10 Largest Optical Telescopes

The word "largest" can be a tricky word. So the list is split into traditional single-instrument optical telescopes on a single mount, and into interferometric "virtual" telescopes consisting of multiple optical instruments whose images are combined. Also see 10 largest radio telescopes.

Single telescopes

These are the 10 largest single-instrument, single-mount optical telescopes in the world:

1. Large Binocular Telescope (LBT) *

11.9 m effective (2 x 8 m)  /  469 inches
Mt Graham, AZ USA

2. Gran Telescopio Canarias (GTC)

10.4 m  /  409 inches
La Palma, Canary Islands, Spain

3. Hobby–Eberly Telescope (HET)

10 m  /  394 inches
Mt Fowlkes TX USA

4. Keck I **

10 m  /  394 inches
Mauna Kea, HI USA

5. Keck II **

10 m  /  394 inches
Mauna Kea, HI USA

6. South African Telescope (SALT)

9.2 m  /  362 inches
Sutherland, South Africa

7. Very Large Telescope (VLT) ***

8.2 m  /  323 inches (UT-1 thru UT-4)
Cerro Paranal, Chile

8. Subaru Telescope

8.2 m  /  323 inches
Mauna Kea, HI USA

9. Gemini North

8.1 m  /  319 inches
Mauna Kea, HI USA

10. Gemini South

8.1 m  /  319 inches
Cerro Pachon, Chile

* Counts as a single instrument, just like a pair of binoculars.

** Keck I and II were originally designed to be used as an optical interferometer, but funding never materialized for the additional equipment needed.

*** The four VLT telescopes can be used either individually or combined as a single optical inteferometer.

Interferometer-array telescopes

Although relatively common in the radio-spectrum domain, optical interferometry where images from multiple telescopes are combined to form a single image is fiendishly difficult and rare. Interferometric telescopes in essence form a large "virtual" telescope with an effective aperture based on the distance between the member telescopes.

1. Navy Precision Optical Interferometer (NPOI)

432 m effective aperture (6 x 1.8 m telescopes)
Flagstaff, AZ USA

2. Magdalena Ridge Observatory Interferometer (MROI)

340 m effective aperture (10 x 1.4 m telescopes - under construction)
Socorro, NM USA

3. CHARA Array

330 m effective aperture (6 x 1 m telescopes)
Mt Wilson, CA USA

4. Very Large Telescope (VLT) **

130 m effective aperture (4 x 8.2 m telescopes)
Cerro Paranal, Chile

*** The four VLT telescopes can be used either individually or combined as a single optical inteferometer.

Future large telescopes

Here are notable large optical telescopes currently under construction:

1. Extremely Large Telescope (ELT)

39.3 m (first light in 2027)
Cerro Armazones, Chile

2. Thirty Meter Telescope (TMT)

30 m (first light in 2027)
Mauna Kea, HI USA

3. Giant Magellan Telescope (GMT)

245 m (first light in 2029)
Cerro Los Campanas, Chile

3. Vera Rubin Observatory

8.4 m (first light in 2023)
Cerro Pachon, Chile