Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile.

The telescope is equipped with adaptive optics, six laser guide star units, and various large-scale scientific instruments.

[5] The ELT is intended to advance astrophysical knowledge by enabling detailed studies of planets around other stars, the first galaxies in the Universe, supermassive black holes, the nature of the Universe's dark sector, and to detect water and organic molecules in protoplanetary disks around other stars.

[11][12] The telescope passed the halfway point in its development and construction in July 2023, with the expected completion and first light set for 2028.

[13][3] On 26 April 2010, the European Southern Observatory (ESO) Council selected Cerro Armazones, Chile, as the baseline site for the planned ELT.

[18]: 15 ESO's Director General commented in a 2011 press release that "With the new E-ELT design we can still satisfy the bold science goals and also ensure that the construction can be completed in only 10–11 years.

"[19] The ESO Council endorsed the revised baseline design in June 2011 and expected a construction proposal for approval in December 2011.

With the 2011 changes in the baseline design (such as a reduction in the size of the primary mirror from 42 m to 39.3 m), in 2017 the construction cost was estimated to be €1.15 billion (including first generation instruments).

Both current fabrication technology and road transportation constraints limit single mirrors to being roughly 8 m (26 ft) per piece.

The ELT uses a similar design, as well as techniques to work around atmospheric distortion of incoming light, known as adaptive optics.

[28] The first three mirrors are curved (non-spherical) and form a three-mirror anastigmat design for excellent image quality over the 10-arcminute field of view (one-third of the width of the full Moon).

The fourth and fifth mirrors also send the light sideways to one of two Nasmyth focal stations at either side of the telescope structure, allowing multiple large instruments to be mounted simultaneously.

2394 position actuators (3 for each segment) use this information to adjust the system, keeping the overall surface shape unchanged against deformations caused by external factors such as wind, gravity, temperature changes and vibrations.

This maintenance set allows segments to be removed, replaced, and recoated on a rotating basis once the ELT is in operation.

The segment support system units for the primary mirror were designed and are produced by CESA (Spain)[37] and VDL (the Netherlands).

The contracts signed with ESO also include the delivery of detailed and complete instructions and engineering drawings for their production.

In January 2017,[32] ESO awarded a contract for the mirror blank to Schott AG, who cast it later the same year from Zerodur.

[47] The AdOptica consortium,[48] partnered with INAF (Istituto Nazionale di Astrofisica) as subcontractors, are responsible for the design and manufacture of the quaternary mirror.

[3] The six laser sources for the adaptive optics system, which will work hand-in-hand with the quaternary mirror, have also been completed and are in testing.

The mirror will include a fast tip-tilt system for image stabilization that will compensate perturbations caused by wind, atmospheric turbulence, and the telescope itself before reaching the ELT instruments.

ESO signed a contract for its construction,[54] together with the main structure of the telescopes, with the Italian ACe Consortium, consisting of Astaldi and Cimolai[55] and the nominated subcontractor, Italy's EIE Group.

[58] One year after signing the contract, and after the laying of the first stone ceremony in May 2017, the site was handed over to ACe, signifying the beginning of the construction of the dome's main structure.

In terms of astronomical performance the dome is required to be able to track about the 1-degree zenithal avoidance locus as well as preset to a new target within 5 minutes.

With such a large opening, the ELT dome requires the presence of a windscreen to protect the telescope's mirrors (apart from the secondary), from direct exposure to the wind.

Two spherical blades, either side of the observing slit doors, slide in front of the telescope aperture to restrict the wind.

[2] Furthermore, the ELT's suite of instruments will allow astronomers to probe the earliest stages of the formation of planetary systems and to detect water and organic molecules in protoplanetary discs around stars in the making.

Observations of these early galaxies with the ELT will give clues that will help understand how these objects form and evolve.

In addition, the ELT will be a unique tool for making an inventory of the changing content of the various elements in the Universe with time, and to understand star formation history in galaxies.

[74] The unaided human eye has an angular resolution of 1 arcminute which corresponds to separating two light sources 30 cm apart from 1 km distance.