A Giant in Moonlight: The Extremely Large Telescope (ELT) under Construction

A Giant in Moonlight: The Extremely Large Telescope (ELT) under Construction

This image shows the Extremely Large Telescope (ELT) construction site in the Atacama desert of Chile in South America, kissed by the white moonlight that fills the air. Although the Moon itself is not in the frame, you can feel its presence nearby. The night sky is a deep blue, becoming slightly whiter as it meets the brown horizon of the desert. Dominating the center of the image is the giant ELT structure under construction, with its circular concrete base at the bottom and its magnificent steel-framed dome on top. The criss-cross steel frame now has two complete arches at the top. On the right and left of the ELT are two cranes leaning in towards the telescope.

It is now close to 80 meters high—the mammoth steel structure of the European Southern Observatory’s Extremely Large Telescope (ELT) dome is taking shape in this image, taken on August 2, 2023, under the light of the full Moon. Right now, engineers and construction workers are assembling the structure of the telescope dome in the Chilean Atacama Desert, with progress visible almost every day. When completed, the ELT will be the world’s biggest eye on the sky for visible and infrared observations.

Overall, the ELT project is now more than 50 percent complete. The dome structure visible here will house a pioneering five-mirror optical design, which includes a giant main mirror (M1), 39 meters wide and made up of 798 hexagonal segments. The telescope mirrors and other components are being built by companies in Europe, where work is progressing well too. All of the other systems needed to complete the ELT, including the control system and the equipment needed to assemble and test the telescope, are also coming along nicely.

When finished, the dome will weigh in at 6100 tonnes, and it will need a mind-boggling 30 million bolts to be held together. This huge structure will shelter the telescope during observations, protecting it from the elements. The entire behemoth will rotate on 36 stationary trolleys, allowing astronomers to observe the southern sky from just about any direction they fancy.

The current largest optical telescopes have diameters of up to ten meters, and the ELT's diameter will thus be four times greater. This diameter was chosen because it is the minimum diameter needed to achieve some of the driving science cases. For example, the ELT will be able to image rocky exoplanets and to characterize their atmospheres, while the existing ESO Very Large Telecope (VLT) can only indirectly detect such Earth-like planets. Moreover, the ELT will be able to directly measure the acceleration of the expansion of the Universe. Adaptive optics systems are fully incorporated into the design of the telescope to compensate for the fuzziness in the stellar images introduced by atmospheric turbulence. The ELT will have more than 5,000 actuators that can change the shape of its mirrors a thousand times per second.

Altitude: 3046 meters

Planned year of technical first light: 2027

Learn more about ESO’s ELT at: https://elt.eso.org

Credit: European Southern Observatory (ESO)

Release Date: Aug. 7, 2023

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