Further refinements History of the telescope

1 further refinements

1.1 refracting telescopes

1.1.1 long focal length refractors
1.1.2 aerial telescopes


1.2 reflecting telescopes
1.3 achromatic refracting telescopes

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.