The future Extremely Large Telescope will have a 39-meter mirror. Engineers blasted the mountain peak flat. It will image exoplanet atmospheres directly when complete.
Swinburne Astronomy Productions/ESO, CC BY 4.0, via Wikimedia Commons
The World's Biggest Eye on the Sky
On the flattened summit of Cerro Armazones, a 3,046-meter peak in Chile's Atacama Desert, construction is underway on the world's largest visible and infrared telescope: the Extremely Large Telescope (ELT). To create a stable platform for this massive observatory, the top of the mountain was leveled in 2014, a process that involved blasting away part of the peak. The construction of the dome and main structure began in 2018. The observatory is a project of the European Southern Observatory (ESO) and is scheduled to achieve "first light" by 2028.
The main feature of the ELT is its enormous primary mirror (M1), which will have a diameter of 39.3 meters. This mirror is too large to be made from a single piece of glass. Instead, it is composed of 798 separate hexagonal segments, each 1.4 meters across and 5 centimeters thick. Together, these segments create a light-collecting area of 978 square meters, which will gather more light than all existing 8–10-meter class telescopes on the planet combined. This will allow the ELT to produce images 15 times sharper than those from the Hubble Space Telescope. The entire moving structure of the telescope weighs around 3,700 tonnes, all housed within a dome 80 meters high and 93 meters in diameter.
A Five-Mirror System with a Deformable Eye
The ELT uses a novel five-mirror optical design to achieve its scientific goals. After light hits the giant M1 primary mirror, it is reflected to a 4.25-meter secondary mirror (M2) and then to a 3.75-meter tertiary mirror (M3). The final two mirrors, M4 and M5, form an advanced adaptive optics system.
The fourth mirror (M4) is a 2.4-meter-wide deformable mirror, the largest of its kind ever built. Its surface, less than 2 millimeters thick, is controlled by 5,000 magnets that adjust its shape up to 1,000 times per second. These rapid, nanometer-scale adjustments counteract the blurring effect of Earth's atmosphere—the phenomenon that makes stars twinkle. The fifth mirror (M5), measuring 2.7 by 2.2 meters, is the world's largest tip-tilt mirror and further stabilizes the image by compensating for wind and mechanical vibrations. This system, often guided by up to eight lasers that create artificial guide stars in the upper atmosphere, allows the telescope to achieve incredibly sharp images. One of the ELT's goals is the direct imaging of Earth-like exoplanets and the characterization of their atmospheres, searching for water and organic molecules.
The location was chosen after years of study. Cerro Armazones offers over 320 clear nights per year, extremely low humidity, and stable atmospheric conditions, making it one of the best sites for astronomy on Earth. Its proximity to ESO's existing Paranal Observatory, just 20 kilometers away, also provides significant logistical and operational advantages.
💡Fun Facts
The ELT will collect 100 million times more light than the human eye.
The surface of the primary mirror, across all 798 segments, must be kept accurate to within tens of nanometers—about 10,000 times thinner than a human hair.
The main telescope structure currently weighs around 3,500 tonnes and will increase to 4,600 tonnes once all mirrors and instruments are installed.
The site on Cerro Armazones was selected over other locations in Chile and Spain after an extensive meteorological investigation.
The construction site is not open for public visits. Media and official visits require special authorization in advance due to safety regulations at the high-altitude construction zone.
Admission
Not applicable. Public access is not permitted.
Accessibility
The site is located at an altitude of 3,046 meters in a remote desert environment. Access is via restricted roads and requires special permission.
