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Thehistory of the telescope can be traced to before the invention of the earliest knowntelescope, which appeared in 1608 in theNetherlands, when a patent was submitted byHans Lippershey, aneyeglass maker. Although Lippershey did not receive his patent, news of the invention soon spread across Europe. The design of these earlyrefracting telescopes consisted of a convexobjective lens and a concaveeyepiece.Galileo improved on this design the following year and applied it to astronomy. In 1611,Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens. By 1655, astronomers such asChristiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.[1]
Isaac Newton is 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 Cassegrain 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.
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 Dollond learned of Hall's invention[2][3] and began producing telescopes using it in commercial quantities, starting in 1758.
Important developments in reflecting telescopes wereJohn Hadley's production of largerparaboloidal mirrors in 1721; the process ofsilvering glass mirrors introduced byLéon Foucault in 1857;[4] and the adoption of long-lasting aluminized coatings on reflector mirrors in 1932.[5] TheRitchey-Chretien variant ofCassegrain reflector was invented around 1910, but not widely adopted until after 1950; many modern telescopes including theHubble Space Telescope use 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 Observatory refractor 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 mount in the 1970s andactive optics in the 1980s enabled a new generation of even larger telescopes, starting with the 10-metre (400 inch)Keck telescopes in 1993/1996, and a number of 8-metre telescopes including theESOVery Large Telescope,Gemini Observatory andSubaru Telescope.
The era ofradio telescopes (along withradio astronomy) was born withKarl Guthe Jansky'sserendipitous 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 togamma-rays. The development ofspace observatories after 1960 allowed accessto several bands impossible to observe from the ground, includingX-rays and longer wavelengthinfrared bands.
Objects resemblinglenses date back 4000 years although it is unknown if they were used for their optical properties or just as decoration.[6]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).[7][unreliable source?]
Actual use of lenses dates back to the widespread manufacture and use ofeyeglasses in Northern Italy beginning in the late 13th century.[8][6][9][10][11] The invention of the use of concave lenses to correctnear-sightedness is ascribed toNicholas of Cusa in 1451.
The first record of a telescope comes from the Netherlands in 1608. It is in a patent filed byMiddelburg spectacle-makerHans Lippershey with theStates General of the Netherlands on 2 October 1608 for his instrument "for seeing things far away as if they were nearby."[12] A few weeks later another Dutch instrument-maker,Jacob Metius also applied for a patent. The States General did not award a patent since the knowledge of the device already seemed to be ubiquitous[12][13] but the Dutchgovernment awarded Lippershey with a contract for copies of hisdesign.
The original Dutch telescopes were composed of aconvex and 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.[14]
In 1655, the Dutch diplomatWilliam de Boreel tried 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", who he remembered as the inventor of the telescope. The magistrate was contacted by a then-unknown claimant– the Middelburg spectacle-maker Johannes Zachariassen, who testified that his father–Zacharias Janssen, invented 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 who he remembered.[17] Boreel's conclusion that Zacharias Janssen invented the telescope a little ahead of another spectacle maker,Hans Lippershey, was adopted byPierre Borel in his 1656 bookDe vero telescopii inventore.[18][19] 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.[20] The "Janssen" claim would continue over the years and be added on to withZacharias Snijder in 1841 presenting 4 iron tubes with lenses in them claimed to be 1590 examples of Janssen's telescope[16] and historianCornelis de Waard's 1906 claim that the man who tried to sell a broken telescope to astronomerSimon Marius at the 1608Frankfurt Book Fair must have been Janssen.[21]
In 1682,[22] the minutes of theRoyal Society in LondonRobert Hooke notedThomas 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 glass in the mid-1500s based on an idea byRoger Bacon.[23][24] 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).[25] 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.[26] The idea of an "Elizabethan Telescope" has been expanded over the years, including astronomer and historianColin Ronan concluding in the 1990s that this reflecting/refractingtelescope was built by Leonard Digges between 1540 and 1559.[27][28][29] This "backwards"reflecting telescope would 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 that it had a very narrow field of view, making it unsuitable for military purposes.[26] 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 time,[30] and it may be that 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.[31]
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.[32]
In 2022, the Italian professor of physics Alessandro Bettini published a paper on whetherLeonardo da Vinci could have invented a telescope.[33] Building upon 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 Fracastoro in 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".[33]
Lippershey's application for a patent was mentioned at the end of a diplomatic report on an embassy to Holland from theKingdom of Siam sent 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.[35]
The Italian polymathGalileo Galilei was inVenice in June 1609[36] and there heard of the "Dutch perspective glass", a militaryspyglass,[37] 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 toPadua from Venice and made his first telescope the next day by using a convex objective lens in one extremity of a leaden tube and a concaveeyepiece lens in the other end, an arrangement that came to be called aGalilean telescope.[38] 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. Thesenate in return settled him for life in his lectureship at Padua and doubled his salary.[39]
Galileo set himself to improving the telescope, producing telescopes of increased magnification. 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.[40] With this last instrument, he began a series of astronomical observations in October or November of 1609, observing thesatellites ofJupiter, hills and valleys on themoon, the phases ofVenus[41] andspots 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 thesun and the tilted path its spots followed for part of the year pointed to the validity of the sun-centeredCopernican system over otherEarth-centered systems such 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 Demisiani at a banquet held on April 14, 1611, by PrinceFederico Cesi to makeGalileo Galilei a member of theAccademia dei Lincei.[42] 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 Bell publishedYuan jing shuo, (遠鏡說,Explanation of the Telescope) in Chinese and Latin.[43]
Johannes Kepler first 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 Scheiner who gives a description of it in hisRosa Ursina (1630).[14]
William Gascoigne 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 commonfocal plane of 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 view was much larger than in theGalilean telescope.[14]
The first powerful telescopes of Keplerian construction were made byChristiaan Huygens after 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,[44] 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.[14]
Thesharpness of the image in Kepler's telescope was limited by thechromatic aberration 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 discoveredSaturn's fifth satellite (Rhea) in 1672 with a telescope 35 feet (11 m) long. Astronomers such asJohannes Hevelius were 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.[45][46]
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.[47] and have been attributed toChristiaan Huygens and his brotherConstantijn Huygens, Jr.[45][48] although it is not clear that they invented it.[49] Christiaan Huygens and his brother made objectives up to 8.5 inches (220 mm) diameter[44] and 210 ft (64 m) focal length and others such asAdrien Auzout 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.[38] Aerial telescopes were employed by several other astronomers. Cassini discovered Saturn's third and fourth satellites in 1684 with aerial telescope objectives made byGiuseppe Campani that were 100 and 136 ft (30 and 41 m) in focal length.[14]
The ability of acurved mirror to form an image may have been known since the time ofEuclid[50] and had been extensively studied byAlhazen in 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.[51]Niccolò Zucchi, an Italian Jesuit astronomer and physicist, wrote in his bookOptica philosophia 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.[52]
In 1636Marin Mersenne 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.[53]James Gregory went 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 aberration as 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.[14]
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 experiments[55] thatall refracting 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.[56] Newton's experiments with mirrors showed that they did not suffer from the chromatic errors of lenses, for all colors of light theangle of incidence reflected in a mirror was equal to theangle of reflection, so as a proof to his theories Newton set out to build a reflecting telescope.[57] Newton completed hisfirst telescope in 1668 and it is the earliest known functional reflecting telescope.[58] After much experiment, he chose analloy (speculum metal) oftin andcopper as the most suitable material for hisobjective 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 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 aneyepiece 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, and fittings. Newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and afocal ratio of f/5.[59] With it he found that he could see the fourGalilean moons ofJupiter and 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 London in December 1671.[14] This type of telescope is still called aNewtonian telescope.
A third form of reflecting telescope, the "Cassegrain reflector" was devised in 1672 byLaurent Cassegrain. The telescope had a small convexhyperboloidal 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 for another 50 years untilJohn Hadley (best known as the inventor of theoctant) developed ways to make precision aspheric andparabolic speculum metal mirrors. In 1721 he showed the first parabolic Newtonian reflector to the Royal Society.[60] 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 Pound andJames Bradley.[61] 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.[62]
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.[63]
The British mathematician, opticianJames Short began 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 designersparabolic andelliptic 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.[64]
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 Lomonosov presented a reflecting telescope before theRussian Academy of Sciences 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 byWilliam Herschel.[65]
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.[66] 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 House inSlough, 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 Rosse built 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.
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.[14] 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 Hall fromEssex, England.[citation needed] He argued that the different humours of the human eye refract rays of light to produce an image on theretina 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 reducedchromatic aberration. One of his instruments had an objective measuring2+1⁄2 inches (6.4 cm) with a relatively short focal length of 20 inches (51 cm).[64]
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 Mansfield that "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."[64]
In 1747,Leonhard Euler sent to thePrussian Academy of Sciences 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 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.[64]
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.[67]John Dollond 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'sexperiments on the refraction of light, and that it was impossible to determine aphysical law from analytical reasoning alone.[64][68]
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 lesserfrequency 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.[64]
Dollond did not reply to this, but soon afterwards he received an abstract of a paper by theSwedish mathematician 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.[64]
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 thedivergence 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.[64][69]
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.[66]
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 concaveflint lens between them. He made many telescopes of this kind.[66]
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 Sciences offered prizes for large perfect disks of optical flint glass.[66]
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 1900 with 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.[70]
In 1856–57,Karl August von Steinheil andLéon Foucault 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. 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 locations[71] such as the60-inch Hale Telescope of 1908, and the 100-inch (2.5 m)Hooker telescope in 1917, both located atMount Wilson Observatory.[72] 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.[73] 1948 saw the completion of the 200-inch (510 cm)Hale reflector atMount Palomar which was the largest telescope in the world up until the completion of the massive 605 cm (238 in)BTA-6 in Russia twenty-seven years later. The Hale reflector introduced several technical innovations used in future telescopes, includinghydrostatic bearings for very low friction, theSerrurier truss for equal deflections of the two mirrors as the tube sags under gravity, and the use ofPyrex low-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.
The 1980s saw the introduction of two new technologies for building larger telescopes and improving image quality,known asactive optics andadaptive optics. In active optics, an image analyser senses the aberrations of a star imagea few times per minute, and a computer adjusts many support forces on the primary mirror and the location of the secondary mirrorto 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 Telescope in 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 telescopes in 1993. Other giant telescopes built since then include: the twoGemini telescopes, the four separate telescopes of theVery Large Telescope, and theLarge Binocular Telescope.
Adaptive optics uses a similar principle, but applying corrections several hundred times per second tocompensate the effects of rapidly changing optical distortion due to the motion of turbulence in Earth's atmosphere. Adaptive optics works by measuring the distortions in a wavefront and then compensating for them by rapid changes ofactuators applied to a small deformable mirror or with aliquid crystal array filter. AO was first envisioned byHorace W. Babcock in 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.[74] 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.
The twentieth century saw the construction of telescopes which could produce images using wavelengths other thanvisible light starting in 1931 whenKarl 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 theelectromagnetic spectrum from radio togamma-rays.
Radio astronomy began in 1931 whenKarl Jansky discovered that theMilky Way was a source of radio emission while doing research on terrestrial static with a direction antenna. Building on Jansky's work,Grote Reber built 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 bank telescope (1957), the 300-foot (91 m)Green Bank Telescope (1962), and the 100-metre (330 ft)Effelsberg telescope (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 asmicrowaves and this has been an important area of astronomy ever since the discovery of thecosmic microwave background radiation in 1964. Many ground-basedradio telescopes can 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 useinterferometry allowing two or more widely separated instruments to simultaneously observe the same source.Very long baseline interferometry extended 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 telescopes and longer wavelength radio telescopes including the microwave band from about 1 mm (1,000 μm) to 1,000 mm (1.0 m) in wavelength.
Although mostinfrared radiation 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 theIRAS satellite 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.
Although optical telescopes can image the near ultraviolet, theozone layer in thestratosphere absorbsultraviolet radiation 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 fluoride orlithium fluoride are used instead. TheOrbiting Solar Observatory satellite 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-rays from space do not reach Earth's surface, so X-ray astronomy must be conducted above Earth's atmosphere. The first X-ray experiments were conducted onsub-orbitalrocket flights, which enabled the first detection of X-rays from thesun (1948), and then from 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 satellites conducted 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 rays are absorbed high inEarth's atmosphere, so most gamma-ray astronomy is conducted withsatellites. Gamma-ray telescopes usescintillation counters,spark chambers and more recently,solid-state detectors. 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 3 satellite 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 radiation that is produced by the passage of the gamma-rays through Earth's atmosphere. Several Cerenkov imaging telescopes have been built around the world, including: theHEGRA (1987),STACEE (2001),HESS (2003), andMAGIC (2004).
In 1868,Fizeau noted that the purpose of the arrangement of mirrors or glass lenses in a conventional telescope was simply to provide an approximation to aFourier transform 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 asastronomical interferometry. It was not until 1891 thatAlbert A. 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 &Francis G. Pease (1921) which was applied by their 20 ft (6.1 m) interferometer mounted on the100 inch Hooker Telescope on Mount Wilson.
The next major development came in 1946, whenMartin Ryle andDerek Vonberg 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 Earth's rotation 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 synthesis 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 themathematical methods to performaperture synthesis Fourier imaging using much larger arrays of telescopes —often spread across more than one continent. In the 1980s, theaperture 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 onan array of separate optical telescopes for 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 theIOTA array.[75]
In 2008,Max Tegmark andMatias Zaldarriaga proposed 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.
John Donavan Strong, a young physicist at the California Institute of Technology, was one of the first to coat a mirror with aluminum. He did it by thermal vacuum evaporation. The first mirror he aluminized, in 1932, is the earliest known example of a telescope mirror coated by this technique.
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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.
