The biggest eyes on the sky: the ten largest operational ground-based optical telescopes, ranked by effective primary mirror aperture.
Quick answer: 1. Gran Telescopio Canarias (GTC), 2. Keck I, 3. Keck II, 4. Hobby-Eberly Telescope (HET), 5. Southern African Large Telescope (SALT), 6. Large Binocular Telescope (LBT), 7. Very Large Telescope Unit Telescope (VLT UT), 8. Subaru Telescope, 9. Gemini North, 10. Gemini South.
A telescope's primary mirror is its light bucket — the wider it is, the fainter and more distant the objects it can see. This list ranks the ten largest currently operational ground-based optical and infrared telescopes by the effective diameter of that mirror, from the 10.4-meter Gran Telescopio Canarias down to the twin Gemini telescopes at 8.1 meters. It's built for astronomy enthusiasts, students, and anyone curious about where the world's most powerful ground telescopes live and how they compare. Note that the next generation of "extremely large" telescopes now under construction — the ELT, GMT, and TMT — will dwarf everything here, but none has yet seen first light, so they're excluded.
The world's largest single-aperture optical telescope. Its 36-hexagon segmented mirror is rated at a full 10.4 m effective aperture, edging out the twin Kecks it was modeled on.
The first of the twin Keck telescopes and the instrument that proved segmented mirrors could scale past 8 m. Its 36-hexagon mirror spans 10 m.
The second Keck telescope, with a mirror identical to its twin. Paired, the two can work together as an interferometer.
Its 91-segment spherical mirror measures 11 m across, but the fixed-altitude design uses only part of it at a time; after a 2015 upgrade this gives a roughly 10 m effective aperture. A specialist survey instrument built for spectroscopy on a budget.
A near-twin of the Hobby-Eberly design and the largest optical telescope in the Southern Hemisphere. Its 91-segment mirror measures 11.1 by 9.8 m, for a 9.2 m effective aperture.
Two 8.4 m monolithic mirrors ride on a single mount, together matching the light-gathering power of an 11.8 m dish. Ranked here by its individual mirror diameter — the largest single monolithic mirrors of any working telescope.
Each of ESO's four Unit Telescopes carries an 8.2 m monolithic mirror; combined via interferometry the array reaches the resolving power of a 16 m instrument. Listed here by a single unit's mirror diameter. Its first unit saw first light in 1998, ahead of Subaru.
A single monolithic 8.2 m mirror cast as one piece of glass, the flagship of the National Astronomical Observatory of Japan. It shares the aperture of the VLT units but saw first light a year later, in 1999.
One half of the twin-hemisphere Gemini Observatory, its 8.1 m monolithic mirror serves the Northern Hemisphere from Mauna Kea.
The southern twin of the Gemini Observatory, with an identical 8.1 m mirror. Together the two Geminis give astronomers full-sky coverage.
| # | Name | mirror_type | location | first_light |
|---|---|---|---|---|
| 1 | Gran Telescopio Canarias (GTC) | Segmented (36 hexagons) | La Palma, Canary Islands, Spain | 2007 |
| 2 | Keck I | Segmented (36 hexagons) | Mauna Kea, Hawaii, USA | 1993 |
| 3 | Keck II | Segmented (36 hexagons) | Mauna Kea, Hawaii, USA | 1996 |
| 4 | Hobby-Eberly Telescope (HET) | Segmented (91 hexagons) | Mount Fowlkes, Texas, USA | 1996 |
| 5 | Southern African Large Telescope (SALT) | Segmented (91 hexagons) | Sutherland, Karoo, South Africa | 2005 |
| 6 | Large Binocular Telescope (LBT) | Two 8.4 m monolithic | Mount Graham, Arizona, USA | 2005 |
| 7 | Very Large Telescope Unit Telescope (VLT UT) | Monolithic (single) | Cerro Paranal, Atacama, Chile | 1998 |
| 8 | Subaru Telescope | Monolithic (single) | Mauna Kea, Hawaii, USA | 1999 |
| 9 | Gemini North | Monolithic (single) | Mauna Kea, Hawaii, USA | 1999 |
| 10 | Gemini South | Monolithic (single) | Cerro Pachon, Chile | 2000 |
Ranked by effective primary mirror aperture — the equivalent single-mirror light-collecting diameter, in meters — for telescopes that are currently operational (first light achieved) and ground-based, observing at optical/infrared wavelengths. Segmented mirrors are rated at their effective aperture rather than their maximum physical span (e.g. the Hobby-Eberly Telescope's 11 m mirror yields a ~10 m usable pupil after its 2015 upgrade, and SALT's 11.1-by-9.8 m mirror an effective 9.2 m). Binocular and multi-mirror instruments are ranked by their individual primary mirror diameter, not their combined collecting area (the Large Binocular Telescope's two 8.4 m mirrors are listed at 8.4 m, not their 11.8 m equivalent). Under-construction giants (the Extremely Large Telescope, Giant Magellan Telescope, and Thirty Meter Telescope) are excluded because they are not yet operational. Figures are drawn from Wikipedia's cross-referenced list of largest optical reflecting telescopes and each instrument's official observatory pages. Ties are broken by first-light date (earlier first).
Among currently operational telescopes, the Gran Telescopio Canarias (GTC) on La Palma, Spain, has the largest single aperture at 10.4 meters, narrowly ahead of the twin 10 m Keck telescopes in Hawaii. Larger telescopes such as the 39 m Extremely Large Telescope are under construction but not yet operational.
This list covers only telescopes that are currently operational — that have achieved first light. The ELT (39 m), Giant Magellan Telescope, and Thirty Meter Telescope are still under construction, so they're excluded even though they will eventually dwarf every telescope here.
A monolithic mirror is a single piece of glass, which becomes extremely heavy and hard to make beyond about 8 meters. A segmented mirror is built from many smaller hexagonal panels — 36 in the case of Keck and GTC — actively aligned to act as one surface, which is how telescopes push past the 10 m mark.
It isn't — this list covers ground-based telescopes only. JWST is a space telescope with a 6.5 m segmented mirror, so even in orbit it is smaller than every instrument ranked here, though its position above Earth's atmosphere gives it advantages ground telescopes can't match.