List of largest optical telescopes in the 20th century

The following is a list of the largest optical telescopes in the 20th century, paying special attention to the diameter of the mirror or lens of the telescope's objective, or aperture. Aperture rank currently goes approximately by the usable physical aperture size and not by aperture synthesis, although interferometers attained some of the highest angular resolutions at visible and infrared wavelengths compared to traditional telescopes. Diverging methods of construction and use for reflecting telescopes in that area make comparing synthesized aperture irregular.

For instance, Keck I or II alone has less angular resolution than the Keck Interferometer (Keck I & II together), however, the Keck Interferometer is used for a much narrower range of type of observations. Ultimately, a valid comparison between two telescopes must take into consideration more specifications, when a general measurement becomes obtuse.

Aperture of the primary mirror alone can be poor measure of a reflective telescope's significance; for example, the Hubble Space Telescope has only a primary mirror. In addition, many large or significant telescopes are not optical and/or reflecting. However, many famous optical telescopes have had large apertures on their primary mirror with corresponding good angular resolution.

The list includes optical observatories, including UV, visible and some optical infrared telescopes, and near infrared. The list covers from about 1901 to 2001, with some flexibility to accommodate ambiguity in classification, for example some 2002 telescopes, if it was nearly operation in 2001. 19th century and earlier telescopes that ceased operation are not included, but the list is not complete.

Table of optical telescopes Multiple mirror telescopes are ranked by their equivalent optical area, not peak interferometric aperture unless it is not relevant for the design. See also List of astronomical interferometers at visible and infrared wavelengths.

See also List of largest optical refracting telescopes, as this list does yet not include such large refractors as the Yerkes Observatory.

Name/Observatory Aperture m Aper. in Mirror type Nationality of Sponsors Site and/or Observatory FL or Built Image Keck 1 10 m 394″ Segmented, 36 United States Mauna Kea Observatory, Hawaii 1993 Keck 2 10 m 394″ Segmented, 36 United States Mauna Kea Observatory, Hawaii 1996 Hobby–Eberly Telescope (HET) 9.2 m 362″ Segmented,91 United States, Germany McDonald Observatory, Texas 1997 Subaru (JNLT) 8.2 m 323″ Single Japan Mauna Kea Observatory, Hawaii 1999 VLT 1 (Antu) 8.2 m 323″ Single ESO Countries + Chile Paranal Observatory, Chile 1998 VLT 2 (Kueyen) 8.2 m 323″ Single ESO Countries + Chile Paranal Observatory, Chile 1999 VLT 3 (Melipal) 8.2 m 323″ Single ESO Countries + Chile Paranal Observatory, Chile 2000* VLT 4 (Yepun) 8.2 m 323″ Single ESO Countries + Chile Paranal Observatory, Chile 2001* Gemini North (Gillett) 8.1 m 318″ Single United States, Canada, Chile, Australia, Argentina, Brazil Mauna Kea Obs., Hawaii 1999 Gemini South 8.1 m 318″ Single United States, Canada, Chile, Australia, Argentina, Brazil Cerro Pachón, Chile 2001 MMT 6.5 m 256″ Single United States F. L. Whipple Obs., Arizona 2000 Magellan 1 (Walter Baade) 6.5 m 256″ Honeycomb United States Las Campanas Obs., Chile 2000 Magellan 2 (Landon Clay) 6.5 m 256″ Honeycomb United States Las Campanas Obs., Chile 2002* BTA-6 6 m 238″ Single USSR + Russia Zelenchukskaya, Caucasus 1976 Hale Telescope (200 inch) 5.08 m 200″ Single United States Palomar Observatory, California 1948 MMT (original) (6 × 1.8 m) optics replaced 4.7 m (6 × 1.8 m) 186″ 6 mirrors United States F. L. Whipple Obs., Arizona 1979–1998 William Herschel Telescope 4.2 m 165″ Single UK, Netherlands, Spain ORM, Canary Islands 1987 SOAR 4.1 m 161″ Single United States, Brazil Cerro Pachón, Chile 2002* Nicholas U. Mayall 4m 4 m 158″ Single USA Kitt Peak National Obs.; Arizona 1973 Victor M. Blanco Telescope 4 m 158″ Single USA Cerro Tololo Inter-American Obs., Chile 1976 Anglo-Australian Telescope (AAT) 3.89 m 154″ Single United Kingdom + Australia Anglo-Australian Obs.; Siding Spring, Australia 1975 United Kingdom Infrared Telescope (UKIRT) 3.8 m 150″ Single IR United Kingdom Mauna Kea Observatories; Mauna Kea, Hawaii 1978 3.67m AEOS Telescope (AEOS) 3.67 m 145″ Single United States Air Force Maui Optical Station; Haleakala, Hawaii 1996 Telescopio Nazionale Galileo (TNG) 3.58 m 138″ Single Italy ORM; La Palma, Canary Islands 1997 New Technology Telescope (NTT)3.58 m 142″ Single ESO countries European Southern Observatory; Cerro La Silla, Chile 1989 Canada-France-Hawaii Telescope (CFHT) 3.58 m 141″ Single Canada, France, United States Mauna Kea Observatories, USA 1979 ESO 3.6 m Telescope 3.57 m 140″ Single ESO countries European Southern Observatory; Cerro La Silla, Chile 1977 MPI-CAHA 3.5mmpia.de "25 Years of Calar Alto Observatory