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Last updated July 12, 2025

Giants of Observation: The Largest Ground-Based Telescopes

The largest ground-based telescopes: VLT

A Quest for Light

Since Galileo (1564-1642) and his 37 mm telescope in 1609, ground-based telescopes have continued to grow in size to collect more light. The performance of an optical telescope depends mainly on the diameter of its primary mirror, which determines its ability to capture light and resolve fine details of distant objects. The largest current instruments, such as the Gran Telescopio Canarias (GTC) or the Very Large Telescopes (VLT), use segmented or coupled mirrors to achieve impressive diameters.

Mechanical and thermal constraints limit the manufacture of monolithic mirrors beyond 8 meters. Modern telescopes therefore adopt segmented mirrors controlled by actuators to maintain a perfect optical surface. The future Extremely Large Telescope (ELT) of the ESO in Chile will embody the pinnacle of this technology with a 39.3-meter mirror composed of 798 hexagonal segments. The table below lists the 20 largest ground-based optical telescopes in operation or under construction, ranked by equivalent diameter of the primary mirror.

The 20 Largest Ground-Based Optical Telescopes

RankNameLocationCountryDiameter (m)YearType
1ELTCerro ArmazonesChile39.32028Segmented
2TMTMauna KeaUnited States30.0~2030Segmented
3GMTCerro Las CampanasChile25.4~2029Segmented
4GTCLa PalmaSpain10.42009Segmented
5Keck IMauna KeaUnited States10.01993Segmented
6Keck IIMauna KeaUnited States10.01996Segmented
7SALTSouth AfricaRSA9.22005Segmented
8VLT UT1 to UT4Cerro ParanalChile8.21998–2001Monolithic
9SubaruMauna KeaJapan8.21999Monolithic
10Gemini NorthMauna KeaUnited States8.11999Monolithic
11Gemini SouthCerro PachónChile8.12000Monolithic
12LBTMount GrahamUnited States8.4×22004Double mirror
13Magellan I & IICerro Las CampanasChile6.52000Monolithic
14MMTArizonaUnited States6.52000Monolithic
15Palomar HaleCaliforniaUnited States5.11948Monolithic
16BaadeCerro Las CampanasChile6.52002Monolithic
17HookerMount WilsonUnited States2.51917Monolithic
18ESO 3.6La SillaChile3.61976Monolithic
19Canada-France-HawaiiMauna KeaCanada/France3.61979Monolithic
20Anglo-AustralianSiding SpringAustralia3.91974Monolithic

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