Refracting telescope 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 an ...

s use a

lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...

to focus light. The largest refracting telescope in the world is the

Yerkes Observatory Yerkes Observatory ( ) is an astronomical observatory located in Williams Bay, Wisconsin, United States. The observatory was operated by the University of Chicago Department of Astronomy and Astrophysics from its founding in 1897 to 2018. Owner ...

40 inch (102 cm) refractor, used for astronomical and scientific observation for over a century. The Swedish 1-m Solar Telescope, which has an actual lens diameter of 43 inches, is technically then larger than the lens of the Yerkes, but only 39 inches are clear for the aperture, and is used today for solar observations .The next largest refractor telescopes are the James Lick telescope, and the

Meudon Great Refractor Meudon Great Refractor (also known as the ''Grande Lunette'') is a double telescope with lenses (83 cm + 62 cm), in Meudon, France. It is a twin refracting telescope built in 1891, with one visual and one photographic, on a single square ...

. Most are classical

great refractors 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 defines an era in modern telescopy in the 19t ...

, which used

achromatic doublet An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus on the same plane. The most comm ...

s on an equatorial mount. However, other large refractors include a 21st-century solar telescope which is not directly comparable because it uses a single element non-achromatic lens, and the short-lived Great Paris Exhibition Telescope of 1900. It used a 78-inch (200 cm) Focault siderostat for aiming light into the

Image-forming optical system In optics, an image-forming optical system is a system capable of being used for imaging. The diameter of the aperture of the main objective is a common criterion for comparison among optical systems, such as large telescopes. The two traditiona ...

part of the telescope, which had a 125 cm diameter lens. Using a siderostat incurs a reflective loss. Larger meniscus lenses have been used in later

catadioptric A catadioptric optical system is one where refraction and reflection are combined in an optical system, usually via lenses (dioptrics) and curved mirrors ( catoptrics). Catadioptric combinations are used in focusing systems such as searchlights, ...

telescopes which mix refractors and reflectors in the image-forming part of the telescope. As with reflecting telescopes, there was an ongoing struggle to balance cost with size, quality, and usefulness. This list includes some additional examples, such as the Great Paris telescope, which also used a mirror, and some solar telescopes which may have more complicated optical configurations. The SST has an optical aperture of 98 cm (39.37"), although the lens itself is 110 cm (43.31"). It is a single element lens whereas most of this list are doublets, with a crown and flint lens elements.

Under construction

See also

References

Further reading