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refracting telescope(ri-frakt -ing) (refractor) A telescope employing an objective lens to bring the light rays to a focus. The first telescopes were simple refractors with single-lens objectives: these were the Galilean and Keplerian telescopes, introduced in the early 17th century. They were supplanted by reflecting telescopes in the last quarter of that century because no-one at that time could overcome their chromatic aberrations, which introduced brilliant false color effects in the images. Although John Dollond discovered how to make an achromatic lens in 1756, he could make only small diameters suitable for terrestrial telescopes; glass makers did not know how to make large uniform disks of crown and flint glass. This obstacle was overcome in the early 1800s by Joseph Fraunhofer, who made the 24-cm refractor at Dorpat and the 16-cm heliometer at Königsberg. His successor, Georg Merz, supplied the 32.4-cm equatorial refractor to the Royal Greenwich Observatory and manufactured the 38-cm refractor that was the first large telescope to be mounted in the USA, at Cambridge, Massachusetts, in 1847.
For years after this, American astronomers leaned strongly toward the refractor. By the 1880s Alvan Clark was making relatively large refractors: a 47-cm diameter instrument for the Dearborn Observatory, a 91-cm one for Lick Observatory, and a 102-cm one for Yerkes Observatory. Other very large refractors built in the late 19th century are the 61-cm at both Lowell Observatory, Arizona, and the Pic du Midi in France, the 75-cm at Pulkovo Observatory, St Petersburg, and the 83-cm at the Meudon Observatory in Paris. There are great technical problems in making large lenses free of imperfections and impurities, and of supporting these lenses (only around the edge) so that distortion of the image is minimal. The desire for ever larger apertures to reach ever farther and fainter objects in space has meant that the major telescopes built in the 20th and 21st centuries have been reflectors.
The surviving large refracting telescopes are mainly used in astrometry for the measurement of stellar positions, proper motions, and parallax. Refractors are often preferred by visual observers because of their long focal length and closed tube; the latter avoids air currents in the tube, which often cause an unsteady image.