Operating optical and infrared telescopes, ranked by effective aperture in metres, as of October 10, 2026. The Large Binocular Telescope leads at 11.8 m combined, with the Gran Telescopio Canarias the largest single mirror at 10.4 m.
Pictured: the “Touring Arizona’s Giant Observatory” video, for the Large Binocular Telescope, #1 with an 11.8 m equivalent aperture.
| Rank | Telescope | Aperture (m) | Country | Site | Altitude (m) | First light | Operator | Type | Note |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Large Binocular Telescope (two 8.4 m mirrors, combined) | 11.8 | United States | Mount Graham, Arizona | 3,221 | 2005 | LBT Corporation (University of Arizona, INAF, German partners) | Ground | First light 2005 on one mirror; both mirrors together from 2008 |
| 2 | Gran Telescopio Canarias (GTC) | 10.4 | Spain | La Palma, Canary Islands | 2,267 | 2007 | Instituto de Astrofísica de Canarias | Ground | Largest single-mirror optical telescope; 36 segments |
| 3 | Keck I | 10.0 | United States | Mauna Kea, Hawaii | 4,145 | 1990 | W. M. Keck Observatory | Ground | First light with 9 of 36 segments; full mirror in 1993 |
| 3 | Keck II | 10.0 | United States | Mauna Kea, Hawaii | 4,145 | 1996 | W. M. Keck Observatory | Ground | Twin of Keck I |
| 5 | Hobby–Eberly Telescope (HET) | 9.2 | United States | Davis Mountains, Texas | 2,025 | 1996 | McDonald Observatory (UT Austin-led consortium) | Ground | 11 m mirror, 9.2 m effective; fixed elevation |
| 5 | Southern African Large Telescope (SALT) | 9.2 | South Africa | Sutherland, Northern Cape | 1,798 | 2005 | South African Astronomical Observatory / SALT Foundation | Ground | 11.1 × 9.8 m mirror, 9.2 m effective |
| 7 | Very Large Telescope (four 8.2 m Unit Telescopes) | 8.2 | Chile | Cerro Paranal | 2,635 | 1998 | European Southern Observatory (ESO) | Ground | Each of the four counts as 8.2 m; Antu first light 1998 |
| 7 | Subaru Telescope | 8.2 | United States | Mauna Kea, Hawaii | 4,139 | 1999 | NAOJ (Japan) | Ground | Engineering first light Dec 1998; science tests Jan 1999 |
| 9 | Gemini North | 8.1 | United States | Mauna Kea, Hawaii | 4,213 | 1999 | NOIRLab (International Gemini Observatory) | Ground | Twin of Gemini South |
| 9 | Gemini South | 8.1 | Chile | Cerro Pachón | 2,722 | 2000 | NOIRLab (International Gemini Observatory) | Ground | Altitude varies by source (2,722–2,737 m) |
| 11 | James Webb Space Telescope | 6.5 | Space (Sun–Earth L2) | L2 point, 1.5 million km from Earth | n/a (space) | 2022 | NASA / ESA / CSA | Space | Launched Dec 25, 2021; 18 segments |
| 11 | Magellan Baade | 6.5 | Chile | Las Campanas | 2,516 | 2000 | Carnegie Institution (Magellan consortium) | Ground | Twin of Clay |
| 11 | Magellan Clay | 6.5 | Chile | Las Campanas | 2,516 | 2002 | Carnegie Institution (Magellan consortium) | Ground | Twin of Baade |
| 11 | Vera C. Rubin Observatory (Simonyi Survey Telescope) | 6.5 | Chile | Cerro Pachón | 2,647 | 2025 | NSF–DOE (NOIRLab / SLAC) | Ground | 8.4 m mirror, about 6.5 m effective; survey began June 29, 2026 |
No matching rows.
As of October 10, 2026. Aperture is the effective, single-circular-mirror-equivalent diameter in metres as stated by the operator: the Large Binocular Telescope counts its two 8.4 m mirrors combined (11.8 m), HET and SALT count their effective 9.2 m rather than the full mirror span, and Rubin counts about 6.5 m rather than its 8.4 m mirror. The largest single mirror is the Gran Telescopio Canarias at 10.4 m. Telescopes with the same aperture share a rank, and the four Very Large Telescope units count as one row. Inclusion rule: operating optical or infrared telescopes only, trimmed to 14 rows; radio telescopes and smaller 6 m class instruments such as the MMT and BTA-6 are not listed, and Hubble (2.4 m) falls outside. Altitudes come from Wikipedia and observatory pages and differ by a few metres between sources;[1] first light is the year of the first light on sky, which for Keck I and the GTC used only part of the final mirror. Under construction, not ranked: ESO’s Extremely Large Telescope (39 m) has a telescope first light of March 2029 and scientific first light in December 2030, per ESO’s March 2025 schedule, the latest I could find; the Giant Magellan Telescope is in its final design phase and still needs federal funding; the Thirty Meter Telescope was left out of NSF’s final-design phase in 2025 and has no confirmed schedule.[12]
Two 8.4 m mirrors share one mount on Mount Graham, and together they collect as much light as a single 11.8 m circular mirror.[2] First light came in October 2005 using one mirror, and the first images with both mirrors working together followed in 2008.[2] Used in interferometric mode, its resolution is comparable to a 22.8 m telescope, which is the fun fact that sets it apart from every single-mirror giant.[2]
The GTC on La Palma is the largest single-mirror optical telescope, with a 10.4 m primary built from 36 hexagonal segments.[3] It cost about €130 million over seven years of construction; its first-light ceremony in 2007 took place with only 12 of the 36 segments installed, and scientific observations began in 2009.[3]
The twin Keck telescopes near the summit of Mauna Kea each have a 10 m mirror of 36 hexagonal segments.[4] Keck I saw first light on November 24, 1990 with just 9 segments, and Keck II followed in 1996; the pair were the largest optical telescopes in the world until 2006 and are now ranked third and fourth by their nominal diameter.[4] Their segmented design became the template for nearly every giant telescope since.[4]
The HET in West Texas has an 11 m mirror of 91 segments, but because it is fixed at one elevation and the instrument tracks across it, only about 9.2 m is used at a time.[5] It reached first light in December 1996, was dedicated in October 1997 and finished commissioning in October 1999, built far more cheaply than a conventional telescope of its size.[5] Other sources round its aperture to 10 m.
SALT is a close relative of the HET, with 91 hexagonal segments forming an 11.1 × 9.8 m mirror and a 9.2 m effective aperture.[6] It is permanently tilted 37 degrees from the zenith and turns only in azimuth, and press coverage calls it the largest optical telescope in the Southern Hemisphere.[6] First light was September 1, 2005 at Sutherland, 1,798 m up on the edge of the Kalahari.[6]
ESO’s flagship on Cerro Paranal has four 8.2 m Unit Telescopes, named Antu, Kueyen, Melipal and Yepun after Mapuche words for sky objects.[7] Antu saw first light on May 25, 1998, and the telescopes can also be combined as an interferometer to see details up to 25 times finer than any one of them.[7]
Japan’s 8.2 m telescope sits at 4,139 m on Mauna Kea and is run by NAOJ.[8] Engineering first light was on December 24, 1998, test observations followed in January 1999, it was inaugurated in September 1999, and it opened to Japanese and international astronomers from December 2000.[8] The name is the Japanese word for the Pleiades star cluster. No suitable verified video was found, so the icon is used.
The two twin 8.1 m telescopes, on Mauna Kea in Hawaii and Cerro Pachón in Chile, give one observatory a view of both hemispheres.[9] Gemini North saw first light in 1999 and Gemini South in 2000.[9] The video shows Gemini North.
Webb’s 6.5 m mirror is 18 gold-plated beryllium segments that unfolded in space, with a collecting area of about 25 square meters, roughly six times Hubble’s.[10] It launched on December 25, 2021 and works near the Sun–Earth L2 point; it is the only space telescope in the table.[10] Counting collecting area rather than the mirror’s widest span, its effective diameter is smaller than 6.5 m.
Rubin’s 8.4 m mirror is a single piece combining primary and tertiary surfaces, with about 6.5 m effective aperture once the central part is accounted for, according to a 2026 Rubin paper (a 2024 deck gave 6.7 m).[11] First test observations came in April 2025, the first public images on June 23, 2025, and the 10-year Legacy Survey of Space and Time began on June 29, 2026.[11]