Part 3
Projects & Landmarks
Physical Sciences & Engineering / Landmark


Extremely Large Telescope

The world’s biggest eye on the sky to revolutionise our perception of the Universe


Extremely Large Telescope (ELT) is a revolutionary new ground-based telescope for the advancement of astrophysical knowledge, allowing detailed studies of objects including planets around other stars, the first objects in the Universe, super-massive black holes, and the nature and distribution of the dark matter and dark energy which dominate the Universe. Equipped with a 39-metre primary mirror, the ELT will be the largest optical/near-infrared telescope in the world: the world’s biggest eye on the sky.

The ELT is an integral part of ESO, the EIROforum organisation operating facilities at a number of sites in Chile. The ELT programme was approved in 2012 and green light for the first phase of construction was given at the end of 2014. It will be located at Cerro Armazones, a 3060-metres high mountain in the central part of Chile’s Atacama Desert, about 20 kilometres from Cerro Paranal, home of ESO’s Very Large Telescope (VLT). The ELT construction is expected to be completed by 2024.


The telescope’s primary mirror will be almost half the length of a soccer pitch in diameter and will gather 15 times more light than today’s largest optical telescopes. The optical design comprises a three-mirror anastigmat with two flat folding mirrors providing the adaptive optics to correct for the turbulent atmosphere, giving unprecedented image quality. One is supported by more than 5.000 actuators operating at a frequency of 1.000 Hz. The primary mirror consists of 798 hexagonal segments, each 1,4 metres wide. The secondary mirror will have a diameter of 4 metres. The telescope will have several science instruments, with switching from one instrument to another within minutes. The ability to observe over a wide range of wavelengths from the optical to mid-infrared will allow scientists to exploit the telescope’s size to the fullest extent.

Science with the ELT covers many areas of astronomy – from the Solar System to extra-solar planets, from nearby galaxies to the furthest observable objects at the edge of the visible Universe, from fundamental physics to cosmology. They include discovering and characterising planets and proto-planetary systems around other stars, resolving stellar populations in a representative sample of the Universe, the study of the physical processes that form and transform galaxies across cosmic time, the discovery and identification of distant type Ia supernovae and constraining dark energy by directly observing the global dynamics of the Universe, as well as searching for possible variations over cosmic time of fundamental physical constants.


The ELT is a major technological challenge and triggers industrial interest and preparedness to deliver extraordinary performance, as it occurred in previous ESO projects (notably the VLT and ALMA). ESO has since many years devolved its instrumentation programme so that science instruments are largely designed and built by national institutes, often in collaboration with industry. In this model, national facilities cover the human resources cost against compensation in guaranteed observing time. The ELT will employ advanced technologies and engineering solutions in a number of areas, from gigantic, lightweight high-precision structures, opto-mechanical systems, optical design and control systems. Many of these technologies will be applicable to other areas of technology development. As regards short-term benefits, these are found in spin-off technologies and the inspirational and educational aspects, strengthening the scientific and engineering recruitment base and public awareness of science.

Concerning the contribution to societal challenges, astronomy is basic science in its most fundamental form and its main purpose is to enhance our understanding of the Universe, its evolution and the role of planet Earth as our cosmic home. Astronomy findings have a profound impact on society in the long run, both in technological and cultural terms. Astronomy is a major theme in education and training, inspiring new generations in STEM topics. It also triggers innovative technology solutions, and international cooperation.

legal status
ESO, EIROforum member
political support
lead entity
The following countries are members of ESO
prospective member countries
The full list of research institutions involved must be found in the website of the RI
Roadmap Entry
Preparation Phase
Implementation/Construction Phase
Operation Start
estimated costs
capital value
1.120 M€
Not Available
Not Available
Not Available
45 M€/year

Garching bei München, Germany