No single mirror 39 meters wide could be cast, transported, or supported. Instead, the ELT's primary mirror is a mosaic of 798 hexagonal segments, each 1.45 meters across and polished to a surface accuracy of 7.5 nanometers -- roughly one ten-thousandth the width of a human hair. German company Schott AG casts the blanks from Zerodur, a ceramic that barely expands or contracts with temperature changes. French firm Safran Reosc then polishes one segment per day to meet the seven-year production schedule. An additional 133 maintenance segments allow individual pieces to be swapped out and recoated on a rotating basis without ever leaving a gap. Three actuators per segment, 2,394 in total, constantly adjust the mosaic against wind, gravity, and thermal distortion, keeping the surface stable to within nanometers.

What makes the ELT truly revolutionary is not just its size but its optical design. Five mirrors work in concert: three curved mirrors form a three-mirror anastigmat that delivers sharp images across a field of view one-third the width of the full Moon, while a fourth mirror -- a deformable surface of six petals rippling with voice-coil actuators -- corrects for atmospheric turbulence in real time. The fifth mirror handles tip-tilt stabilization. Together, they deliver adaptive optics that improve resolution by a factor of 500 compared to the best natural seeing conditions. Six laser guide star units shoot beams into the upper atmosphere, creating artificial reference points that let the system measure and cancel the blurring caused by turbulent air above the desert.

At 93 meters in diameter, the ELT dome dwarfs the Roman Colosseum. Its total mass exceeds 6,100 tonnes, yet it must rotate smoothly enough to track celestial targets, accelerating to 2 degrees per second -- about the speed of a fast walk along its circumference. A pair of curved sliding doors open a 41-meter slit to the sky. Behind them, a windscreen of four spherical blades shields the mirrors from the Atacama's persistent winds. By day, an air-conditioning system cools the interior so that the telescope reaches thermal equilibrium with the night air before sunset, preventing heat shimmer from degrading observations. The Italian ACe Consortium, led by Astaldi and Cimolai, won the construction contract in 2016. As of early 2026, the dome structure is nearing completion, with full dome closure expected in 2027.

The ELT's science goals read like a wish list for the next century of astronomy. Its ANDES spectrograph will search for biosignatures in the atmospheres of Earth-like exoplanets. MICADO, its first imaging camera, will resolve individual stars in galaxies tens of millions of light-years away. HARMONI will function as the workhorse spectroscope, dissecting light from the most distant objects in the universe. The telescope will probe the nature of dark energy by measuring the acceleration of cosmic expansion, test whether the fundamental constants of physics have changed over time, and image the environments around supermassive black holes. Compared to the largest optical telescope operating today, the 10.4-meter Gran Telescopio Canarias, the ELT will collect nearly fifteen times more light.

The Atacama Desert was chosen for the same reasons it has attracted observatories for decades: extreme aridity, high altitude, and some of the clearest skies on Earth. Cerro Armazones sits 23 kilometers from ESO's existing Paranal Observatory, home of the Very Large Telescope. The site receives fewer than ten millimeters of rainfall per year. The atmosphere is so thin and dry at 3,046 meters that infrared and submillimeter radiation passes through with minimal absorption -- precisely the wavelengths needed to study forming stars and distant galaxies. Costing approximately 1.4 billion euros, the ELT is the most expensive ground-based telescope ever constructed. Two rival projects, the Giant Magellan Telescope and the Thirty Meter Telescope, are also targeting the 2030s, but the ELT's 39-meter aperture will give it the largest light-collecting area of any optical telescope in history.