History of the telescope

Early depiction of a "Dutch telescope" from 1624.

Thehistory of the telescopecan be traced to before the invention of the earliest knowntelescope,which appeared in 1608 in theNetherlands,when a patent was submitted byHans Lippershey,aneyeglassmaker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these earlyrefracting telescopesconsisted of a convexobjectivelens and a concaveeyepiece.Galileoimproved on this design the following year and applied it to astronomy. In 1611,Johannes Keplerdescribed how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such asChristiaan Huygenswere building powerful but unwieldy Keplerian telescopes with compound eyepieces.

Isaac Newtonis credited with building the first 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 Cassegrainin 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.

Theachromatic lens,which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made byChester Moore Hall,who did not publicize it.John Dollondlearned of Hall's inventionand began producing telescopes using it in commercial quantities, starting in 1758.

Important developments in reflecting telescopes wereJohn Hadley's production of largerparaboloidalmirrors in 1721; the process ofsilveringglass mirrors introduced byLéon Foucaultin 1857;and the adoption of long-lasting aluminized coatings on reflector mirrors in 1932.TheRitchey-Chretienvariant ofCassegrain reflectorwas invented around 1910, but not widely adopted until after 1950; many modern telescopes including theHubble Space Telescopeuse this design, which gives a wider field of view than a classic Cassegrain.

During the period 1850–1900, reflectors suffered from problems with speculum metal mirrors, and a considerable number of "Great Refractors" were built from 60 cm to 1 metre aperture, culminating in theYerkes Observatoryrefractor in 1897; however, starting from the early 1900s a series of ever-larger reflectors with glass mirrors were built, including the Mount Wilson 60-inch (1.5 metre), the 100-inch (2.5 metre)Hooker Telescope(1917) and the 200-inch (5 metre)Hale Telescope(1948); essentially all major research telescopes since 1900 have been reflectors. A number of 4-metre class (160 inch) telescopes were built on superior higher altitude sites including Hawaii and the Chilean desert in the 1975–1985 era. The development of the computer-controlledalt-azimuth mountin the 1970s andactive opticsin the 1980s enabled a new generation of even larger telescopes, starting with the 10-metre (400 inch)Keck telescopesin 1993/1996, and a number of 8-metre telescopes including theESOVery Large Telescope,Gemini ObservatoryandSubaru Telescope.

The era ofradio telescopes(along withradio astronomy) was born withKarl Guthe Jansky'sserendipitousdiscovery 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 togamma-rays.The development ofspace observatoriesafter 1960 allowed access to several bands impossible to observe from the ground, includingX-raysand longer wavelengthinfraredbands.

Optical telescopes

Optical foundations

See also:History of optics

Optical diagram showing light being refracted by a spherical glass container full of water, fromRoger Bacon,De multiplicatione specierum

Further information:Lens (optics) § History

Objects resemblinglensesdate back 4000 years although it is unknown if they were used for their optical properties or just as decoration. Greek accounts of the optical properties of water-filled spheres (5th century BC) were followed by many centuries of writings on optics, includingPtolemy(2nd century) in hisOptics,who wrote about the properties of light includingreflection,refraction,andcolor,followed byIbn Sahl(10th century) andIbn Al-Haytham(11th century).

Actual use of lenses dates back to the widespread manufacture and use ofeyeglassesin Northern Italy beginning in the late 13th century.The invention of the use of concave lenses to correctnear-sightednessis ascribed toNicholas of Cusain 1451.

Invention

Notes on Hans Lippershey's unsuccessful telescope patent in 1608

The first record of a telescope comes from the Netherlands in 1608. It is in a patent filed byMiddelburgspectacle-makerHans Lippersheywith theStates General of the Netherlandson 2 October 1608 for his instrument "for seeing things far away as if they were nearby."A few weeks later another Dutch instrument-maker,Jacob Metiusalso applied for a patent. The States General did not award a patent since the knowledge of the device already seemed to be ubiquitousbut the Dutchgovernmentawarded Lippershey with a contract for copies of hisdesign.

The original Dutch telescopes were composed of aconvexand aconcave lens—telescopes that are constructed this way do not invert the image. Lippershey's original design had only 3xmagnification.Telescopes seem to have been made in the Netherlands in considerable numbers soon after this date of "invention", and rapidly found their way all over Europe.

Claims of prior invention

Reproduction of one of the four optical devices that Zacharias Snijder in 1841 claimed were early telescopes built byZacharias Janssen.Its actual function and creator has been disputed over the years.

In 1655 Dutch diplomatWilliam de Boreeltried to solve the mystery of who invented the telescope. He had a local magistrate in Middelburg follow up on Boreel's childhood and early adult recollections of a spectacle maker named "Hans" whom he remembered as the inventor of the telescope. The magistrate was contacted by a then unknown claimant, Middelburg spectacle maker Johannes Zachariassen, who testified that his father,Zacharias Jansseninvented the telescope and the microscope as early as 1590. This testimony seemed convincing to Boreel, who now recollected that Zacharias and his father, Hans Martens, must have been whom he remembered.Boreel's conclusion that Zacharias Janssen invented the telescope a little ahead of another spectacle maker,Hans Lippershey,was adopted byPierre Borelin his 1656 bookDe vero telescopii inventore.Discrepancies in Boreel's investigation and Zachariassen's testimony (including Zachariassen misrepresenting his date of birth and role in the invention) has led some historians to consider this claim dubious.The "Janssen" claim would continue over the years and be added on to withZacharias Snijderin 1841 presenting 4 iron tubes with lenses in them claimed to be 1590 examples of Janssen's telescopeand historianCornelis de Waard's 1906 claim that the man who tried to sell a broken telescope to astronomerSimon Mariusat the 1608Frankfurt Book Fairmust have been Janssen.

In 1682,the minutes of theRoyal Societyin LondonRobert HookenotedThomas Digges' 1571Pantometria,(a book on measurement, partially based on his fatherLeonard Digges' notes and observations) seemed to support an English claim to the invention of the telescope, describing Leonard as having afare seeing glassin the mid-1500s based on an idea byRoger Bacon.Thomas described it as "by proportional Glasses duly situate in convenient angles, not only discovered things far off, read letters, numbered pieces of money with the very coin and superscription thereof, cast by some of his friends of purpose upon downs in open fields, but also seven miles off declared what hath been done at that instant in private places."Comments on the use of proportional or" perspective glass "are also made in the writings ofJohn Dee(1575) andWilliam Bourne(1585).Bourne was asked in 1580 to investigate the Diggs device byQueen Elizabeth I's chief advisorLord Burghley.Bourne's is the best description of it, and from his writing it seemed to consist of peering into a large curved mirror that reflected the image produced by a large lens.The idea of an "Elizabethan Telescope" has been expanded over the years, including astronomer and historianColin Ronanconcluding in the 1990s that this reflecting/refractingtelescopewas built by Leonard Digges between 1540 and 1559.This "backwards"reflecting telescopewould have been unwieldy, it needed very large mirrors and lens to work, the observer had to stand backwards to look at an upside down view, and Bourne noted it had a very narrow field of view making it unsuitable for military purposes.The optical performance required to see the details of coins lying about in fields, or private activities seven miles away, seems to be far beyond the technology of the timeand it could be the "perspective glass" being described was a far simpler idea, originating with Bacon, of using a single lens held in front of the eye to magnify a distant view.

A 1959 research paper by Simon de Guilleuma claimed that evidence he had uncovered pointed to the French born spectacle makerJuan Roget(died before 1624) as another possible builder of an early telescope that predated Hans Lippershey's patent application.

Leonardo da Vinci's purported "telescope", with the described eyepiece-lens drawn in.

In 2022 Italian Professor of Physics Alessandro Bettini published a paper on whetherLeonardo da Vincicould have invented a telescope.Building on 1939 observations by Domenico Argentieri of what look like lenses arranged like a telescope in da Vinci drawings, Bettini superimposed Argentieri's lens arrangement on an adjacent drawing of diverging rays, coming up with an arrangement that also looked like a telescope. Bettini also noted the writings of Italian scholar and professorGirolamo Fracastoroin 1538 about combining lenses in eyeglasses to make the "moon or at another star" "so near that they would appear not higher than the towers".

Spread of the invention

Lippershey's application for a patent was mentioned at the end of a diplomatic report on an embassy to Holland from theKingdom of Siamsent by the Siamese kingEkathotsarot:Ambassades du Roy de Siam envoyé à l'Excellence du Prince Maurice, arrivé à La Haye le 10 Septemb. 1608(Embassy of the King of Siam sent to his Excellency Prince Maurice, arrived at The Hague on 10 September 1608). This report was issued in October 1608 and distributed across Europe, leading to experiments by other scientists, such as the ItalianPaolo Sarpi,who received the report in November, and the English mathematician and astronomerThomas Harriot,who used a six-powered telescope by the summer of 1609 to observe features on the moon.

19th-century painting depictingGalileo Galileidisplaying histelescopetoLeonardo Donatoin 1609.

The Italian polymathGalileo Galileiwas inVenicein June 1609and there heard of the "Dutch perspective glass", a militaryspyglass,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 toPaduafrom Venice and made his first telescope the next day by using a convex objective lens in one extremity of a leaden tube and a concaveeyepiecelens in the other end, an arrangement that came to be called aGalilean telescope.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 thedogeLeonardo Donato,who was sitting in full council. Thesenatein return settled him for life in his lectureship at Padua and doubled his salary.

In 1610Galileo Galileiobserved with his telescope thatVenus showed phases,despite remaining near the Sun in Earth's sky (first image). This proved that it orbits theSunand notEarth,as predicted byCopernicus'sheliocentric modeland disproved the then conventionalgeocentric model(second image).

Galileo set himself to improving the telescope, producing telescopes of increased power. His first telescope had a 3x magnification, but he soon made instruments which magnified 8x and finally, one nearly a meter long with a 37mm objective (which he would stop down to 16mm or 12mm) and a 23x magnification.With this last instrument he began a series of astronomical observations in October or November 1609, observing thesatellitesofJupiter,hills and valleys on theMoon,the phases ofVenusandspots on the sun(using the projection method rather than direct observation). Galileo noted that the revolution of the satellites of Jupiter, the phases of Venus, rotation of theSunand the tilted path its spots followed for part of the year pointed to the validity of the sun-centeredCopernican systemover otherEarth-centered systemssuch as the one proposed byPtolemy.

Galileo's instrument was the first to be given the name "telescope". The name was invented by the Greek poet/theologianGiovanni Demisianiat a banquet held on April 14, 1611, by PrinceFederico Cesito makeGalileo Galileia member of theAccademia dei Lincei.The word was created from theGreektele= 'far' andskopein= 'to look or see';teleskopos= 'far-seeing'.

By 1626 knowledge of the telescope had spread to China when German Jesuit and astronomerJohann Adam Schall von BellpublishedYuan jing shuo,( xa kính nói,Explanation of the Telescope) in Chinese and Latin.

Further refinements

Refracting telescopes

Johannes Keplerfirst explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in hisCatoptrics(1611). The first person who actually constructed a telescope of this form was theJesuitChristoph Scheinerwho gives a description of it in hisRosa Ursina(1630).

William Gascoignewas 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 commonfocal planeof the objective and the eyepiece. This led to his invention of themicrometer,and his application of telescopic sights to precision astronomical instruments. It was not until 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 itsfield of viewwas much larger than in theGalilean telescope.

The first powerful telescopes of Keplerian construction were made byChristiaan Huygensafter much labor—in which his brother assisted him. With one of these: an objective diameter of 2.24 inches (57 mm) and a 12 ft (3.7 m) focal length,he discovered the brightest of Saturn's satellites (Titan) in 1655; in 1659, he published his "Systema Saturnium"which, for the first time, gave a true explanation of Saturn'sring—founded on observations made with the same instrument.

Long focal length refractors

Engraved illustration of a 45 m (148 ft) focal length Keplerian astronomical refracting telescope built byJohannes Hevelius.From his book, "Machina coelestis"(first part), published in 1673.

Thesharpnessof the image in Kepler's telescope was limited by thechromatic aberrationintroduced 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 CassinidiscoveredSaturn'sfifth satellite (Rhea) in 1672 with a telescope 35 feet (11 m) long. Astronomers such asJohannes Heveliuswere constructing telescopes with focal lengths as long as 150 feet (46 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

Main article:Aerial telescope

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 handheld or mounted on a stand at the focus, and the image was found by trial and error. These were consequently termedaerial telescopes.and have been attributed toChristiaan Huygensand his brotherConstantijn Huygens, Jr.although it is not clear that they invented it.Christiaan Huygens and his brother made objectives up to 8.5 inches (220 mm) diameterand 210 ft (64 m) focal length and others such asAdrien Auzoutmade 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 byGiuseppe Campanithat were 100 and 136 ft (30 and 41 m) in focal length.

Reflecting telescopes

See also:Reflecting telescope

The ability of acurved mirrorto form an image may have been known since the time ofEuclidand had been extensively studied byAlhazenin the 11th century. Galileo,Giovanni Francesco Sagredo,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,an Italian Jesuit astronomer and physicist, wrote in his bookOptica philosophiaof 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.

Light path in aGregorian telescope.

In 1636Marin Mersenneproposed 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 Gregorywent into further detail in his bookOptica Promota(1663), pointing out that a reflecting telescope with a mirror that was shaped like the part of aconic section,would correctspherical aberrationas well as the chromatic aberration seen in refractors. The design he came up with bears his name: the "Gregorian telescope";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.

Light path in aNewtonian telescope.

A replica of Newton's second reflecting telescope which was presented to theRoyal Societyin 1672.

In 1666Isaac 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 concluded that light could not be refracted through a lens without causing chromatic aberrations, although he incorrectly concluded from some rough experimentsthatallrefracting substances would diverge the prismatic colors in a constant proportion to their mean refraction. From these experiments Newton concluded that no improvement could be made in the refracting telescope.Newton's experiments with mirrors showed that they did not suffer from the chromatic errors of lenses, for all colors of light theangle of incidencereflected in a mirror was equal to theangle of reflection,so as a proof to his theories Newton set out to build a reflecting telescope.Newton completed hisfirst telescopein 1668 and it is the earliest known functional reflecting telescope.After much experiment, he chose analloy(speculum metal) oftinandcopperas the most suitable material for hisobjectivemirror. 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 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 aneyepiecemounted 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,and fittings. Newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and afocal ratioof f/5.With it he found that he could see the fourGalilean moonsofJupiterand thecrescent phase of the planet Venus.Encouraged by this success, he made a second telescope with a magnifying power of 38x which he presented to theRoyal Society of Londonin December 1671.This type of telescope is still called aNewtonian telescope.

Light path in aCassegrain telescope.

A third form of reflecting telescope, the "Cassegrain reflector"was devised in 1672 byLaurent Cassegrain.The telescope had a small convexhyperboloidalsecondary 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 for another 50 years untilJohn Hadley(best known as the inventor of theoctant) developed ways to make precision aspheric andparabolicspeculum metal mirrors. In 1721 he showed the first parabolic Newtonian reflector to the Royal Society.It had a 6-inch (15 cm) diameter,62+3⁄4-inch (159 cm) focal length speculum metal objective mirror. The instrument was examined byJames PoundandJames Bradley.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 inches (190 mm) 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 andSamuel Molyneux,having been instructed by Hadley in his methods of polishing speculum metal, succeeded in producing large reflecting telescopes of their own, one of which had a focal length of 8 ft (2.4 m). These methods of fabricating mirrors were passed on by Molyneux to two London opticians —Scarlet and Hearn— who started a business manufacturing telescopes.

The British mathematician, opticianJames Shortbegan experimenting with building telescopes based on Gregory's designs in the 1730s. He first tried making his mirrors out of glass as suggested by Gregory, but he later switched to speculum metal mirrors creating Gregorian telescopes with original designersparabolicandellipticfigures. 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 1762Mikhail Lomonosovpresented a reflecting telescope before theRussian Academy of Sciencesforum. 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 byWilliam Herschel.

William Herschel's 49-inch (1,200 mm) "40-foot" telescope of 1789. Illustration fromEncyclopædia Britannica Third Editionpublished in 1797.

About the year 1774 William Herschel (then a teacher of music inBath,England) began to occupy his leisure hours with the construction of reflector telescope mirrors, finally devoted himself entirely to their construction and use in astronomical research. In 1778, he selected a6+1⁄4-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 (6.1 m) focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times. In 1789 Herschel finished building his largest reflecting telescope with a mirror of 49 inches (120 cm) and a focal length of 40 ft (12 m), (commonly known as his40-foot telescope) at his new home, atObservatory HouseinSlough,England. 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 theHerschelian telescope.He discovered Saturn's sixth known moon,Enceladus,the first night he used it (August 28, 1789), and on September 17, its seventh known moon, Mimas. This telescope was world's largest telescope for over 50 years. However, this large scope was difficult to handle and thus less used than his favorite 18.7-inch reflector.

In 1845William Parsons, 3rd Earl of Rossebuilt his 72-inch (180 cm) Newtonian reflector called the "Leviathan of Parsonstown"with which he discovered the spiral form ofgalaxies.

All of these larger reflectors suffered from the poor reflectivity and fast tarnishing nature of their speculum metal mirrors. This meant they need more than one mirror per telescope since mirrors had to be frequently removed and re-polished. This was time-consuming since the polishing process could change the curve of the mirror, so it usually had to be "re-figured"to the correct shape.

Achromatic refracting telescopes

See also:Achromatic lens

Light path through anachromatic lens.

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";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 wasChester Moore HallfromEssex, England.He argued that the different humours of the human eye refract rays of light to produce an image on theretinawhich 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 reducedchromatic aberration.One of his instruments had an objective measuring2+1⁄2inches (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 toJohn Dollond(Watkin v. Dollond), Hall was admitted to be the first inventor of the achromatic telescope. However, it was ruled byLord Mansfieldthat "it was not the person who locked his invention in his scrutoire who ought to profit for such invention, but the one who brought it forth for the benefit of mankind."

In 1747,Leonhard Eulersent to thePrussian Academy of Sciencesa 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 telescopeobjective.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 Dollondagreed 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'sexperimentson the refraction of light, and that it was impossible to determine aphysical lawfrom 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 lesserfrequencyof 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 theSwedishmathematician and astronomer,Samuel Klingenstierna,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.

Dollond telescope.

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 thedivergenceof 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 hisbravado,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 concaveflintlens 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 theFrench Academy of Sciencesoffered prizes for large perfect disks of optical flint glass.

The difficulties with the impractical metal mirrors of reflecting telescopes led to the construction of large refracting telescopes. By 1866 refracting telescopes had reached 18 inches (46 cm) in aperture with many larger "Great refractors"being built in the mid to late 19th century. In 1897, the refractor reached its maximum practical limit in a research telescope with the construction of theYerkes Observatorys' 40-inch (100 cm) refractor (although a larger refractorGreat Paris Exhibition Telescope of 1900with an objective of 49.2 inches (1.25 m) diameter was temporarily exhibited at theParis 1900 Exposition). No larger refractors could be built because ofgravity'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.

Large reflecting telescopes

See also:List of largest optical telescopes historically

The 200-inch (5.1 m)Hale TelescopeatMount Palomar

In 1856–57,Karl August von SteinheilandLéon Foucaultintroduced 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. Towards the end of the 19th century very large silver on glass mirror reflecting telescopes were built.

The beginning of the 20th century saw construction of the first of the "modern" large research reflectors, designed for precision photographic imaging and located at remote high altitude clear sky locationssuch as the60-inch Hale Telescopeof 1908, and the 100-inch (2.5 m)Hooker telescopein 1917, both located atMount Wilson Observatory.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 theCalifornia Institute of Technology,developed a technique for coating a mirror with a much longer lasting aluminum coating using thermalvacuum evaporation.In 1932, he became the first person to "aluminize" a mirror; three years later the 60-inch (1,500 mm) and 100-inch (2,500 mm) telescopes became the first large astronomical telescopes to have their mirrors aluminized.1948 saw the completion of the 200-inch (510 cm)Hale reflectoratMount Palomarwhich was the largest telescope in the world up until the completion of the massive 605 cm (238 in)BTA-6in Russia twenty-seven years later. The Hale reflector introduced several technical innovations used in future telescopes, includinghydrostatic bearingsfor very low friction, theSerrurier trussfor equal deflections of the two mirrors as the tube sags under gravity, and the use ofPyrexlow-expansion glass for the mirrors. 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.

Active and adaptive optics

See also:Adaptive opticsandList of largest optical reflecting telescopes

The 1980s saw the introduction of two new technologies for building larger telescopes and improving image quality, known asactive opticsandadaptive optics.In active optics, an image analyser senses the aberrations of a star image a few times per minute, and a computer adjusts many support forces on the primary mirror and the location of the secondary mirror to maintain the optics in optimal shape and alignment. This is too slow to correct for atmospheric blurring effects, but enables the use of thin single mirrors up to 8 m diameter, or even larger segmented mirrors. This method was pioneered by the ESONew Technology Telescopein the late 1980s.

The 1990s saw a new generation of giant telescopes appear using active optics, beginning with the construction of the first of the two 10 m (390 in)Keck telescopesin 1993. Other giant telescopes built since then include: the twoGemini telescopes,the four separate telescopes of theVery Large Telescope,and theLarge Binocular Telescope.

ESO'sVLTboasts advancedadaptive opticssystems, which counteract the blurring effects of the Earth's atmosphere.

Adaptive optics uses a similar principle, but applying corrections several hundred times per second to compensate the effects of rapidly changing optical distortion due to the motion of turbulence in the Earth's atmosphere. Adaptive optics works by measuring the distortions in a wavefront and then compensating for them by rapid changes ofactuatorsapplied to a small deformable mirror or with aliquid crystalarray filter. AO was first envisioned byHorace W. Babcockin 1953, but did not come into common usage in astronomical telescopes until advances in computer and detector technology during the 1990s made it possible to calculate the compensation needed inreal time.In adaptive optics, the high-speed corrections needed mean that a fairly bright star is needed very close to the target of interest (or an artificial star is created by a laser). Also, with a single star or laser the corrections are only effective over a very narrow field (tens of arcsec), and current systems operating on several 8-10m telescopes work mainly in near-infrared wavelengths for single-object observations.

Developments of adaptive optics include systems with multiple lasers over a wider corrected field, and/or working above kiloHertz rates for good correction at visible wavelengths; these are currently in progress but not yet in routine operation as of 2015.

Other wavelengths

The twentieth century saw the construction of telescopes which could produce images using wavelengths other thanvisible lightstarting in 1931 whenKarl Janskydiscovered 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 theelectromagnetic spectrumfrom radio togamma-rays.

Radio telescopes

See also:Radio telescopeandRadio astronomy

The 250-foot (76 m)Lovell radio telescopeatJodrell BankObservatory.

Radio astronomy began in 1931 whenKarl Janskydiscovered that theMilky Waywas a source of radio emission while doing research on terrestrial static with a direction antenna. Building on Jansky's work,Grote Reberbuilt a more sophisticated purpose-built radio telescope in 1937, with a 31.4-foot (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-foot (76 m)Jodrell banktelescope (1957), the 300-foot (91 m)Green Bank Telescope(1962), and the 100-metre (330 ft)Effelsbergtelescope (1971). The huge 1,000-foot (300 m)Arecibo telescope(1963) was so large that it was fixed into a natural depression in the ground; the central antenna could be steered to allow the telescope to study objects up to twenty degrees from thezenith.However, not every radio telescope is of the dish type. For example, theMills Cross Telescope(1954) was an early example of an array which used two perpendicular lines of antennae 1,500 feet (460 m) in length to survey the sky.

High-energy radio waves are known asmicrowavesand this has been an important area of astronomy ever since the discovery of thecosmic microwave background radiationin 1964. Many ground-basedradio telescopescan study microwaves. Short wavelength microwaves are best studied from space because water vapor (even at high altitudes) strongly weakens the signal. TheCosmic Background Explorer(1989) revolutionized the study of the microwave background radiation.

Because radio telescopes have low resolution, they were the first instruments to useinterferometryallowing two or more widely separated instruments to simultaneously observe the same source.Very long baseline interferometryextended the technique over thousands of kilometers and allowed resolutions down to a fewmilli-arcseconds.

A telescope like theLarge Millimeter Telescope(active since 2006) observes from 0.85 to 4 mm (850 to 4,000 μm), bridging between the far-infrared/submillimeter telescopesand longer wavelength radio telescopes including the microwave band from about 1 mm (1,000 μm) to 1,000 mm (1.0 m) in wavelength.

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

See also:Infrared telescope,Infrared astronomy,andFar-infrared astronomy

Although mostinfraredradiation is absorbed by the atmosphere, infrared astronomy at certain wavelengths can be conducted on high mountains where there is little absorption by atmosphericwater vapor.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-metre (150 in)UKIRT,and the 3-metre (120 in)IRTF— both onMauna Kea— are dedicated infrared telescopes. The launch of theIRASsatellite in 1983 revolutionized infrared astronomy from space. This reflecting telescope which had a 60-centimetre (24 in) mirror, operated for nine months until its supply of coolant (liquid helium) ran out. It surveyed the entire sky detecting 245,000 infrared sources—more than 100 times the number previously known.

Ultra-violet telescopes (10 nm – 400 nm)

See also:Ultraviolet astronomy

Although optical telescopes can image the near ultraviolet, theozone layerin thestratosphereabsorbsultravioletradiation shorter than 300 nm so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes resemble optical telescopes, but conventionalaluminium-coated mirrors cannot be used and alternative coatings such asmagnesium fluorideorlithium fluorideare used instead. TheOrbiting Solar Observatorysatellite carried out observations in the ultra-violet as early as 1962. TheInternational Ultraviolet Explorer(1978) systematically surveyed the sky for eighteen years, using a 45-centimetre (18 in) aperture telescope with twospectroscopes.Extreme-ultraviolet astronomy (10–100 nm) is a discipline in its own right and involves many of the techniques of X-ray astronomy; theExtreme Ultraviolet Explorer(1992) was a satellite operating at these wavelengths.

X-ray telescopes (0.01 nm – 10 nm)

See also:X-ray telescopeandX-ray astronomy

X-raysfrom 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 onsub-orbitalrocketflights which enabled the first detection of X-rays from theSun(1948) and the first galactic X-ray sources:Scorpius X-1(June 1962) and theCrab Nebula(October 1962). Since then, X-ray telescopes (Wolter telescopes) have been built using nested grazing-incidence mirrors which deflect X-rays to a detector. Some of theOAO satellitesconducted X-ray astronomy in the late 1960s, but the first dedicated X-ray satellite was theUhuru(1970) which discovered 300 sources. More recent X-ray satellites include: theEXOSAT(1983),ROSAT(1990),Chandra(1999), andNewton(1999).

Gamma-ray telescopes (less than 0.01 nm)

See also:Gamma-ray astronomy

Gamma raysare absorbed high in theEarth's atmosphereso most gamma-ray astronomy is conducted withsatellites.Gamma-ray telescopes usescintillation counters,spark chambersand more recently,solid-statedetectors. The angular resolution of these devices is typically very poor. There wereballoon-borne experiments in the early 1960s, but gamma-ray astronomy really began with the launch of theOSO 3satellite in 1967; the first dedicated gamma-ray satellites wereSAS B(1972) andCos B(1975). TheCompton Gamma Ray Observatory(1991) was a big improvement on previous surveys. Very high-energy gamma-rays (above 200 GeV) can be detected from the ground via theCerenkov radiationproduced by the passage of the gamma-rays in the Earth's atmosphere. Several Cerenkov imaging telescopes have been built around the world including: theHEGRA(1987),STACEE(2001),HESS(2003), andMAGIC(2004).

Interferometric telescopes

See also:Astronomical interferometry

In 1868,Fizeaunoted that the purpose of the arrangement of mirrors or glass lenses in a conventional telescope was simply to provide an approximation to aFourier transformof 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 asastronomical interferometry.It was not until 1891 thatAlbert A. Michelsonsuccessfully 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 &Francis G. Pease(1921) which was applied by their 20 ft (6.1 m) interferometer mounted on the100 inch Hooker Telescopeon Mount Wilson.

The next major development came in 1946 whenRyleand Vonberg (Ryle and Vonberg 1946) located a number of new cosmic radio sources by constructing a radio analogue of theMichelson interferometer.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 firstaperture synthesisimaging 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 themathematical methodsto performaperture synthesisFourier imaging using much larger arrays of telescopes —often spread across more than one continent. In the 1980s, theaperture synthesistechnique 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 onan array of separate optical telescopesfor the first time, allowing a further improvement in resolution, and also allowing even higher resolutionimaging of stellar surfaces.The same techniques have now been applied at a number of other astronomical telescope arrays including: theNavy Prototype Optical Interferometer,theCHARA array,and theIOTAarray. A detailed description of the development of astronomical optical interferometry can be found here [https://web.archive.org/web/20091018192226/http://geocities /CapeCanaveral/2309/page1.html

In 2008,Max TegmarkandMatias Zaldarriagaproposed a "Fast Fourier Transform Telescope"design in which the lenses and mirrors could be dispensed with altogether when computers become fast enough to perform all the necessary transforms.

See also

• 400 Years of the Telescopedocumentary
• History of astronomy
• Astronomical interferometer
• Timeline of telescope technology
• Timeline of telescopes, observatories, and observing technology
• International Year of Astronomy,2009 marking the 400th anniversary of Galileo's first astronomical observations using his telescope
• List of optical telescopes
• List of largest optical refracting telescopes
• List of space telescopes
• List of telescope types
• Visible-light astronomy

References

Sources

• This article incorporates text from a publication now in thepublic domain:Taylor, Harold Dennis;Gill, David(1911). "Telescope".InChisholm, Hugh(ed.).Encyclopædia Britannica.Vol. 26 (11th ed.). Cambridge University Press. pp. 557–573.
• Crawford, David Livingstone, ed. (1966),The Construction of Large Telescopes(International Astronomical Union. Symposium no. 27 ed.), London, New York: Academic Press, p. 234
• Elliott, Robert S. (1966),Electromagnetics,McGraw-Hill
• Fizeau, H. 1868 C. R. Hebd. Seanc. Acad. Sci. Paris 66, 932
• King, Henry C., ed. (1955),The History of the Telescope,London: Charles Griffin & Co. Ltd
• Lindberg, D. C. (1976),Theories of Vision from al-Kindi to Kepler,Chicago:University of Chicago Press
• Michelson, A. A. 1891 Publ. Astron. Soc. Pac. 3, 274
• Michelson, A. A. & Pease, F. G. 1921 Astrophys. J. 53, 249
• Rashed, Roshdi; Morelon, Régis (1996),Encyclopedia of the History of Arabic Science,vol. 1 & 3,Routledge,ISBN0-415-12410-7
• Ryle, M. & Vonberg, D., 1946 Solar radiation on 175Mc/s, Nature 158 pp 339
• Wade, Nicholas J.; Finger, Stanley (2001), "The eye as an optical instrument: from camera obscura to Helmholtz's perspective",Perception,30(10): 1157–1177,doi:10.1068/p3210,PMID11721819,S2CID8185797
• Van Helden, Albert (1977), "The Invention of the Telescope",Transactions of the American Philosophical Society,Vol. 67, No. 4 – reprinted with corrections in 2008
• Van Helden, Albert; Dupré, Sven; van Gent, Rob & Zuidervaart, Huib, eds. (2010),The Origins of the Telescope,Amsterdam: KNAW Press [=History of Science and Scholarship in the Netherlands,vol. 12]pdf link
• Watson, Fred, ed. (2004),Star Gazer: The Life and History of the Telescope,Sydney, Cambridge: Allen & Unwin, Da Capo Press

External links

History of optics articles

• Best Idea; Eyes Wide Open

History of telescope articles

• The Galileo Project – The Telescopeby Al Van Helden
• 400th Anniversary of the Invention of the Telescope
• Articles on the history of the telescope and related subjects
• The Prehistory of the Invention of the Telescope
• A Brief History of the Telescope and Ideas for Use in the High School Physics Classroom
• A History Of The Telescope
• Physics 1040 – Beginning Astronomy – The Telescope
• An early history of the telescope – From 3500 B.C. until about 1900 A.D.
• Reflecting telescopesHistorical Introduction– The Early Period (1608–1672)

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 Diggesby Colin A Ronan, M.Sc., F.R.A.S. - originally published in theJournal 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