Optical telescopes History of the telescope

1 optical telescopes

1.1 optical foundations
1.2 invention

1.2.1 claims of prior invention
1.2.2 spread through europe


1.3 further refinements

1.3.1 refracting telescopes

1.3.1.1 long focal length refractors
1.3.1.2 aerial telescopes


1.3.2 reflecting telescopes
1.3.3 achromatic refracting telescopes


1.4 large reflecting telescopes

1.4.1 active , adaptive optics

optical telescopes
optical foundations


optical diagram showing light being refracted spherical glass container full of water, roger bacon, de multiplicatione specierum.

objects resembling lenses date 4000 years although unknown if used optical properties or decoration. greek accounts of optical properties of water filled spheres (5th century bc) followed many centuries of writings on optics, including ptolemy (2nd century) in optics, wrote properties of light including reflection, refraction, , color, followed ibn sahl (10th century) , ibn al-haytham (11th century).

actual use of lenses dates widespread manufacture , use of eyeglasses in northern italy beginning in late 13th century. invention of use of concave lenses correct near-sightedness ascribed nicholas of cusa in 1451.

invention

notes on hans lipperhey s unsuccessfully requested patent on telescope in 1608

the first record of telescope comes netherlands in 1608. in patent filed middelburg spectacle-maker hans lippershey states general of netherlands on 2 october 1608 instrument seeing things far away if nearby . few weeks later dutch instrument-maker, jacob metius applied patent. states general did not award patent since knowledge of device seemed ubiquitous dutch government awarded lippershey contract copies of design.

the original dutch telescopes composed of convex , concave lens—telescopes constructed way not invert image. lippershey s original design had 3x magnification. telescopes seem have been made in netherlands in considerable numbers after date of invention , , rapidly found way on europe.

claims of prior invention

reproduction of 1 of 4 optical devices zacharias snijder in 1841 claimed telescopes built zacharias janssen. actual function , creator has been disputed on years.

in 1655 dutch diplomat william de boreel tried solve mystery of invented telescope. had local magistrate in middelburg follow on boreel s childhood , adult recollections of spectacle maker named hans remembered inventor of telescope. magistrate contacted unknown claimant, middelburg spectacle maker johannes zachariassen, testified father, zacharias janssen invented telescope , microscope 1590. testimony seemed convincing boreel, recollected zacharias , father, hans martens, must have been remembered. boreel s conclusion zacharias janssen invented telescope little ahead of spectacle maker, hans lippershey, adopted pierre borel in 1656 book de vero telescopii inventore. discrepancies in boreel s investigation , zachariassen s testimony (including zachariassen misrepresenting date of birth , role in invention) has led historians consider claim dubious. janssen claim continue on years , added on zacharias snijder in 1841 presenting 4 iron tubes lenses in them claimed 1590 examples of janssen s telescope , historian cornelius de waard s 1906 claim man tried sell broken telescope astronomer simon marius @ 1608 frankfurt book fair must have been janssen.

in 1682, minutes of royal society in london robert hooke noted thomas digges 1571 pantometria, (a book on measurement, partially based on father leonard digges notes , observations) seemed support english claim invention of telescope, describing leonard having fare seeing glass in mid 1500s based on idea roger bacon. thomas described proportional glasses duly situate in convenient angles, not discovered things far off, read letters, numbered pieces of money coin , superscription thereof, cast of friends of purpose upon downs in open fields, 7 miles off declared hath been done @ instant in private places. comments on use of proportional or perspective glass made in writings of john dee (1575) , william bourne (1585). bourne asked in 1580 investigate diggs device queen elizabeth s chief advisor lord burghley. bourne s best description of it, , writing seemed consist of peering large curved mirror reflected image produced large lens. idea of elizabethan telescope has been expanded on years, including astronomer , historian colin ronan 1990s explorations concluding reflecting/refracting telescope built leonard digges between 1540 , 1559. backwards reflecting telescope have been unwieldy, needed large mirrors , lens work, observer had stand backwards @ upside down view, , bourne noted had narrow field of view making unsuitable military purposes. optical performance required see details of coins lying in fields, or private activities 7 miles away, seems far beyond technology of time , perspective glass being described far simpler idea, originating bacon, of using single lens held in front of eye magnify distant view.

a 1959 research paper simon de guilleuma claimed evidence had uncovered pointed french born spectacle maker juan roget (died before 1624) possible builder of telescope predated hans lipperhey s patent application. inventing telescope-like devices.

spread through europe

lippershey s application patent mentioned @ end of diplomatic report on embassy holland kingdom of siam sent siamese king ekathotsarot: ambassades du roy de siam envoyé à l excellence du prince maurice, arrivé à la haye le 10 septemb. 1608 (embassy of king of siam sent excellency prince maurice, arrived @ hague on 10 september 1608). report issued in october 1608 , distributed across europe, leading experiments other scientists, such italian paolo sarpi, received report in november, , english mathematician , astronomer thomas harriot, used six-powered telescope summer of 1609 observe features on moon.

19th-century painting depicting galileo galilei displaying telescope leonardo donato in 1609.

the italian polymath galileo galilei in venice in june 1609 , there heard of dutch perspective glass means of distant objects appeared nearer , larger. galileo states solved problem of construction of telescope first night after return padua venice , made first telescope next day fitting convex lens in 1 extremity of leaden tube , concave lens in other one. few days afterwards, having succeeded in making better telescope first, took venice communicated details of invention public , presented instrument doge leonardo donato, sitting in full council. senate in return settled him life in lectureship @ padua , doubled salary.

portrait of galileo galilei.

galileo spent time improving telescope, producing telescopes of increased power. first telescope had 3x magnification, made instruments magnified 8x , finally, 1 meter long 37mm objective (which stop down 16mm or 12mm) , 23x magnification. last instrument began series of astronomical observations in october or november 1609, discovering satellites of jupiter, hills , valleys on moon, phases of venus , observed spots on sun. galileo noted revolution of satellites of jupiter, phases of venus, rotation of sun , tilted path spots followed part of year pointed validity of sun centered copernican system on other earth centered systems such 1 proposed ptolemy. galileo s instrument first given name telescope . name invented greek poet/theologian giovanni demisiani @ banquet held on april 14, 1611 prince federico cesi make galileo galilei member of accademia dei lincei. word created greek tele = far , skopein = or see ; teleskopos = far-seeing .

these observations, galileo s improvement of instrument, led adoption of name of galilean telescope these forms of telescope employed negative lens.

further refinements
refracting telescopes

johannes kepler first explained theory , of practical advantages of telescope constructed of 2 convex lenses in catoptrics (1611). first person constructed telescope of form jesuit christoph scheiner gives description of in rosa ursina (1630).

william gascoigne first commanded chief advantage of form of telescope suggested kepler: small material object placed @ common focal plane of objective , eyepiece. led invention of micrometer, , application of telescopic sights precision astronomical instruments. not till middle of 17th century kepler s telescope came general use: not because of advantages pointed out gascoigne, because field of view larger in galilean telescope.

the first powerful telescopes of keplerian construction made christiaan huygens after labor—in brother assisted him. 1 of these: objective diameter of 2.24 inches (57 mm) , 12 ft (3.7 m) focal length, discovered brightest of saturn s satellites (titan) in 1655; in 1659, published systema saturnium which, first time, gave true explanation of saturn s ring—founded on observations made same instrument.

long focal length refractors

woodcut illustration of 45 m (148 ft) focal length keplerian astronomical refracting telescope built johannes hevelius. book, machina coelestis (first part), published in 1673.

the sharpness of image in kepler s telescope limited chromatic aberration introduced non-uniform refractive properties of objective lens. way overcome limitation @ high magnifying powers create objectives long focal lengths. giovanni cassini discovered saturn s fifth satellite (rhea) in 1672 telescope 35 feet (11 m) long. astronomers such johannes hevelius constructing telescopes focal lengths long 150 feet (46 m). besides having long tubes these telescopes needed scaffolding or long masts , cranes hold them up. value research tools minimal since telescope s frame tube flexed , vibrated in slightest breeze , collapsed altogether.

aerial telescopes

in of long refracting telescopes constructed after 1675, no tube employed @ all. objective mounted on swiveling ball-joint on top of pole, tree, or available tall structure , aimed means of string or connecting rod. eyepiece handheld or mounted on stand @ focus, , image found trial , error. these consequently termed aerial telescopes. , have been attributed christiaan huygens , brother constantijn huygens, jr. although not clear invented it. christiaan huygens , brother made objectives 8.5 inches (220 mm) diameter , 210 ft (64 m) focal length , others such adrien auzout made telescopes focal lengths 600 ft (180 m). telescopes of such great length naturally difficult use , must have taxed utmost skill , patience of observers. aerial telescopes employed several other astronomers. cassini discovered saturn s third , fourth satellites in 1684 aerial telescope objectives made giuseppe campani 100 , 136 ft (30 , 41 m) in focal length.

reflecting telescopes

the ability of curved mirror form image may have been known since time of euclid , had been extensively studied alhazen in 11th century. galileo, giovanni francesco sagredo, , others, spurred on knowledge curved mirrors had similar properties lenses, discussed idea of building telescope using mirror image forming objective. niccolò zucchi, italian jesuit astronomer , physicist, wrote in book optica philosophia of 1652 tried replacing lens of refracting telescope bronze concave mirror in 1616. zucchi tried looking mirror hand held concave lens did not satisfactory image, possibly due poor quality of mirror, angle tilted at, or fact head partially obstructed image.

light path in gregorian telescope.

in 1636 marin mersenne proposed telescope consisting of paraboloidal primary mirror , paraboloidal secondary mirror bouncing image through hole in primary, solving problem of viewing image. james gregory went further detail in book optica promota (1663), pointing out reflecting telescope mirror shaped part of conic section, correct spherical aberration chromatic aberration seen in refractors. design came bears name: gregorian telescope ; according own confession, gregory had no practical skill , find no optician capable of realizing ideas , after fruitless attempts, obliged abandon hope of bringing telescope practical use.

light path in newtonian telescope.



a replica of newton s second reflecting telescope presented royal society in 1672.

in 1666 isaac newton, based on theories of refraction , color, perceived faults of refracting telescope due more lens s varying refraction of light of different colors lens s imperfect shape. concluded light not refracted through lens without causing chromatic aberrations, although incorrectly concluded rough experiments refracting substances diverge prismatic colors in constant proportion mean refraction. these experiments newton concluded no improvement made in refracting telescope. newton s experiments mirrors showed did not suffer chromatic errors of lenses, colors of light angle of incidence reflected in mirror equal angle of reflection, proof theories newton set out build reflecting telescope. newton completed first telescope in 1668 , earliest known functional reflecting telescope. after experiment, chose alloy (speculum metal) of tin , copper suitable material objective mirror. later devised means grinding , polishing them, chose spherical shape mirror instead of parabola simplify construction. added reflector hallmark of design of newtonian telescope , secondary diagonal mirror near primary mirror s focus reflect image @ 90° angle eyepiece mounted on side of telescope. unique addition allowed image viewed minimal obstruction of objective mirror. made tube, mount, , fittings. newton s first compact reflecting telescope had mirror diameter of 1.3 inches , focal ratio of f/5. found see 4 galilean moons of jupiter , crescent phase of planet venus. encouraged success, made second telescope magnifying power of 38x presented royal society of london in december 1672. type of telescope still called newtonian telescope.

light path in cassegrain telescope.

a third form of reflecting telescope, cassegrain reflector devised in 1672 laurent cassegrain. telescope had small convex hyperboloidal secondary mirror placed near prime focus reflect light through central hole in main mirror.

no further practical advance appears have been made in design or construction of reflecting telescopes 50 years until john hadley (best known inventor of octant) developed ways make precision aspheric , parabolic speculum metal mirrors. in 1721 showed first parabolic newtonian reflector royal society. had 6-inch (15 cm) diameter, 62 ⁄4-inch (159 cm) focal length speculum metal objective mirror. instrument examined james pound , james bradley. after remarking newton s telescope had lain neglected fifty years, stated hadley had sufficiently shown invention did not consist in bare theory. compared performance of 7.5 inches (190 mm) diameter aerial telescope presented royal society constantijn huygens, jr. , found hadley s reflector, bear such charge make magnify object many times latter due charge , , represents objects distinct, though not altogether clear , bright.

bradley , samuel molyneux, having been instructed hadley in methods of polishing speculum metal, succeeded in producing large reflecting telescopes of own, 1 of had focal length of 8 ft (2.4 m). these methods of fabricating mirrors passed on molyneux 2 london opticians —scarlet , hearn— started business manufacturing telescopes.

the british mathematician, optician james short began experimenting building telescopes based on gregory s designs in 1730s. first tried making mirrors out of glass suggested gregory, later switched speculum metal mirrors creating gregorian telescopes original designers parabolic , elliptic figures. short adopted telescope-making profession practised first in edinburgh, , afterward in london. short s telescopes of gregorian form. short died in london in 1768, having made considerable fortune selling telescopes.

since speculum metal mirror secondaries or diagonal mirrors reduced light reached eyepiece, several reflecting telescope designers tried away them. in 1762 mikhail lomonosov presented reflecting telescope before russian academy of sciences forum. had primary mirror tilted @ 4 degrees telescope s axis image viewed via eyepiece mounted @ front of telescope tube without observer s head blocking incoming light. innovation not published until 1827, type came called herschelian telescope after similar design william herschel.

william herschel s 49-inch (1,200 mm) 40-foot telescope of 1789. illustration encyclopædia britannica third edition published in 1797.

about year 1774 william herschel (then teacher of music in bath, england) began occupy leisure hours construction of reflector telescope mirrors, devoted himself entirely construction , use in astronomical research. in 1778, selected 6 ⁄4-inch (16 cm) reflector mirror (the best of 400 telescope mirrors had made) , it, built 7-foot (2.1 m) focal length telescope. using telescope, made brilliant astronomical discoveries. in 1783, herschel completed reflector of approximately 18 inches (46 cm) in diameter , 20 ft (6.1 m) focal length. observed heavens telescope twenty years, replacing mirror several times. in 1789 herschel finished building largest reflecting telescope mirror of 49 inches (120 cm) , focal length of 40 ft (12 m), (commonly known 40-foot telescope) @ new home, @ observatory house in slough, england. cut down on light loss poor reflectivity of speculum mirrors of day, herschel eliminated small diagonal mirror design , tilted primary mirror view formed image directly. design has come called herschelian telescope. discovered saturn s sixth known moon, enceladus, first night used (august 28, 1789), , on september 17, seventh known moon, mimas. telescope world s largest telescope on 50 years. however, large scope difficult handle , less used favorite 18.7-inch reflector.

in 1845 william parsons, 3rd earl of rosse built 72-inch (180 cm) newtonian reflector called leviathan of parsonstown discovered spiral form of galaxies.

all of these larger reflectors suffered poor reflectivity , fast tarnishing nature of speculum metal mirrors. meant need more 1 mirror per telescope since mirrors had removed , re-polished. time consuming since polishing process change curve of mirror had re-figured correct shape.

achromatic refracting telescopes


light path through achromatic lens.

from time of invention of first refracting telescopes supposed chromatic errors seen in lenses arose errors in spherical figure of surfaces. opticians tried construct lenses of varying forms of curvature correct these errors. isaac newton discovered in 1666 chromatic colors arose un-even refraction of light passed through glass medium. led opticians experiment lenses constructed of more 1 type of glass in attempt canceling errors produced each type of glass. hoped create achromatic lens ; lens focus colors single point, , produce instruments of shorter focal length.

the first person succeeded in making practical achromatic refracting telescope chester moore hall essex, england. argued different humours of human eye refract rays of light produce image on retina free color, , reasonably argued might possible produce result combining lenses composed of different refracting media. after devoting time inquiry found combining 2 lenses formed of different kinds of glass, make achromatic lens effects of unequal refractions of 2 colors of light (red , blue) corrected. in 1733, succeeded in constructing telescope lenses exhibited reduced chromatic aberration. 1 of instruments had objective measuring 2 ⁄2 inches (6.4 cm) relatively short focal length of 20 inches (51 cm).

hall man of independent means , seems have been careless of fame; @ least took no trouble communicate invention world. @ trial in westminster hall patent rights granted john dollond (watkin v. dollond), hall admitted first inventor of achromatic telescope. however, ruled lord mansfield not original inventor ought profit such invention, 1 brought forth benefit of mankind.

in 1747, leonhard euler sent prussian academy of sciences paper in tried prove possibility of correcting both chromatic , spherical aberration of lens. gregory , hall, argued since various humours of human eye combined produce perfect image, should possible suitable combinations of lenses of different refracting media construct perfect telescope objective. adopting hypothetical law of dispersion of differently colored rays of light, proved analytically possibility of constructing achromatic objective composed of lenses of glass , water.

all of euler s efforts produce actual objective of construction fruitless—a failure attributed solely difficulty of procuring lenses worked precisely requisite curves. john dollond agreed accuracy of euler s analysis, disputed hypothesis on grounds purely theoretical assumption: theory opposed results of newton s experiments on refraction of light, , impossible determine physical law analytical reasoning alone.

in 1754, euler sent berlin academy further paper in starting hypothesis light consists of vibrations excited in elastic fluid luminous bodies—and difference of color of light due greater or lesser frequency of these vibrations in given time— deduced previous results. did not doubt accuracy of newton s experiments quoted dollond.

dollond did not reply this, afterwards received abstract of paper swedish mathematician , astronomer, samuel klingenstierna, led him doubt accuracy of results deduced newton on dispersion of refracted light. klingenstierna showed purely geometrical considerations (fully appreciated dollond) results of newton s experiments not brought harmony other universally accepted facts of refraction.

dollond telescope.

as practical man, dollond @ once put doubts test of experiment: confirmed conclusions of klingenstierna, discovered difference far beyond hopes in refractive qualities of different kinds of glass respect divergence of colors, , rapidly led construction of lenses in first chromatic aberration—and afterwards—the spherical aberration corrected.

dollond aware of conditions necessary attainment of achromatism in refracting telescopes, relied on accuracy of experiments made newton. writings show exception of bravado, have arrived sooner @ discovery mind prepared. dollond s paper recounts successive steps arrived @ discovery independently of hall s earlier invention—and logical processes these steps suggested mind.

in 1765 peter dollond (son of john dollond) introduced triple objective, consisted of combination of 2 convex lenses of crown glass concave flint lens between them. made many telescopes of kind.

the difficulty of procuring disks of glass (especially of flint glass) of suitable purity , homogeneity limited diameter , light gathering power of lenses found in achromatic telescope. in vain french academy of sciences offered prizes large perfect disks of optical flint glass.

the difficulties impractical metal mirrors of reflecting telescopes led construction of large refracting telescopes. 1866 refracting telescopes had reached 18 inches (46 cm) in aperture many larger great refractors being built in mid late 19th century. in 1897, refractor reached maximum practical limit in research telescope construction of yerkes observatorys 40-inch (100 cm) refractor (although larger refractor great paris exhibition telescope of 1900 objective of 49.2 inches (1.25 m) diameter temporarily exhibited @ paris 1900 exposition). no larger refractors built because of gravity s effect on lens. since lens can held in place edge, center of large lens sag due gravity, distorting image produces.

large reflecting telescopes


the 200-inch (5.1 m) hale telescope @ mount palomar

in 1856–57, karl august von steinheil , léon foucault introduced process of depositing layer of silver on glass telescope mirrors. silver layer not more reflective , longer lasting finish on speculum mirrors, had advantage of being able removed , re-deposited without changing shape of glass substrate. towards end of 19th century large silver on glass mirror reflecting telescopes built.

the beginning of 20th century saw construction of first of modern large research reflectors, designed precision photographic imaging , located @ remote high altitude clear sky locations such 60-inch hale telescope of 1908, , 100-inch (2.5 m) hooker telescope in 1917, both located @ mount wilson observatory. these , other telescopes of size had have provisions allow removal of main mirrors re-silvering every few months. john donavan strong, young physicist @ california institute of technology, developed technique coating mirror longer lasting aluminum coating using thermal vacuum evaporation. in 1932, became first person aluminize mirror; 3 years later 60-inch (1,500 mm) , 100-inch (2,500 mm) telescopes became first large astronomical telescopes have mirrors aluminized. 1948 saw completion of 200-inch (510 cm) hale reflector @ mount palomar largest telescope in world until completion of massive 605 cm (238 in) bta-6 in russia twenty-seven years later. hale reflector introduced several technical innovations used in future telescopes, including hydrostatic bearings low friction, serrurier truss equal deflections of 2 mirrors tube sags under gravity, , use of pyrex low-expansion glass mirrors. arrival of substantially larger telescopes had await introduction of methods other rigidity of glass maintain proper shape of mirror.

active , adaptive optics

the 1980s saw introduction of 2 new technologies building larger telescopes , improving image quality, known active optics , adaptive optics. in active optics, image analyser senses aberrations of star image few times per minute, , computer adjusts many support forces on primary mirror , location of secondary mirror maintain optics in optimal shape , alignment. slow correct atmospheric blurring effects, enables use of thin single mirrors 8 m diameter, or larger segmented mirrors. method pioneered eso new technology telescope in late 1980s.

the 1990s saw new generation of giant telescopes appear using active optics, beginning construction of first of 2 10 m (390 in) keck telescopes in 1993. other giant telescopes built since include: 2 gemini telescopes, 4 separate telescopes of large telescope, , large binocular telescope.

eso s vlt boasts advanced adaptive optics systems, counteract blurring effects of earth s atmosphere.

adaptive optics uses similar principle, applying corrections several hundred times per second compensate effects of rapidly changing optical distortion due motion of turbulence in earth s atmosphere. adaptive optics works measuring distortions in wavefront , compensating them rapid changes of actuators applied small deformable mirror or liquid crystal array filter. ao first envisioned horace w. babcock in 1953, did not come common usage in astronomical telescopes until advances in computer , detector technology during 1990s made possible calculate compensation needed in real time. in adaptive optics, high-speed corrections needed mean bright star needed close target of interest (or artificial star created laser). also, single star or laser corrections effective on narrow field (tens of arcsec), , current systems operating on several 8-10m telescopes work in near-infrared wavelengths single-object observations.

developments of adaptive optics include systems multiple lasers on wider corrected field, and/or working above kilohertz rates correction @ visible wavelengths; these in progress not yet in routine operation of 2015.