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ESO’s Extremely Large Telescope under construction in the Atacama Desert in January 2025. | Photo credit: ESO/G. Vecchia
The frame of the dome, which will house the world’s largest telescope, has been completed, marking another important milestone in the observatory’s construction.
The European Southern Observatory’s (ESO) Extremely Large Telescope (ELT) – the world’s largest visible and infrared telescope – is currently being developed on the Cerro Armazones mountain in Chile’s Atacama Desert. The powerful telescope is expected to see its “first light” in 2028.
New photos from ESO show that the frame of the dome is now complete, while the outer shell that will completely enclose the telescope is still under construction. According to a statement from the observatory, aluminum panels will be attached to the outside of the frame to protect the telescope from the extreme environmental conditions of the Atacama Desert, including fluctuating temperatures.
The dome is 305 feet (93 meters) in diameter, about the size of a football field, and is 263 feet (80 meters) high. The dome will house the ELT, whose goal is to observe terrestrial exoplanets and their atmospheres and measure the expansion of the universe.
ESO’s Extremely Large Telescope under construction in the Atacama Desert in January 2025. | Photo credit: ESO/G. Vecchia
The latest progress photos from January 2025 show cranes assembling the outer aluminum layers, also known as cladding. Part of the dome will have large sliding doors that remain closed during the day and open at night, allowing the telescope to observe the sky.
The dome and skeleton of ESO’s Extremely Large Telescope, which will be built in the Atacama Desert in January 2025. The central structure will support the telescope’s large central mirror and secondary mirror. | Photo credit: ESO/G. Vecchia
Progress has also been made on the polygon structure at the base inside the dome, which will support the telescope’s primary mirror (M1), and on the “spider” structure on top, which will hold the secondary mirror (M2) center. M1 will be a whopping 128 feet (39 meters) in diameter, while M2, expected to be completed later this year, will be 14 feet (4.25 meters) in diameter. The telescope will have three smaller mirrors, bringing the total to five when completed.
ESO’s Extremely Large Telescope under construction in the Atacama Desert in January 2025. | Photo credit: ESO/G. Vecchia
ESO also shared a close-up of the inner frame of the “spider” structure, named for its arched shape over the base and with six arms radiating from the center. While the skeleton itself is finished, it waits to receive all of the individual segments that will make up the five mirrors. M1 alone will consist of 798 hexagonal glass-ceramic segments, each about 2 inches (5 centimeters) thick and about 5 feet (1.5 meters) wide. When fully assembled, M1 will be the largest mirror ever built for an optical telescope.
The inner frame of ESO’s Extremely Large Telescope (ELT), which will support all five of the ELT’s mirrors when completed. | Photo credit: ESO/G. Vecchia
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The central tower below the “spider” structure will support the remaining three mirrors of the ELT. These components – the central tower, the spider structure and the polygon base – are all housed on the so-called height structure. This metal frame is 164 feet (50 meters) high and is used to support all five of the ELT’s mirrors and rotate so that the telescope can be aimed at different parts of the sky.
