Human interest in space exploration leads to the development of advanced ground-based telescopes, which only escalated during the late 20th century. The ground-based telescopes, as you may know, have limited use since they can only observe a small section of the electromagnetic spectrum (optical), and that’s why we have space-based telescopes.
However, unlike space telescopes, the ground-based ones can be made in sheer size. For example, the primary mirror of the largest space-based telescope (currently in construction), the James Webb telescope is 6.5-meters – only 60% of the largest operational ground-based telescopes.
Below, we have compiled a list of the twelve largest telescopes in the world. The list includes both operational and planned telescopes, sorted by their effective aperture (light gathering limit of an optical instrument).
Diameter: 6.5 Meters
Location: Mount Hopkins, Arizona, The United States
MMT (formerly Multi-Mirror Telescope) is part of the Fred Lawrence Whipple Observatory situated on Mount Hopkins, Arizona. Its original name, Multi-Mirror Telescope, was actually inspired by the six small honeycombs-styled mirrors which were once used for light gathering. The current one is a single piece primary mirror and was installed in 1999.
The telescope introduced a few groundbreaking changes in the field. Its adaptive optics system influenced the revolutionary design of the Large Binocular Telescope. Apart from optics, the telescope was able to obtain improved results in infrared studies by eliminating almost every possible warm surface from its light path.
11. Gemini Observatory Telescopes
Gemini North Telescope
Diameter: 8.1 Meters
Location: Mauna Kea, Hawaii and Cerro Pachon, Chile
The Gemini Observatory, owned and maintained by five major research organizations from different nations, consists of two identical telescopes that are located in two separate locations. Both the telescopes can operate in infrared wavelengths with the help of wide-field adaptive optics technology.
One of its instruments, the Gemini Planet Imager (GPI), basically a high contrast spectrometer, allows the telescopes to take images of exoplanets revolving around extremely bright stars. The GPI successfully discovered 51 Eridani b, which is understood to be a million times fainter than its parent 51 Eridani.
10. Very Large Telescope
The Very Large Telescope | Image Courtesy: European Southern Observatory
Diameter: 8.2 Meters
Location: Atacama Desert, Chile
The Very Large Telescope (VLT for short) is perhaps one of the most popular telescope facility in the world. VLT is actually comprised of four independent telescopes, all having a single 8.2 m primary mirror. They can either be used separately or as a unit to attain much higher angular resolution.
The telescope(s) can operate in both visual and infrared wavelengths. All four telescopes are linked with advanced interferometric instruments (VLTI), which allow researchers to study bright astronomical objects, including stars and nebulas, through Interferometry.
After NASA’s Hubble Space Telescope, VLT is perhaps the most productive research facility (operating in visual wavelength) in terms of total peer-reviewed papers published so far. In 2017, more than 600 published scientific papers were based on data provided by the VLT/VLTI.
It became the first telescope to take a direct image of an exoplanet (Beta Pictoris b). VLT is one of the few observatories tracking stars revolving around the supermassive black hole at the center of the Milky Way.
9. Subaru Telescope
Diameter: 8.4 Meters
Location: Mauna Kea, Hawaii, The United States
The Subaru Telescope, located at the famous Mauna Kea Observatory, is operated and controlled by the National Astronomical Observatory of Japan. It’s named after a popular open star cluster ‘Pleiades.’
It’s a single mirror-type telescope, almost identical to Gemini Telescopes, which are slightly larger. A number of state-of-the-art technologies, including Multi-Object Infrared Camera and Spectrograph (MOIRCS) and Cooled Mid Infrared Camera and Spectrometer (COMICS), allow astronomers to investigate multiple targets at once, including the cool interstellar dust.
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO), an advanced high contrast imaging system is capable of taking direct images of exoplanets.
The Subaru Telescope is one of the few operational telescopes to have been used with an unaided eye. Due to its large field of view and remarkable light-gathering power, Subaru is mostly used for deep wide-field surveys. For similar reasons, Subaru is also used for hunting predicted planet nine in our solar system.
8. Large Binocular Telescope
The Large Binocular Telescope | Image Courtesy: NASA
Diameter: 8.4 Meters
Location: Pinaleno Mountains, Arizona, The United States
The Large Binocular Telescope (LBT) is a unique optical telescope that has two identical 8.4 m wide primary mirrors, with a combined circular aperture of 11.8 m.
Theoretically, it’s greater than any single telescope in operation today, but since LBT collects light at a much lower diffraction limit, it cannot be seen in the same regard. Nevertheless, it’s currently the largest non-segmented telescope in the world.
LBT’s rather unique design combined with light-adaptive optics allows it to reduce atmospheric phase errors, have a low thermal background, high angular resolution, and high sensitivity to detect faint, distant objects.
Back in 2008, LBT, along with a space-based telescope, successfully discovered a distant galaxy cluster designated as 2XMM J083026+524133, located about 6 billion light-years away from the Earth.
7. Southern African Large Telescope
Image Courtesy: SALT
Diameter: 9.2 Meters
Location: Sutherland, South Africa
The South African Large Telescope (SALT), at the moment, is the largest optical telescope in the southern hemisphere. It features an unusual mirror design, which is fixed at a 37° angle and is based on the Hobby-Eberly Telescope (at McDonald Observatory). A fixed zenith angle allows the telescope to access a large section of the sky. Its primary mirror is composed of 91 hexagonal segments.
Its location makes it possible for researchers to conduct spectroscopic and polarimetric analyses of astronomical objects which cannot be seen from the northern hemisphere. Over the course of the next few years, the SALT will focus on distant quasars and faint galaxies
6. Keck 1 and 2
The W.M Keck Observatory | Image Courtesy: NASA
Diameter: 10 Meters
Location: Mauna Kea, Hawaii, The United States
The famous twin telescope of W.M Keck Observatory situated on Mauna Kea is among the most advanced telescopes in the world. The primary mirrors on both telescopes are 10 meters wide and are composed of 36 hexagonal segments.
They are equipped with state-of-the-art instruments, including a laser guide star adaptive optics. One of its instruments, the Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) can gather light from more than 130 galaxies in one single exposure.
Another instrument, the Near Infrared Camera (NIRC), is so sensitive it could technically detect a tiny flame on the Moon’s surface. This allows Keck telescopes to gather data from distant galaxies/protogalaxies, quasars in order to study their formation and evolution.
5. Hobby-Eberly Telescope
Diameter: 10 meters
Location: Davis Mountain, Texas, The United States
Located at renowned McDonald Observatory in Texas, the Hobby-Eberly Telescope (HET) is currently the second-largest optical telescope in the world with a usable optical aperture of 10 meters (its actual diameter is 11 m). Like most other large telescopes, HET’s primary mirror is made up of multiple small hexagonal segments, 91 to be exact.
HET is mostly used to detect/study distant galaxies and various stellar objects through spectroscopy. Over the years, the telescope has been able to detect a number of extrasolar planets and successfully calculate the rotation speed of few galaxies.
Unlike many telescopes, HET’s main mirror is fixed at an angle of 55° (can rotate around its base). This allows the telescope to have access to about 70-81% of the night sky.
The facility is named after the former lieutenant governor of Texas, Bill Hobby, and a distinguished Penn State alumnus Robert E. Eberly.
4. Gran Telescopio Canarias
Gran Telescopio Canarias in La Palma
Diameter: 10.4 meters
Location: La Palma, Canary Islands, Spain
The Gran Telescopio Canarias (GranTeCan) is perhaps the largest segmented primary mirror telescope in operation today. The entire GranTeCan project is supported by universities and institutes from more than one nation and is led by a Spanish astrophysical research institute IAC.
During its initial trial phase, the telescope was launched with just 12 hexagonal segments but was increased to 36 segments fully powered by an adaptive control system.
It features three main imaging instruments; MEGARA, a multi-wavelength spectrograph, CanariCam, an advanced mid-infrared imager with polarimetric capabilities, and OSIRIS, a low resolution integrated spectroscopy. The telescope became fully operational in 2009 and cost around €130 million.
Telescopes Currently Under Construction
3. The Giant Magellan Telescope
Artist’s concept of Giant Magellan Telescope | Image Courtesy: GMTO Corporation
Diameter: 24.5 m
Location: Vallenar, Chile
Estimated Completion: 2025
At the moment, there are about a dozen extremely large telescopes under construction, and the Giant Magellan Telescope is one of them.
It will eventually have seven identical 8.4 m wide segments forming the primary mirror. However, it will begin with just four. These segments are going to be arranged in a symmetrical manner with one at the center.
The telescope is expected to achieve an image resolving power near ten times greater than that of the Hubble Space Telescope. The entire project is expected to cost somewhere around $1 billion.
2. Thirty Meter Telescope
Artist’s conception of Thirty Meter Telescope | Image Courtesy: TMT.org
Diameter: 30 Meters
Location: Mauna Kea, Hawaii
Estimated Completion: 2027
The Thirty Meter Telescope (TMT) is a highly ambitious astronomical telescope project including a 30 meter wide segmented primary mirror and two smaller, subsequent mirrors to extend its overall capacity. Once completed, it will possibly be the second-largest telescope in the world.
The telescope is designed to operate from near-ultraviolet to mid-infrared wavelengths and will feature a Multi-Conjugate Adaptive Optics system, which will allow researchers to observe astronomical objects without most of the atmospheric disturbances.
The project is carried out by a number of international private and government research institutes, including Caltech and National Astronomical Observatory of Japan.
The project’s location has caused some serious socio-political unrest all across Hawaii. At the moment, Mauna Kea hosts 13 different observatories occupying more than 500 acres of protected land (which have cultural significance among locals).
1. European Extremely Large Telescope
The proposed 5-mirror optical system of ELT | Image Courtesy: European Southern Observatory
Diameter: 39.3 Meters
Location: Cerro Armazones, Chile
Estimated Completion: 2024
If all goes as planned, the European Extremely Large Telescope (ELT) will be the largest telescope in the world by 2024. It will be able to gather 13 times more light than any other optical telescope existing today, and the resulting images would be 16 times sharper than those captured by the Hubble Space Telescope.
Apart from the giant 39-meter primary mirror (composed of 798 hexagonal segments), the telescope will use four additional mirrors to improve image quality and adaptive optics. The ELT will search for distant extrasolar planets, analyze the supermassive black holes, earliest galaxies in the universe with more depth and accuracy.
Its advanced set of instruments will allow astronomers to detect organic molecules and water near young stars, which will help them explore more about planetary evolution. The first phase of the telescope is likely to cost around €1 billion.