The European Southern Observatory’s (ESO) Extremely Large Telescope (ELT), the world’s largest visible- and infrared-light telescope, is nearing completion. The dome’s frame has been completed, marking a key milestone in the observatory’s construction. The dome measures 305 feet in diameter and stands 263 feet tall.
To protect the ELT from extreme temperatures in the Atacama Desert, aluminum plates will be added to the exterior of the frame. These plates are part of an outer shell that is still under construction. The dome features large sliding doors that will remain closed during the day and open at night to allow the telescope to survey the sky.
The ELT’s main mirror (M1) will measure 128 feet across, while its secondary mirror (M2) will measure 14 feet in diameter. Three smaller mirrors will complete the telescope once construction is finished. The central tower below the “spider” structure will support the remaining three mirrors.
Construction progress has been made on the polygon structure at the base and the “spider” structure on top, which will hold M2 in place. These structures are part of the altitude structure, a metal frame that stands 164 feet tall and will carry all five mirrors and rotate to point the telescope at different parts of the sky.
The ELT is expected to see its first light by 2028 and aims to observe terrestrial exoplanets and their atmospheres, as well as measure the expansion of the universe.
Source: https://www.space.com/the-universe/worlds-largest-telescope-gets-its-protective-shell-photos