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QuotesNov 12

Observatories you can visit

The big telescopes of the world, and the smaller ones near you, that let the public in: what is there, how to visit, and a link to each one's own website. 99 observatories in 27 countries, searchable by country and state. Start with the three record holders, or jump to the search.

The big three, oldest first

The largest refractor, the largest single-mirror reflector, and the largest space telescope. The three drawings use one scale; the person is 1.8 m (5 ft 11 in) tall.

1 · LARGEST REFRACTOR · 1897

Yerkes Observatory

Williams Bay, Wisconsin, a 50-acre campus on Geneva Lake · 334 m above sea level

The 40-inch refracting telescope at Yerkes Observatory, a long white tube on a tall mount under the dome.
Photo: user Kb9vrg (the picture's source, opens in a new tab), Public domain
40-inch (1 m) lensabout 19 m (63 ft) from lens to eyepieceeyepiece end
  • How it started: in 1892 astronomer George Ellery Hale learned that two 40-inch glass lens blanks were sitting unused. University of Chicago president William Rainey Harper backed him, and Chicago businessman Charles T. Yerkes paid. It was dedicated on October 21, 1897.
  • The 40-inch: the largest refracting telescope ever used for astronomy. The Alvan Clark & Sons lens weighs 225 kg and it took 18 tries to cast the glass. It focuses light 19.3 m (63 ft) behind the lens.
  • Why it matters: often called the birthplace of modern astrophysics. The Astrophysical Journal was edited here until the 1960s, and a meeting of astronomers here in October 1897 led to the American Astronomical Society.
  • Today: the University of Chicago ran it for 121 years and closed it on October 1, 2018. The Yerkes Future Foundation, a nonprofit started by local residents, took ownership in 2020, restored it and reopened it in 2022.
Who worked here and what they found
  • 1897George Ellery Hale opens Yerkes. He invented the spectroheliograph for photographing the Sun, then founded Mount Wilson Observatory and planned Palomar's 200-inch.
  • 1900sE. E. Barnard photographs the Milky Way; his niece and assistant Mary Ross Calvert worked as a computer and astrophotographer from 1905 to 1946. S. W. Burnham, a court reporter by day, is credited with 1,340 double stars.
  • 1914-17Edwin Hubble is a graduate student here. Later he showed there are other galaxies and that the universe is expanding.
  • 1921Otto Struve arrives, a refugee from the Russian Revolution; he later became director. Albert Einstein visits in May, on his first trip to the United States.
  • 1936-50Subrahmanyan Chandrasekhar works here; Nobel Prize in Physics, 1983. NASA's Chandra X-ray Observatory is named for him.
  • 1943William W. Morgan, Philip Keenan and Edith Kellman begin the MKK system for sorting stars by their spectra, still used today.
  • 1944Gerard Kuiper finds that Saturn's moon Titan has an atmosphere. In 1951 he proposed the icy belt beyond Neptune now called the Kuiper Belt.
  • 1946-54Nancy Grace Roman studies here; she became NASA's first Chief of Astronomy.
  • 1951Morgan, with Donald Osterbrock and Stewart Sharpless, discovers the Milky Way's spiral arms.
  • 1950sMargaret Burbidge helps show in 1957 that most heavy elements are made inside stars. Carl Sagan is a graduate student here, 1956-60.
Einstein, Pluto and a movie
Albert Einstein with the staff of Yerkes Observatory in front of the 40-inch refractor, May 1921.
Photo: Photo Credit: Yerkes Observatory (the picture's source, opens in a new tab), Public domain

1921: Albert Einstein visits. He was photographed in front of the 40-inch. His theory predicted that the Sun's gravity bends starlight, and eclipse photographs in 1919 had just shown that it does. Telescopes were how his ideas got tested.

Pluto, from old glass to New Horizons. In 1909 a Yerkes photograph caught Pluto, 21 years before Clyde Tombaugh discovered it at Lowell Observatory; nobody noticed. Nearly a century later NASA borrowed glass plates from the Yerkes archive to help pin down Pluto's path for the New Horizons mission, which flew past Pluto on July 14, 2015.

Lights, camera, Yerkes. Scenes for the 1996 movie Chain Reaction, with Keanu Reeves, Morgan Freeman and Rachel Weisz, were filmed here.

2 · LARGEST SINGLE-MIRROR REFLECTOR · 2009

Gran Telescopio Canarias (GTC)

Roque de los Muchachos Observatory, La Palma, Canary Islands, Spain · about 2,300 m

Inside the dome of the Gran Telescopio Canarias on La Palma: the telescope's steel frame over the mirror.
Photo: H. Raab ( User:Vesta ) (the picture's source, opens in a new tab), CC BY-SA 3.0
10.4 m mirror,36 segments5 m
  • The mirror: 10.4 m across, made of 36 hexagonal segments that computer-controlled actuators keep aligned (active optics).
  • Timeline: first light July 13, 2007 with 12 segments; science from May 2009; opened by King Juan Carlos I of Spain on July 24, 2009.
  • Problem it solves: the bigger the mirror, the fainter and farther the objects it can study. La Palma is one of the best observing sites in the Northern Hemisphere.
  • Close rivals: the twin Keck telescopes in Hawaii (10 m each, also 36 segments). The Extremely Large Telescope (39 m, 798 segments) is being built in Chile.

3 · LARGEST SPACE TELESCOPE · 2021

James Webb Space Telescope

In orbit around the Sun-Earth L2 point, about 1.5 million km from Earth

The James Webb Space Telescope's gold primary mirror, 18 hexagonal segments, standing in the clean room at NASA Goddard.
Photo: NASA's James Webb Space Telescope from Greenbelt, MD, USA (the picture's source, opens in a new tab), Public domain
sunshield 21 by 14 m (tennis-court size)6.5 m gold mirror, 18 segments5 m
  • The mirror: 6.5 m across, 18 hexagonal beryllium segments coated with a thin layer of gold. It is 2.7 times wider than Hubble's mirror.
  • The sunshield: 21.2 m by 14.2 m, the size of a tennis court: five layers of aluminum-coated film that keep the telescope cold enough to see infrared.
  • Launch: December 25, 2021, on an Ariane 5 rocket from Kourou, French Guiana. A NASA, European and Canadian mission.
  • Problem it solves: it sees infrared light, which passes through dust clouds and carries the light of the very first galaxies, stretched into infrared by the expanding universe.
  • Visit: you can't, it is 1.5 million km away. Its pictures are free at science.nasa.gov/mission/webb.

FIND AN OBSERVATORY

All 99 observatories.

Websites and visiting notes were checked on Nov 12, 2026. Hours, tickets and tours change, so open the observatory's own page before you travel.

TAKE IT WITH YOU

PDF

Telescope travel guide. The three biggest telescopes drawn to one scale, and the major observatories of the world. 6 pages, 280 KB.

New to telescopes? Telescopes: which kind does what explains the three main kinds and what radio, infrared and X-ray telescopes are for. Working on the Astronomy merit badge? A visit to an observatory or planetarium is requirement 8a; the Scouts page has the rest.

The first 27 observatories come from the telescope travel guide, whose facts were checked against Wikipedia, NASA and the observatories' own sites; the rest were added for the search. Each official website and visiting note was confirmed by web search on Nov 12, 2026; a note that says something was not confirmed means exactly that. The photographs are real: NASA's own, or Wikimedia Commons files under a free licence, each credited under the picture with a link to its source. SpaceDew is not connected to any observatory listed. Nothing you type in the search leaves this page.

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