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Great refractor refers to a large telescope with a lens, usually the largest refractor at an observatory with an equatorial mount. The preeminence and success of this style in observational astronomy was an era in telescope use〔(【引用サイトリンク】title=Era: Great Refractors )〕 in the 19th and early 20th century.〔 Great refractors were large refracting telescopes using achromatic lenses (as opposed to mirrors). They were often the largest in the world, or largest in a region. Despite typical designs having smaller apertures than reflectors, Great refractors offered a number of advantages and were favored for astronomy.〔 It was not until the 20th century that they were gradually superseded by large reflecting telescopes for professional astronomy. A great refractor was often the centerpiece of a new 19th century observatory, but was typically used with an entourage of other astronomical instruments such as a Meridian Circle, a Heliometer, and Astrograph, and a smaller refractor such as a Comet Seeker or Equatorial. Great refractors were often used for observing double stars and equipped with a Filar micrometer. Numerous discoveries of minor planets, satellites, the planet Neptune, and double stars were made, and pioneering work on astrophotography was done, with great refractors. ==Era of large refractors== The choice of building large refractors over reflectors was a technological one. The difficulties of fabricating two disks of optical glass required to make a large achromatic lens were formidable. But reflecting telescopes had larger problems. The material their primary mirror was made of was speculum metal, a substance that only reflected up to 66 percent of the light that hit it and tarnished in months. They had to be removed, polished, and re-figured to the correct shape. This sometimes proved difficult, with the telescope mirrors sometimes having to be abandoned.〔(【引用サイトリンク】title=1956ASPL 7..249P Page 253 )〕 Because of this, large refractors seemed to be the better choice. Although there had been very large (and unwieldy) Non-achromatic aerial telescopes of the late 17th century, and Chester Moore Hall and others had experimented with small achromatic telescopes in the 18th century, John Dollond (1706–1761) invented and created an achromatic object glass and lens which permitted achromatic telescopes up to 3–5 in (8–13 cm) aperture.〔(【引用サイトリンク】title=History - British History in depth: The Airy Transit Circle )〕〔(【引用サイトリンク】title=the lens )〕 The Swiss Pierre Louis Guinand (1748–1824) discovered and developed a way to make much larger crown and flint glass blanks.〔 He worked with instrument maker Joseph von Fraunhofer (1787–1826) to use this technology for instruments in the early 19th century.〔 The era of ''great refractors'' started with the first modern, achromatic, refracting telescopes built by Joseph von Fraunhofer in the early 1820s.〔(adsabs link ) ''Fraunhofer and the Great Dorpat Refractor'', Waaland, J. Robert, American Journal of Physics, Volume 35, Issue 4, pp. 344-350 (1967)〕〔(【引用サイトリンク】title=Fraunhoferi refraktor )〕〔''Resolute and undertaking characters: the lives of Wilhelm and Otto Struve'' By Alan Henry Batten; Page 46-49; (Google Books )〕〔(The great nineteenth century refractors ) by James Lequeux, Page 1〕 The first of these was the Dorpat Great Refractor, also known as the Fraunhofer 9-inch, at what was then Dorpat Observatory in the Governorate of Estonia (Estland) (which later became Tartu Observatory in southern Estonia). This telescope made by Fraunhofer had a 9 Paris inch (about 9.6 in (24 cm)) aperture achromatic lens and a 4 m (13.4 ft) focal length. It was also equipped with the first modern equatorial mount type called a "German equatorial mount" developed by Fraunhofer, a mount that became standard for most large refractors from then on. A Fraunhofer 9-inch (24 cm) at Berlin Observatory was used by Johann Gottfried Galle in the discovery of Neptune. Refracting telescopes would quadruple in size by the end of the century, culminating with the largest practical refractor ever built, the Yerkes Observatory 40-inch (1 meter) aperture of 1895.〔(【引用サイトリンク】title=Telescope: Yerkes 40-inch Refractor )〕 This great refractor pushed the limits of technology of the day; the fabrication of the two element achromatic lens (the largest lens ever made at the time), required 18 attempts and cooperation between Alvan Clark & Sons and Charles Feil of Paris.〔 To achieve its optical aperture it was actually slightly bigger physically, at 41 3/8 in.〔 Refractors had reached their technological limit; the problems of lens sagging from gravity meant refractors would not exceed around 1 meter,〔("Physics Demystified" By Stan Gibilisco, ISBN 0-07-138201-1, page 515 ) Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces. The largest practical lens size in a refracting telescope is around 1 meter,〕 although Alvan G. Clark, who had made the Yerkes 40-inch objective, said a 45-inch (114 cm) would be possible before he died.〔(''Six Thousand Years of History: Achievements of the nineteenth century'' Edgar Sanderson, John Porter Lamberton, Charles Morris, Page 286 (Google Books 2010) )〕 In addition to the lens, the rest of the telescope needed to be a practical and high precision instrument, despite the size. For example, the Yerkes tube alone weighed 75 tons, and had to track stars just as accurately as a smaller instrument.〔 The Great Paris Exhibition Telescope of 1900, which was fixed in horizontal position to overcome gravitational distortion on its 1.25 m (49.2 in) lens, and aimed with a 2 m siderostat, but this demonstration telescope was scrapped after the Exposition Universelle closed. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Great refractor」の詳細全文を読む スポンサード リンク
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