Reflectors In England in the 17th century, attention turned to mirror, or reflecting,
telescopes, which usually consisted
of two mirrors because of the mistaken belief that lens aberrations were irreparable. In 1663 the Scottish mathematician James Gregory proposed a configuration in which a primary concave parabolic mirror converges the light to one focus of a concave ellipsoidal mirror. Reflection of light rays at its surface causes convergence to the ellipsoid's second focus, located behind the main mirror and reached through a central hole in it. The tube length is thus less than the sum of the focal lengths of the two mirrors. Because of construction difficulties, only a few so-called Gregorian
telescopes have ever been made. The largest, 50 in (130 cm) in diameter, is at Canberra, Australia. Isaac Newton, in 1668, proposed a
Telescope in which a primary parabolic mirror reflects light rays to an inclined "flat"Ñin small instruments, a totally reflecting prismÑplaced centrally in the tube. This flat, in turn, reflects the rays to an observer located at the side of the upper end of the tube. An instrument of this kind, built by Newton, still exists at the Royal Society of London. Many large and medium-size reflectors are are still basically Newtonian in design, although for diameters less than 1 m (3.3 ft) the weights of instrumentation capable of being carried far from the support axes are small. For instruments with a larger diameter, elaborate carriages are needed to give an observer access to the focus. For large Newtonians the focal ratioÑthe ratio of aperture size to focal lengthÑis typically near f/4. In some casesÑthe Hale 200-in (5-m) telescope at Palomar Observatory and the 120-in (3-m) telescope at Lick Observatory, for instanceÑonly a single mirror is used, and access to the prime focus is from a central cage containing the observer. Some large telescopes, such as the 156-in (4-m) one at Kitt Peak National Observatory and the 153-in (3.9-m) Anglo-Australian Telescope, carry observers at the so-called Cassegrain focus (see Cassegrainian telescope). Except for work done at the prime focus, both old and recently built large telescopes use television viewing systems and electronic devices instead of human observers at the focus. Early reflectors had mirrors made of speculum metalÑa brittle alloy consisting of copper and tinÑwhich required repolishing and refiguring when it became tarnished. Sir William Herschel used such mirrors for making telescopes, including one 48 in (1.2 m) in diameter. The Herschelian configuration has the main mirror slightly tilted to feed an eyepiece mounted at one side of the upper end of the tube. In 1845, at Birr Castle in Ireland, William Parsons, 3d earl of Rosse, built a 72-in (1.8-m) reflector with a speculum-metal mirror and used it to observe galaxies. Glass was first used for the manufacture of telescope mirrors in 1856, by Carl August von Steinheil and Jean Foucault. Such optical surfaces were silvered by chemical processes developed by Justus von Liebig and John A. Brashear. The use of a silver coating on glass was later superseded by vacuum-deposited aluminum and sometimes by other metals, using a technique developed in 1934. Between the world wars Pyrex, a glass with a low coefficient of thermal expansion, came into use. Since World War II such glass has largely been replaced by quartz (fused or vapor-condensed) and in some cases by special ceramics noted for their low thermal expansion.