Reflecting TelescopesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSReflecting TelescopesAll the earliest optical telescopes were REFRACTORS, usinglenses to form an image; they date back to at least 1608.Since light of different wavelengths is bent or ‘refracted’unequally as it passes through glass, a refractor alwaysgives a certain amount of false color round an image suchas that of a star. Light of whatever color is howeverreflected in the same direction, so aREFLECTOR does notsuffer from this chromatic aberration. The first practicaloptical reflecting telescope was made by Isaac Newtonaround 1668.In a Newtonian reflector, there is no lens to collectthe light. The light passes down a tube and falls upon aconcave mirror. The mirror sends the light back up the tubeonto a smaller, flat mirror placed at an angle of 45◦. Thelight is then directed into the side of the tube and is broughtto focus. The image is formed outside the incoming lightbeam and is enlarged by an eyepiece lens—the observer’shead does not obstruct the incoming beam (seeNEWTONIANTELESCOPE).The Newtonian pattern is still widely used byamateur reflectors of the 6 inch to 20 inch (15 to 50 cm)class. The lower limit to the useful size range is set by theproportion of light to the main mirror which is obstructedby the flat mirror and its support. The upper limit isweaker and is set by the convenience of access to theeyepiece, say at the height of a standing man or woman,although larger telescopes may have a Newtonian focusaccessed by ladders. The 72 inch (1.9 meter) RadcliffeTelescope, originally erected in 1948 at Pretoria in SouthAfrica and later moved to Sutherland in 1974, was thelargest Newtonian telescope constructed (equal to thesize of the Rosse reflector of 1845). Its Newtonian focusis accessed from a cage attached to the dome shuttermechanism—this facility (intended for photography) isalmost unused nowadays.For larger apertures than, say, 50 cm, theCASSEGRAINTELESCOPE, invented in 1673 is more convenient. Here thesecondary mirror is convex and is placed between themain mirror and its prime focus, reflecting the light to themain focus of the telescope, through a hole in the mainmirror. The combined action of the two mirrors lengthensthe focal length of the telescope above the focal length ofthe primary mirror alone, but makes the telescope morecompact. The combination of concave and convex mirrorsalso reduces the off-axis aberrations introduced by theprimary mirror. The focus of a Cassegrain telescope isclose to the observing floor, and as convenient to accessas the telescope mounting will allow. The reduction inoverall length of a Cassegrain telescope (compared toa Newtonian telescope) directly reduces the size of thebuilding in which it is housed, with consequent savingof costs. Thus, Cassegrain telescopes quickly becamepopular for astronomical use, and still are, largely becausethey are much shorter and more compact than Newtoniansand are therefore much easier to handle and less expensive.The next major developments in making reflectingtelescopes were due to William Herschel. He made hisown mirrors and telescopes, of altazimuth design. Withone of these, a reflector of 6.2 in (16 cm) aperture and 7 foot(2.1 m) focal length, he discovered the planet Uranus in1781. His largesttelescope had a mirror49 in (1.5 m) across.It was not surpassed in size until 1845, when the thirdEarl of Rosse, in Ireland, produced a Newtonian telescopewith a 72 in (1.9 m) mirror. Despite the limitationsof its extraordinary mounting (it swung on ropes andchains between two massive stone walls on which theobserving platform was mounted, to access the eyepiece),and its cylindrical tubular mounting (which trapped heatand, potentially, degraded seeing), the Rosse telescopediscovered the spiral forms of the objects we now knowto be external galaxies. The telescope was restored toworking order (although not with its original mirror) in1998.The early reflectors had ‘speculum’ mirrors of lowreflectivity and did not produce bright images. Themetal mirrors tarnished quickly and had to be regularlyre-polished, whereas refractors required practically nomaintenance at all. It became possible to make glassmirrors, which could be coated with a highly reflectivesubstance such as silver or aluminum. In modern times,the reflection coating can be optimized to the wavelengthbands used by the telescope, e.g. the infrared.As telescopes, whether refractors or reflectors, tracka star, or point to different stars in the sky, their attitudechanges relative to the vertical and thus to the direction ofgravity. This produces mechanical flexure not only of thetelescope structure but also of the glass lenses or mirrors,causing degradation of the images. The mechanicalrigidity of glass held at the edges limits the size of lensesto about the 40 inch (1 m) size of the Yerkes refractor,made 1895. A mirror, on the other hand, can be supportedover the whole of the back of the mirror. Supportingmechanisms or mirror mounts were developed to spreadthe support evenly and to be effective at all attitudes of themirror.Reflecting telescopes are typically made with aparaboloidal primary mirror, which, if perfectly formed,correctly focuses light from a star on its optical axis toa diffraction-limited point image. Off-axis, however, theimages are aberrated by coma. A large reflecting telescopethus has a small field of view, perhaps only an arc minuteor two, unless a correcting lens is used just ahead ofthe prime focus. A corrector lens combined with its3.9 m hyperboloidal primary mirror enables the Anglo-Australian Telescope to have a field of view of up to1◦. The hyperboloidal primary mirror is combined withan ellipsoidal secondary mirror in the AAT’s Ritchey–Chr´etien optical design.For survey purposes, such as to make a photographicatlas of the whole sky, an even larger field of view is a greatadvantage. In 1930, an entirely new optical system wasdeveloped by an Estonian researcher, Bernhard Schmidt,with a modified system invented in 1941 by DmitriCopyright © Nature Publishing Group 2001Brunel Road, Houndmills, Basingstoke, Hampshire, RG21 6XS, UK Registered No. 785998and Institute of Physics Publishing 2001Dirac House, Temple Back, Bristol, BS1 6BE, UK1Reflecting TelescopesENCYCLOPEDIA OF ASTRONOMY AND ASTROPHYSICSMaksutov. In these reflecting telescopes, the incominglight is collected by a main mirror which is spherical ratherthan paraboloidal. Normally