hypothesis that light consists of vibrations excited in an elastic fluid by luminous bodies—and that the difference of color of light is due to the greater or lesser frequency

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....

of these vibrations in a given time— he deduced his previous results. He did not doubt the accuracy of Newton's experiments quoted by Dollond.

Dollond did not reply to this, but soon afterwards he received an abstract of a paper by the Swedish

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

mathematician and astronomer, Samuel Klingenstierna

Samuel Klingenstierna

Samuel Klingenstierna was a very renowned Swedish mathematician and scientist. He started his career as a lawyer but soon moved to natural philosophy. He was the first to enunciate errors in Newton's theories of refraction, geometrical notes that were used by John Dollond in his experiments...

, which led him to doubt the accuracy of the results deduced by Newton on the dispersion of refracted light. Klingenstierna showed from purely geometrical considerations (fully appreciated by Dollond) that the results of Newton's experiments could not be brought into harmony with other universally accepted facts of refraction.

As a practical man, Dollond at once put his doubts to the test of experiment: he confirmed the conclusions of Klingenstierna, discovered a difference far beyond his hopes in the refractive qualities of different kinds of glass with respect to the divergence

Refraction

of colors, and was thus rapidly led to the construction of lenses in which first the chromatic aberration—and afterwards—the spherical aberration were corrected.

Dollond was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, but relied on the accuracy of experiments made by Newton. His writings show that with the exception of his bravado

Courage

Courage is the ability to confront fear, pain, danger, uncertainty, or intimidation...

, he would have arrived sooner at a discovery for which his mind was fully prepared. Dollond's paper recounts the successive steps by which he arrived at his discovery independently of Hall's earlier invention—and the logical processes by which these steps were suggested to his mind.

In 1765 Peter Dollond (son of John Dollond) introduced the triple objective, which consisted of a combination of two convex lenses of crown glass with a concave flint

Flint glass

Flint glass is optical glass that has relatively high refractive index and low Abbe number. Flint glasses are arbitrarily defined as having an Abbe number of 50 to 55 or less. The currently known flint glasses have refractive indices ranging between 1.45 and 2.00...

lens between them. He made many telescopes of this kind.

The difficulty of procuring disks of glass (especially of flint glass) of suitable purity and homogeneity limited the diameter and light gathering power of the lenses found in the achromatic telescope. It was in vain that the French Academy of Sciences

French Academy of Sciences

The French Academy of Sciences is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research...

offered prizes for large perfect disks of optical flint glass. Not until 1866 did refracting telescopes reach 18 inches (45 cm) in aperture.

Giant optical telescopes

The first giant reflecting telescope can be said to be William Herschel

William Herschel

's great reflector with a mirror of 49 inches (124 cm) and a focal length of 40 ft (12 m) built in 1789. To cut down on the light loss from the poor reflectivity of the speculum mirrors of that day, Herschel eliminated the small diagonal mirror from his design and tilted his primary mirror so he could view the formed image directly. This design has come to be called the Herschelian telescope. The telescope suffered from other problems of scale that were not altogether solved in Herschel's century. This was followed in 1845 by Lord Rosse

William Parsons, 3rd Earl of Rosse

William Parsons, 3rd Earl of Rosse, Knight of the Order of St Patrick was an Irish astronomer who had several telescopes built. His 72-inch telescope "Leviathan", built 1845, was the world's largest telescope until the early 20th century.-Life:He was born in Yorkshire, England, in the city of...

's 72 inch (183 cm) Newtonian reflector called the "Leviathan of Parsonstown

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...

" with which he discovered the spiral form of the galaxies. Both telescopes suffered from the poor reflectivity and fast tarnishing nature of their speculum metal mirrors. This meant the mirrors had to be frequently removed and re-polished. This could change the curve of the mirror so it usually had to be "re-figured

Figuring

Figuring is the process of final polishing of an optical surface to remove imperfections or modify the surface curvature to achieve the shape required for a given application.-Types of figuring:...

" to the correct shape.

The development of the achromatic lenses and the difficulties with the impractical metal mirrors of reflectors led to a boom in the construction of large refracting telescopes culminating in the Great refractor

Great refractor

Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use in the 19th and early 20th century. Great refractors were large...

s of the mid to late 19th century. In 1897, the refractor reached its maximum practical limit in a research telescope with the construction of the Yerkes Observatory

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...

s' 40 inch (101.6 cm) refractor (although a larger refractor 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...

with an objective of 49.2 inch (1.25 m) diameter was temporarily exhibited at the Paris 1900 Exposition). No larger refractors could be built because of gravity's effect on the lens. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces.

In 1856-57, Karl August von Steinheil and Léon Foucault

Léon Foucault

Jean Bernard Léon Foucault was a French physicist best known for the invention of the Foucault pendulum, a device demonstrating the effect of the Earth's rotation...

introduced a process of depositing a layer of silver on glass telescope mirrors. The silver layer was not only much more reflective and longer lasting than the finish on speculum mirrors, it had the advantage of being able to be removed and re-deposited without changing the shape of the glass substrate. Very large silver on glass mirror reflecting telescopes were built such as the 36 inch (91 cm) Crossley Reflector (1895), 60 inch (150 cm) 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...

Hale telescope of 1908, and the 100 inch (2.5 m) Mount Wilson Hooker telescope in 1917. These and other telescopes of this size had to have provisions to allow for the removal of their main mirrors for re-silvering every few months. John Donavan Strong, a young physicist at the California Institute of Technology

California Institute of Technology

The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...

, developed a technique for coating a mirror with a much longer lasting aluminum coating using thermal vacuum evaporation

Vacuum evaporation

Vacuum evaporation is the process of causing the pressure in a liquid-filled container to be reduced below the vapor pressure of the liquid, causing the liquid to evaporate at a lower temperature than normal...

. In 1932, he became the first person to "aluminize" a mirror; three years later the 60 inches (1,524 mm) and 100 inches (2,540 mm) telescopes became the first large astronomical telescopes to have their mirrors aluminized. The rise of 1948 saw the completion of the 200 inch (508 cm) Hale reflector

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...

at Mount Palomar

Palomar Observatory

Palomar Observatory is a privately owned observatory located in San Diego County, California, southeast of Pasadena's Mount Wilson Observatory, in the Palomar Mountain Range. At approximately elevation, it is owned and operated by the California Institute of Technology...

which was the largest telescope in the world up until the completion of the massive 605 cm (238 in) BTA-6

BTA-6

The BTA-6 is a 6 m aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District on the north side of the Caucasus Mountains in southern Russia...

in Russia seventeen years later. The arrival of substantially larger telescopes had to await the introduction of methods other than the rigidity of glass to maintain the proper shape of the mirror.

The era of adaptive optics

The 1990s saw a new generation of giant telescopes appear, beginning with the construction of the first of the two 10 m (394 in) Keck telescopes in 1993. Other giant telescopes built since then include: the two Gemini telescopes, the four separate telescopes of 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...

, and the Large Binocular Telescope

Large Binocular Telescope

Large Binocular Telescope is an optical telescope for astronomy located on Mount Graham in the Pinaleno Mountains of southeastern Arizona, and is a part of the Mount Graham International Observatory...

.

These telescopes all depend on 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...

(AO), the latest technology

Technology

Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

used to improve the performance of telescopes. It reduces the effects of rapidly changing optical distortion due to the motion of air currents in the Earth's atmosphere. Adaptive optics works by measuring the distortions in a wavefront usually with a laser and then compensating for them by rapid changes of actuator

Actuator

An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...

s applied to a deformable mirror or with a liquid crystal

Liquid crystal

Liquid crystals are a state of matter that have properties between those of a conventional liquid and those of a solid crystal. For instance, an LC may flow like a liquid, but its molecules may be oriented in a crystal-like way. There are many different types of LC phases, which can be...

array filter. AO was first envisioned by Horace W. Babcock

Horace W. Babcock

Horace Welcome Babcock was an American astronomer. He was the son of Harold D. Babcock.He invented and built a number of astronomical instruments, and in 1953 was the first to propose the idea of adaptive optics. He specialized in spectroscopy and the study of magnetic fields of stars...

in 1953, but did not come into common usage in astronomical telescopes until advances in computer technology during the 1990s made it possible to calculate the compensation needed in real time

Real-time computing

In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...

Other wavelengths

The twentieth century saw the construction of telescopes which could produce images using wavelengths other than visible light starting in 1931 when Karl Jansky discovered astronomical objects gave off radio emissions; this prompted a new era of observational astronomy after World War II, with telescopes being developed for other parts of the electromagnetic spectrum

Electromagnetic spectrum

from radio to gamma-rays.

Radio telescopes

Radio astronomy began in 1931 when Karl Jansky discovered that the Milky Way

Milky Way

The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...

was a source of radio emission while doing research on terrestrial static with a direction antenna. Building on Jansky's work, Grote Reber

Grote Reber

Grote Reber was an amateur astronomer and pioneer of radio astronomy. He was instrumental in investigating and extending Karl Jansky's pioneering work, and conducted the first sky survey in the radio frequencies...

built a more sophisticated purpose-built radio telescope in 1937, with a 31.4 feet (9.6 m) dish; using this, he discovered various unexplained radio sources in the sky. Interest in radio astronomy grew after the Second World War when much larger dishes were built including: the 250 feet (76.2 m) Jodrell bank

Jodrell Bank

The Jodrell Bank Observatory is a British observatory that hosts a number of radio telescopes, and is part of the Jodrell Bank Centre for Astrophysics at the University of Manchester...

telescope (1957), the 300 feet (91.4 m) Green Bank Telescope

Green Bank Telescope

The Robert C. Byrd Green Bank Telescope is the world's largest fully steerable radio telescope and the world's largest land-based movable structure. It is part of the National Radio Astronomy Observatory site at Green Bank, West Virginia, USA. The telescope honors the name of the late Senator...

(1962), and the 100 metres (328.1 ft) Effelsberg

Effelsberg

The Effelsberg 100-m Radio Telescope is a radio telescope in the Ahrgebirge in Bad Münstereifel, district of Euskirchen, North Rhine-Westphalia, Germany.- Geography :...

telescope (1971). The huge 1000 feet (304.8 m) Arecibo telescope (1963) is so large that it is fixed into a natural depression in the ground; the central antenna can be steered to allow the telescope to study objects up to twenty degrees from the zenith

Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...

. However, not every radio telescope is of the dish type. For example, the Mills Cross Telescope

Mills Cross Telescope

Bernard Mills built the two-dimensional Mills Cross Telescope in 1954 at the Fleurs field station of the Australian Commonwealth Scientific and Industrial Research Organisation in the area known now as Badgerys Creek, about 40km west of Sydney, New South Wales, Australia.Each arm of the cross was...

(1954) was an early example of an array which used two perpendicular lines of antennae 1500 feet (457.2 m) in length to survey the sky.

High-energy radio waves are known as microwaves and this has been an important area of astronomy ever since the discovery of the cosmic microwave background radiation

Cosmic microwave background radiation

In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....

in 1964. Many ground-based radio telescopes can study microwaves. Short wavelength microwaves are best studied from space because water vapor (even at high altitudes) strongly weakens the signal. The Cosmic Background Explorer (1989) revolutionized the study of the microwave background radiation.

Because radio telescopes have low resolution, they were the first instruments to use interferometry

Interferometry

Interferometry refers to a family of techniques in which electromagnetic waves are superimposed in order to extract information about the waves. An instrument used to interfere waves is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy,...

allowing two or more widely separated instruments to simultaneously observe the same source. Very long baseline interferometry

Very Long Baseline Interferometry

Very 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...

extended the technique over thousands of kilometers and allowed resolutions down to a few milli-arcseconds.

A telescope like the Large Millimeter Telescope

Large Millimeter Telescope

The Large Millimeter Telescope was inaugurated in Mexico on 22 November 2006. It is the world's largest and most sensitive single-aperture telescope in its frequency range, built for observing radio waves in the wave lengths from approximately 0.85 to 4 mm...

(active since 2006) observes from 0.85 millimetre, bridging between the far-infrared/submillimeter telescopes

Submillimetre astronomy

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...

and longer wavelength radio telescopes including the microwave band from about 1 mm (1000 µm) to 1000 mm (1 meter) in wavelength.

Infrared telescopes (700 nm/ 0.7 µm - 1000 µm/1 mm)

Although most infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

radiation is absorbed by the atmosphere, infrared astronomy at certain wavelengths can be conducted on high mountains where there is little absorption by atmospheric water vapor

Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...

. Ever since suitable detectors became available, most optical telescopes at high-altitudes have been able to image at infrared wavelengths. Some telescopes such as the 3.8 metres (149.6 in) UKIRT, and the 3 metres (118.1 in) IRTF — both on Mauna Kea

Mauna Kea

Mauna Kea is a volcano on the island of Hawaii. Standing above sea level, its peak is the highest point in the state of Hawaii. However, much of the mountain is under water; when measured from its oceanic base, Mauna Kea is over tall—significantly taller than Mount Everest...

— are dedicated infrared telescopes. The launch of the IRAS

IRAS

The Infrared Astronomical Satellite was the first-ever space-based observatory to perform a survey of the entire sky at infrared wavelengths....

satellite in 1983 revolutionized infrared astronomy from space. This reflecting telescope which had a 60 centimetres (23.6 in) mirror, operated for nine months until its supply of coolant (liquid helium

Liquid helium

Helium exists in liquid form only at extremely low temperatures. The boiling point and critical point depend on the isotope of the helium; see the table below for values. The density of liquid helium-4 at its boiling point and 1 atmosphere is approximately 0.125 g/mL Helium-4 was first liquefied...

) ran out. It surveyed the entire sky detecting 245,000 infrared sources—more than 100 times the number previously known.

Visible-light telescopes (400 nm - 700 nm)

See main article

Ultra-violet telescopes (10 nm - 400 nm)

Although optical telescopes can image the near ultraviolet, the ozone layer

Ozone layer

The ozone layer is a layer in Earth's atmosphere which contains relatively high concentrations of ozone . This layer absorbs 97–99% of the Sun's high frequency ultraviolet light, which is potentially damaging to the life forms on Earth...

in the stratosphere

Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...

absorbs 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...

radiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes resemble optical telescopes, but conventional aluminium

Aluminium

Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....

-coated mirrors cannot be used and alternative coatings such as magnesium fluoride

Magnesium fluoride

Magnesium fluoride is an inorganic compound with the formula MgF2. The compound is a white crystalline salt and is transparent over a wide range of wavelengths, with commercial uses in optics.-Production and structure:...

or lithium fluoride

Lithium fluoride

Lithium fluoride is an inorganic compound with the formula LiF. It is the lithium salt of hydrofluoric acid. This white solid is a simple ionic compound. Its structure is analogous to that of sodium chloride, but it is much less soluble in water. It is mainly used as a component of molten...

are used instead. The OSO 1

Orbiting Solar Observatory

The Orbiting Solar Observatory Program was the name of a series of nine American science satellites primarily intended to study the Sun, though they also included important non-solar experiments. Eight were launched successfully by NASA between 1962 and 1975 using Delta rockets...

satellite carried out observations in the ultra-violet as early as 1962. The International Ultraviolet Explorer

International Ultraviolet Explorer

The International Ultraviolet Explorer was an astronomical observatory satellite primarily designed to take ultraviolet spectra. The satellite was a collaborative project between NASA, the UK Science Research Council and the European Space Agency...

(1978) systematically surveyed the sky for eighteen years, using a 45 centimetres (17.7 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10–100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy; the Extreme Ultraviolet Explorer

Extreme Ultraviolet Explorer

The Extreme Ultraviolet Explorer was a space telescope for ultraviolet astronomy, launched on June 7, 1992. With instruments for UV radiation between wavelengths of 7 and 76 nm, the EUVE was the first satellite mission especially for the short-wave ultraviolet range...

(1992) was a satellite operating at these wavelengths.

X-ray telescopes (0.01 nm - 10 nm)

X-rays from space do not reach the Earth's surface so X-ray astronomy has to be conducted above the Earth's atmosphere. The first X-ray experiments were conducted on sub-orbital rocket

Rocket

A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...

flights which enabled the first detection of X-rays from the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

(1948) and the first galactic X-ray sources: Scorpius X-1

Scorpius X-1

Scorpius X-1 is an X-ray source located roughly 9000 light years away in the constellation Scorpius. Scorpius X-1 was the first extrasolar X-ray source discovered, and, aside from the Sun, it is the strongest source of X-rays in the sky...

(June 1962) and the Crab Nebula

Crab Nebula

The Crab Nebula is a supernova remnant and pulsar wind nebula in the constellation of Taurus...

(October 1962). Since then, X-ray telescopes (Wolter telescope

Wolter telescope

A 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) have been built using nested grazing-incidence mirrors which deflect X-rays to a detector. Some of the OAO satellites

Orbiting Astronomical Observatory

The Orbiting Astronomical Observatory satellites were a series of four American space observatories launched by NASA between 1966 and 1972, which provided the first high-quality observations of many objects in ultraviolet light...

conducted X-ray astronomy in the late 1960s, but the first dedicated X-ray satellite was the Uhuru

Uhuru (satellite)

Uhuru was the first satellite launched specifically for the purpose of X-ray astronomy. It was also known as the X-ray Explorer Satellite, SAS-A , SAS 1, or Explorer 42.The observatory was launched on 12 December 1970 into an initial orbit of about 560 km apogee, 520 km...

(1970) which discovered 300 sources. More recent X-ray satellites include: the EXOSAT

EXOSAT

The European X-ray Observatory Satellite , originally named HELOS, was operational from May 1983 until April 1986 and in that time made 1780 observations in the X-ray band of most classes of astronomical object including active galactic nuclei, stellar coronae, cataclysmic variables, white dwarfs,...

(1983), ROSAT

ROSAT

ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...

(1990), Chandra

Chandra X-ray Observatory

The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...

(1999), and Newton

XMM-Newton

The XMM-Newton is an orbiting X-ray observatory launched by ESA in December 1999 on a Ariane 5 rocket...

(1999).

Gamma-ray telescopes (less than 0.01 nm)

Gamma rays are absorbed high in the Earth's 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...

so most gamma-ray astronomy is conducted with satellites. Gamma-ray telescopes use scintillation counters, spark chambers and more recently, solid-state

Solid state (electronics)

Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...

detectors. The angular resolution of these devices is typically very poor. There were balloon

Balloon

A balloon is an inflatable flexible bag filled with a gas, such as helium, hydrogen, nitrous oxide, oxygen, or air. Modern balloons can be made from materials such as rubber, latex, polychloroprene, or a nylon fabric, while some early balloons were made of dried animal bladders, such as the pig...

-borne experiments in the early 1960s, but gamma-ray astronomy really began with the launch of the OSO 3

Orbiting Solar Observatory

satellite in 1967; the first dedicated gamma-ray satellites were SAS B

Second Small Astronomy Satellite

The Small Astronomy Satellite 2, also known also as SAS-2, SAS B or Explorer 48, was a NASA gamma ray telescope. It was launched on 15 November 1972 into low Earth orbit with a periapsis of 443 km and an apoapsis of 632 km...

(1972) and Cos B

Cos-B

Cos-B was the first European Space Research Organisation mission to study gamma-ray sources. COS-B was first put forward by the European scientific community in the mid 1960s and approved by the ESRO council in 1969. The mission consisted of a satellite containing gamma-ray detectors, which was...

(1975). The Compton Gamma Ray Observatory

Compton Gamma Ray Observatory

The Compton Gamma Ray Observatory was a space observatory detecting light from 20 KeV to 30 GeV in Earth orbit from 1991 to 2000. It featured four main telescopes in one spacecraft covering x-rays and gamma-rays, including various specialized sub-instruments and detectors...

(1991) was a big improvement on previous surveys. Very high-energy gamma-rays (above 200 GeV) can be detected from the ground via the Cerenkov radiation produced by the passage of the gamma-rays in the Earth's atmosphere. Several Cerenkov imaging telescopes have been built around the world including: the HEGRA

HEGRA

HEGRA, which stands for High-Energy-Gamma-Ray Astronomy, was an atmospheric Cherenkov telescope for Gamma-ray astronomy. With its various types of detectors, HEGRA took data between 1987 and 2002, at which point it was dismantled in order to build its successor, MAGIC, at the same site.It was...

(1987), STACEE

STACEE

The Solar Tower Atmospheric Cherenkov Effect Experiment , is a gamma ray detector located near Albuquerque, New Mexico. Observations with STACEE began in October 2001 and concluded in June 2007. Gamma rays were observed from objects such as the Crab Nebula, a supernova remnant, and Markarian 421,...

(2001), HESS

High Energy Stereoscopic System

High Energy Stereoscopic System or H.E.S.S. is a next-generation system of Imaging Atmospheric Cherenkov Telescopes for the investigation of cosmic gamma rays in the 100 GeV and TeV energy range...

(2003), and MAGIC

MAGIC (telescope)

MAGIC is a system of two Imaging Atmospheric Cherenkov telescopes situated at the Roque de los Muchachos Observatory on La Palma, one of the Canary Islands, at about 2200 m above sea level...

(2004).

Interferometric telescopes

In 1868, Fizeau

Hippolyte Fizeau

Armand Hippolyte Louis Fizeau was a French physicist.-Biography:Fizeau was born in Paris. His earliest work was concerned with improvements in photographic processes. Following suggestions by François Arago, Léon Foucault and Fizeau collaborated in a series of investigations on the interference of...

noted that the purpose of the arrangement of mirrors or glass lenses in a conventional telescope was simply to provide an approximation to a Fourier transform

Fourier transform

In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...

of the optical wave field entering the telescope. As this mathematical transformation was well understood and could be performed mathematically on paper, he noted that by using an array of small instruments it would be possible to measure the diameter of a star with the same precision as a single telescope which was as large as the whole array— a technique which later became known as astronomical interferometry. It was not until 1891 that Michelson successfully used this technique for the measurement of astronomical angular diameters: the diameters of Jupiter's satellites (Michelson 1891). Thirty years later, a direct interferometric measurement of a stellar diameter was finally realized by Michelson & Pease

Francis Gladheim Pease

Francis Gladheim Pease was an American astronomer.He joined the Yerkes Observatory in Wisconsin, where he was an observer and an optician. There he assisted George W. Ritchey who built many of America's first large reflecting telescopes. In 1908 he became an astronomer and instrument maker at the...

(1921) which was applied by their 20 ft (6.1 m) interferometer mounted on the 100 inch Hooker Telescope on Mount Wilson.

The next major development came in 1946 when Ryle

Martin Ryle

Sir Martin Ryle was an English radio astronomer who developed revolutionary radio telescope systems and used them for accurate location and imaging of weak radio sources...

and Vonberg (Ryle and Vonberg 1946) located a number of new cosmic radio sources by constructing a radio analogue of the Michelson interferometer

Michelson interferometer

The Michelson interferometer is the most common configuration for optical interferometry and was invented by Albert Abraham Michelson. An interference pattern is produced by splitting a beam of light into two paths, bouncing the beams back and recombining them...

. The signals from two radio antennas were added electronically to produce interference. Ryle and Vonberg's telescope used the rotation of the Earth to scan the sky in one dimension. With the development of larger arrays and of computers which could rapidly perform the necessary Fourier transforms, the first 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...

imaging instruments were soon developed which could obtain high resolution images without the need of a giant parabolic reflector to perform the Fourier transform. This technique is now used in most radio astronomy observations. Radio astronomers soon developed the mathematical methods

Closure phase

The closure phase is an observable quantity in imaging astronomical interferometry, which allowed the use of interferometry with very long baselines. It forms the basis of the self-calibration approach to interferometric imaging...

to perform aperture synthesis

Aperture synthesis

Fourier imaging using much larger arrays of telescopes —often spread across more than one continent. In the 1980s, the aperture synthesis

Aperture synthesis

technique was extended to visible light as well as infrared astronomy, providing the first very high resolution optical and infrared images of nearby stars.

In 1995 this imaging technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and also allowing even higher resolution imaging of stellar surfaces. The same techniques have now been applied at a number of other astronomical telescope arrays including: 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...

, the CHARA 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...

, and the IOTA

Infrared Optical Telescope Array

The Infrared Optical Telescope Array began with an agreement in 1988 among five Institutions, the Smithsonian Astrophysical Observatory, Harvard University, the University of Massachusetts, the University of Wyoming, and MIT/Lincoln Laboratory, to build a two-telescope stellar interferometer for...

array. A detailed description of the development of astronomical optical interferometry can be found here.

In 2008, Max Tegmark

Max Tegmark

Max Tegmark is a Swedish-American cosmologist. Tegmark is a professor at the Massachusetts Institute of Technology and belongs to the scientific directorate of the Foundational Questions Institute.-Early life:...

and Matias Zaldarriaga

Matias Zaldarriaga

Matias Zaldarriaga is an Argentine cosmologist. Born in Coghlan neighbourhood, Buenos Aires, at the present time he works in the Institute for Advanced Study located in Princeton, New Jersey, United States. He is known especially for his work on the cosmic microwave background...

proposed a "Fast Fourier Transform Telescope

Fast Fourier Transform Telescope

Fast Fourier Transform Telescope is Tegmark and Zaldarriaga's name for a design for an all-digital synthetic-aperture telescope. It is a type of interferometer designed to be cheaper than standard telescope interferometers currently in use....

" design in which the lenses and mirrors could be dispensed with altogether when computers become fast enough to perform all the necessary transforms.

External links

History of optics articles

Best Idea; Eyes Wide Open

History of telescope articles

Other media

"Eyes on the Skies" - documentary available online about the history and future of the telescope

Other possible telescope inventors

Leonard Digges (1520–1559) Did the reflecting telescope have English origins? Leonard and Thomas Digges by Colin A Ronan, M.Sc., F.R.A.S. - originally published in the Journal of the British Astronomical Association, 101, 6, 1991
Juan Roget (died before 1624) - Historian Nick Pelling says Juan Roget, a Burgundian spectacle maker who died between 1617 and 1624 could have invented an early telescope. Controversy over telescope origin - BBC News 16 September 2008

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

Optical foundations

Pre 17th century developments

The first known telescopes

Refracting telescopes

Long focal length refractors

Aerial telescopes

Reflecting telescopes

Achromatic refracting telescopes

Giant optical telescopes

The era of adaptive optics

Other wavelengths

Radio telescopes

Infrared telescopes (700 nm/ 0.7 µm - 1000 µm/1 mm)

Visible-light telescopes (400 nm - 700 nm)

Ultra-violet telescopes (10 nm - 400 nm)

X-ray telescopes (0.01 nm - 10 nm)

Gamma-ray telescopes (less than 0.01 nm)

Interferometric telescopes

See also

External links