Reflecting telescopes History of the telescope

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.