At 9,993 feet, high on Cerro Armazones in the Atacama Desert of Chile, the foundation for the Extremely Large Telescope is being laid. The ELT, a project from the European Southern Observatory, promises to be the largest optical reflecting telescope in the world (meaning it's not a radio dish, but uses a mirror to collect light).



The primary mirror of this behemoth will span 39.3 meters, or 130 feet from end to end. The first of 798 hexagonal mirror segments that will make up the ELT, each 4.2 meters (14 feet) wide, were cast early this year. Now, workers with the ACe Consortium, consisting of Astaldi and Cimolai, have begun to build the mountaintop foundation for the world's biggest telescope, which is slated for first light in 2024.

Construction on the foundation of the Extremely Large Telescope. Not the size of the work trucks on the site for a sense of scale. ESO/G. Hüdepohl

Drone shots of the worksite released by ESO show the outline of what will become the Extremely Large Telescope. An 80-meter-tall dome (262 feet) will house the observatory, covering the entire site. The 55-meter-diameter (180 feet) circular pit visible in the photos will house the foundation for structures to support the telescope's extremely large primary mirror.

The site for the ELT is only a 30-minute drive from ESO's current flagship telescope, the Very Large Telescope (which is actually four telescopes working together), perched on Cerro Paranal. The close proximity of support and maintenance facilities should help ESO get the ELT up an running. The location high in the mountains of Chile is desirable because the air is so dry and thin here that atmospheric refraction, which obscures astronomers' views of the cosmos, is less of a hindrance than almost anywhere else on the planet.

Following first light, this telescope of extremely large size will allow astronomers to probe the earliest ages of the universe, study ancient galaxies, measure exoplanet atmospheres, and answer dozens of lingering questions in astronomy. While discussing the work of future large telescopes with NASA's Director of Astrophysics, Paul Hertz, just before the TESS launch, he had a simple way of putting it: "size matters."

Source: European Southern Observatory

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