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generally do not suffer from the size limitations of refract- ing telescopes. The mirrors in reflecting telescopes can be as thick as necessary and can be supported by mechanisms that prevent sagging and thus inhibit excessive distortion. In ad- dition, mirrors benefit from materials with vanishingly small expansion coefficients, and from ribbing techniques that allow rapid equalization of thermal gradients in a mirror. The larg- est reflecting telescopes use segmented mirrors, composed of many separate pieces, using lasers and computer calculations to maintain accurate alignment of the segments. The largest optical reflecting telescope currently in operation is the 10.4 m (34.1 ft) Gran Telescopio Canarias on the Spanish island of La Palma, Spain. Its primary mirror consists of 36 hexagonal seg- ments (Fig. 8). A next generation of optical telescopes is planned for the 2020s, including the Giant Magellan Telescope, utilizing an array of 8.4 m mirrors with a combined resolving power of a single 24.5 m (965 in) mirror (Fig. 9); the Thirty Meter Telescope; and the Euro- pean Extremely Large Telescope, whose 798 segmented mirrors will act together as a 39 m (1535 in) primary mirror. Catadioptric telescopes Catadioptric telescopes combine both mirrors and lenses. This combination is generally used to image wide fields. Catadiop- tric telescopes with apertures of 20 – 35 cm (8 – 14 in.) are popu- lar with amateur astronomers. One variant of the catadioptric system is the Schmidt-Cassegrain design, and Ritchie-Chrétien designs have also appeared in amateur-grade systems. Telescope (continued) + ward ' s science Fig. 9 An artist's impression of the completed Giant Magellan Telescope, to be located at the Las Campanas Observatory in Chile. (Credit: GMTO Corporation) Fig. 8 The largest single telescope in the world, the 10.4-m (34.1-ft) Gran Telescopio Canarias, on La Palma, Canary Islands, Spain. (Credit: Instituto de Astrofísica de Canarias) Fig. 7 Diagrams of reflecting telescopes. (a) Cassegrain telescope, with either classical or Ritchey-Chrétien optics. (b) Newtonian telescope. The eyepieces or detectors are not shown.