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A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spyglasses and astronomical telescopes but is also used for long-focus camera lenses .
Lens sag is a problem that sometimes afflicts very large refracting telescopes. It is the equivalent of mirror sag in reflecting telescopes. It occurs when the physical weight of the glass causes a distortion in the shape of the lens because the lens can only be supported by the edges. A mirror on the other hand can be effectively supported by ...
The Swedish 1-m Solar Telescope, with a lens diameter of 43 inches, is technically the largest, with 39 inches clear for the aperture.The second largest refracting telescope in the world is the Yerkes Observatory 40 inch (102 cm) refractor, used for astronomical and scientific observation for over a century.
The 28-inch telescope was installed on the same mounting as an older 12.8 inch aperture refractor at the same location. [1] The 12.8 inch was moved to be a guide telescope for the Thompson 26-inch refractor. [1] The mounting dates to the 1850s. [2] [7] The mounting is an English equatorial mount built by Ransomes and Sims, and Williams Sims. [4]
This happened during testing of an 18.5-inch (470 mm) aperture great refractor telescope for Dearborn Observatory, which was one of the largest refracting telescope lens in existence at the time, and the largest telescope in the United States. [20] A 25-inch (63.5 cm) objective refractor was installed in the Newall telescope. [18]
Two-element refracting telescopes were extensively used in 19th century observatories despite their smaller apertures than metal, and later glass, mirror telescopes. The technology for silver-coating glass mirrors, more reflective than speculum metal and not subject to tarnishing, was developed in the mid-19th century but was slow to be adopted.
A reflecting telescope by James Short; this English telescope maker produced almost 1400 Gregorian reflectors in the mid-1700s. Mobile versions were used to observe the Transit of Venus . List of largest optical telescopes in the 18th century includes various refractors and reflectors that were active some time between about 1699 to 1801.
The Great Refractor was built in 1896 for the Great Industrial Exposition of Berlin. With an aperture of 68 cm (27 in), a focal length of 21 m (69 ft) and a movable mass of 130 metric tons, it is considered a masterpiece of technology. The lenses were made by C. A. Steinheil & Söhne. Since 1967, the telescope is a protected monument.
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