Great refractor

Vienna Observatory's Große Refraktor (Great Refractor)^[1] of 1880, with 69 cm aperture

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^[2] in the 19th and early 20th century.^[2] 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.^[2]

It was not until the 20th century that they were gradually superseded by large reflecting telescopes for professional astronomy.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, double stars, and pioneering work on astrophotgraphy was done great refractors.^[3]

Era of large refractors

Percival Lowell observing Venus in the daytime from the observer's chair of the 24-inch (61 cm) Alvan Clark refracting telescope in Flagstaff, Arizona.

The choice of building large refractors over reflectors was a technological one.^[4] 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.^[5] 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.^[6][7] The Swiss Pierre Louis Guinand (1748–1824) discovered and developed a way to make much larger crown and flint glass blanks.^[7] He worked with instrument maker Joseph von Fraunhofer (1787–1826) to use this technology for instruments in the early 19th century.^[7]

The era of great refractors started with the first modern, achromatic, refracting telescopes built by Joseph von Fraunhofer in the early 1820s.^{[8][9][10][11]} 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 northern 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,^[12] 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.^[13]

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.^[14][15] 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.^[15] To achieve its optical aperture it was actually slightly bigger physically, at 41 3/8 in.^[16] Refractors had reached their technological limit; the problems of lens sagging from gravity meant refractors would not exceed around 1 meter,^[17] although Alvan G. Clark, who had made the Yerkes 40-inch objective, said a 45-inch (114 cm) would be possible before he died.^[16] 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.^[16]

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.

End of the era

The 40-inch (1.02 m) Refractor, at Yerkes Observatory, the largest achromatic refractor ever put into practical astronomical use.

The era slowly came to end as large reflecting telescopes superseded the great refractors. In 1856-57, Carl August von Steinheil and Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors. Silvered glass mirrors were a vast improvement over speculum metal and made reflectors a practical instrument. The era of large reflectors had begun, with telescopes such as the 36-inch (91 cm) Crossley Reflector (1895), 60-inch (1.5 m) Mount Wilson Observatory Hale telescope of 1908, and the 100-inch (2.5 m) Mount Wilson Hooker telescope in 1917.^[18] (See also)

Examples

Great refractors were admired for their quality, durability, and usefulness which correlated to features such as lens quality, mount quality, aperture, and also length. Length was important because unlike reflectors (which can be folded and shorted), the focal length of glass lens correlated to the physical length of the telescope and offered some optical and image quality advantages.

Aperture

Approximate historical progression:

The progression of largest refracting telescopes in the 19th century, including some telescopes at Private Observatories that were not really used very much or had problems.^[19][20]

Some of the 2nd largest refractors, or otherwise notable.

Focal length

Approximate historical progression in the late 19th century:

As long as these were, they were actually much shorter than the longest singlet refractors in aerial telescopes.^[31]

Gallery

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  Modern day video of Großer Refraktor of 1896 at Treptow Observatory, later renamed Archenhold Observatory

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  Grande Lunette of Nice Observatory of 1886, with 77 cm aperture^[1]

• 

  James Lick telescope of 1888, with 91 cm aperture

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  Four astronomical instruments of the Strasbourg Observatory, including its Großer Refraktor

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See also

• List of largest optical refracting telescopes
• Aerial telescopes
• List of largest optical telescopes in the 18th century
• List of largest optical telescopes in the 19th century
• List of largest optical telescopes in the 20th century
• Extremely large telescope
• Achromat
• Apochromat

References

Further reading

• "The History of the Development of the Telescope", Authors: Schirach, W. F. H.
• "The Great 19th Century Refractors"

See also

• List of telescope types

External links

• Great Refractors Era
• Fraunhofer's refractor
• Google Books: Great Refractor
• Modern Day Image of the Harvard Great Refractor
• High-school astronomy By Hiram Mattison (U.S. Examples circa 1856) circa 1872