For centuries, optical telescopes have allowed us to peer into the abyssal depths of space and develop modern astronomy beyond our wildest dreams. But while they come in many sizes, the biggest ground-based telescopes can't function without occupying areas so vast that only the most remote places on Earth will do.
While technical problems pose a challenge to the construction of bigger telescopes, the cost of building colossal telescopes has yet to outweigh the value of driving forward our expanding knowledge of the universe.
Quick tour of the world's biggest optical telescopes
Of course, a quick list of the world's biggest optical telescopes is far from exhaustive — but this one is sorted according to effective aperture size, in decreasing order. We'll consider only ground-based, optical telescopes.
1. The Large Binocular Telescope (LBT) leaves a deep impression
The Large Binocular Telescope at the Mount Graham International Observatory in the state of Arizona is one of the world's biggest and best telescopes. First active in the mid-2000s, it has an effective aperture of 11.9 meters.
Located at an altitude of 3,200 meters (roughly 10,500 feet), the telescope consists of two identical 8.4-meter (27.5-foot) telescopes mounted side-by-side.
"LBTO has a dual mission: (1) to be the first Extremely Large Telescope (ELT), experimenting with new technologies that will be used for the ELT generation now currently under construction by enabling 23m-aperture class science and (2) to be one of the leading 8-m class telescopes," according to the LBTO website.
2. The Gran Telescopio Canarias (GTC) is a daunting telescopic masterpiece
Another of the world's biggest telescopes is the Gran Telescopio Canarias on the Canary Islands, in Spain. The GTC first went active in the late-2000s and has an effective aperture of 10.4 meters.
Jointly-funded by Spain, Mexico, and the United States of America, the GTC has a segmented mirror of 36 hexagonal pieces.
"The ultimate aim of the GTC is to facilitate world-class science observations. Being the largest telescope in the world and thanks to its location at the Roque de Los Muchachos Observatory, the telescope will allow the study of key questions in astrophysics such as the nature of black holes, the formation history of stars and galaxies in the early universe, the physics of distant planets around other stars, and the nature of dark matter and dark energy in the universe," according to the GTC website.
3. The Hobby-Eberly Telescope (HET), an enigmatic telescope
Yet another of the world's biggest and best telescopes is the Hobby-Eberly Telescope (HET). Situated at McDonald Observatory in Texas, USA, it has an effective aperture of 1`0 meters (32.8 feet).
First opened in the late-1990s, the HET has an equally giant 11-meter mirror.
"It was designed specifically for spectroscopy, the decoding of light from stars and galaxies to study their properties. This makes it ideal in searching for planets around other stars, studying distant galaxies, exploding stars, black holes, and more. The telescope is especially suited to conduct large survey projects using spectroscopy," according to the McDonald Observatory website.
4. Keck 1, 2 are among the world's largest telescopes
Keck 1 and Keck 2 are also among the world's biggest and best telescopes. Each having an effective aperture of 10 meters (32.8 feet), they are both located at Mauna Kea Observatories in the U.S. state of Hawaii.
Located at an elevation of 4,145 meters (roughly 13,600 feet), they are officially recognized as the third- and fourth-largest telescopes in the world, respectively. Keck 1 was dedicated first in 1993, with Keck 2 following shortly after, in 1996.
5. The South-African Large Telescope is an engineering marvel
The Southern African Large Telescope (SALT) located at the South African Astronomical Observatory, Northern Cape, South Africa is also among the world's largest telescopes. With an effective aperture of 9.2 meters (roughly 30.2 feet), its mirror consists of more than 90 segmented hexagonal segments.
"SALT is funded by a consortium of international partners from South Africa, the United States, Germany, Poland, India, the United Kingdom, and New Zealand. The telescope has been in full science operation since 2011 and is realizing its huge potential as Africa’s Giant Eye on the Universe," according to the SALT website.
6. The Japanese-funded Subaru telescope
Subaru (JNLT), located at the Mauna Kea Observatories in Hawaii, is also one of the world's biggest telescopes. It was first dedicated in 1999 and was primarily funded by Japan.
With an effective aperture of 8.2 meters (roughly 26.9 feet), its mirror comprises a single piece of polished metal rather than segmented like previous telescopes on this list. Like Keck 1 and 2, Mauna Kea, Hawaii was an ideal location for the telescope.
"The air pressure upon Maunakea is only two-third of what it is at the sea level. Clouds typically form below the summit where an inversion layer keeps the clouds from rising to the summit. Because Hawaii is isolated from any other landmass, trade winds blow smoothly over the islands, and there are few cities to pollute its dark skies," according to the Subaru website.
7. Very Large Telescope Array UT 1 through 4
Located at the Paranal Observatory in the Antofagasta Region, Chile, are some of the world's biggest telescopes. Called the VLT (Very Large Telescope Array) UT 1, 2, 3, and 4, each one has an effective aperture of 8.2 meters (roughly 26.9 feet).
Dedicated consecutively between the late 1990s and early 2000s, these telescopes each make use of a single mirror. Each telescope can work either independently or jointly, the latter joint mode gives the VLT the same total light-collecting power of a 16-meter (52.4-foot) telescope.
Of the VLT, the European Space Observatory (who funded their construction) said: "[T]he telescopes may also be used in interferometric mode providing high-resolution imaging. The useful wavelength range extends from the near UV up to 25 µm in the infrared."
8. Gemini North and South telescopes also rank high in telescope size
Gemini North and South telescopes — located at the Mauna Kea Observatories in Hawaii, and Cerro Pachón (CTIO), Coquimbo Region, Chile — also rank among the largest telescopes in the world. Each has an effective aperture of 8.1 meters (roughly 26.6 feet), and both were funded by multi-national donors.
"Both of the Gemini telescopes have been designed to excel in a wide variety of optical and infrared capabilities. By incorporating technologies such as laser guide star adaptive optics and multi-object spectroscopy, astronomers in the Gemini partnership explore the universe in unprecedented depth and detail," according to NASA's Gemini website.
9. The Multiple Mirror Telescope Observatory in Arizona
And finally, the Multiple Mirror Telescope Observatory (MMT) is a distinct last-but-not-least addition to our list of the world's best and biggest telescopes. With an effective aperture of 6.5 meters (21.3 feet), MMTO was first dedicated in the early 2000s.
A joint venture between the Smithsonian Institution and the University of Arizona, MMT is located on the summit of Mt. Hopkins in southeastern Arizona.
The Encyclopedia Brittanica says the MMT gained its name in 1979 from function: it combines light collected via six 180-centimeter- (70-inch-) diameter telescopes into one, single image.
More recent innovations and enhancements in mirror design have enabled the production of larger, higher-quality mirrors. This is why the original MMT's mirror was eventually replaced with a single 6.5-meter (21.3-foot) mirror in the late 1990s.
After the refit, the MMT was renamed the MMT Observatory.
The 2020s will see the launch of the most advanced space telescopes ever, but also some of the largest (like the James Webb Space Telescope). So while we're impressed with the biggest Earth-based telescopes — and their delightful environs — it's also important to note that the largest available space for basing telescopes will always be outer-space itself.