Great refractor
Despite typical designs having smaller apertures than reflectors, great refractors offered a number of advantages and were popular for astronomy.
[3] In modern times many large refractors have become important historical items, and are often used for public astronomy outreaches.
In the early 19th century a young Edward Joshua Cooper built in Ireland one of the most richly furnished astronomical observatories of the period.
[4] Cooper had acquired the largest lens in the world, made by Cauchoix of Paris, with an objective of 13.3 inches (~34.8 cm) for 1200 pounds, and he placed it as the centerpiece of the observatory.
This was the largest refractor in the world in the early 1830s, and Cooper used the telescope to sketch Halley's comet in 1835 and to view the solar eclipse of 15 May 1836.
[6] The telescope was used for over a century with some updates, but the original was an "achromatic doublet of 11.6 inches clear aperture and focal length 19ft 6in".
[8] The era of great refractors started with the first modern, achromatic, refracting telescopes built by Joseph von Fraunhofer in the early 1820s.
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.
[18] On January 31, 1862, American telescope-maker and astronomer Alvan Graham Clark first observed the faint companion, which is now called Sirius B, or affectionately "the Pup".
[21] 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.
For reflectors in much of the 19th century, the preferred material of a primary mirror was speculum metal, a substance that reflected up to 66 percent of the light that hit it and tarnished in months.
The Great Paris Exhibition Telescope of 1900 was fixed in a horizontal position to overcome gravitational distortion on its 1.25 m (49.2 in) lens and was aimed with a 2 m siderostat.
In 1856–57, Carl August von Steinheil and Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors.
The largest refractor in Europe, with the exhibition scope dismantled, would be the double telescope, with 33-inch (84 cm) primary, La Grande Lunette at Meudon (later part of Paris Observatory).
[30] The advent of chemical-based astrophotography in the late 19th century brought difficulties in adapting great refractors to this application.
Approximate historical progression of some of the Great refactors of the late 19th century: As long as these were, they were actually much shorter than the longest singlet refractors in aerial telescopes.
