A refracting telescope is an optical instrument in which the light rays from an object is gathered and brought to a focus by an objective lens and then it is magnified and viewed through the eyepiece lens. It is called 'refracting' because the objective refracts the light rays, which are parallel, meaning bending them into focus. The first telescopes in history were simple refractors, which were fitted with a single-lens objective and a single-lens eyepiece. On a Galilean refracting telescope, the objective lens was convex, while the eyepiece was concave. The Keplerian telescope, on the other hand, utilized a plano-convex lens for the both the objective and the eyepiece. Both types had been developed in the 17th century. However, due to their chromatic aberrations, they would be replaced by reflecting telescopes, which use a concave mirror, instead of a convex lens, to bring the light into focus.
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| Schematic drawing of a simple refracting telescope. |
The major advance in the evolution of refracting telescopes was the invention of the achromatic lens, which is a lens made up of multiple elements. This invention helped solve problems with chromatic aberrations, allowing shorter focal lengths. It was invented in 1733 by an English barrister named Chester Moore Hall. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to reduce chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane.
For years, European and American astronomers leaned strongly toward the refracting telescope, whose objective lens had been improved. 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. Nevertheless, lens makers managed to minimized these imperfection, producing lenses minimal optical distortion. 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.
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| Above, a 1897 photo of a refractor telescope at Charles Yerkes Observatory, Wisconsin, USA. |