Posted on by Nancy Atkinson

The Biggest Telescope in the World is Half Built

The European Southern Observatory continues to build the largest telescope in the world, the Extremely Large Telescope (ELT). Construction of the telescope began in 2014 with flattening the top of a mountain named Cerro Armazones in Chile’s Atacama Desert.

ESO just announced that progress on construction has crossed the 50% mark.  The remaining work should take another five years. When it finally comes online in 2028, the telescope will have a 39-meter (128 ft) primary mirror of 798 hexagonal segments, making it the largest telescope in the world for visible and infrared light. The new telescope should help to answer some of the outstanding questions about our Universe, such as how the first stars and galaxies formed, and perhaps even be able to take direct images of extrasolar planets.  

“The ELT is the largest of the next generation of ground-based optical and near-infrared telescopes and the one that is most advanced in its construction,” said ESO Director General Xavier Barcons, in an ESO press release. “Reaching 50% completion is no small feat, given the challenges inherent to large, complex projects, and it was only possible thanks to the commitment of everyone at ESO, the continued support of the ESO Member States and the engagement of our partners in industry and instrument consortia. I am extremely proud that the ELT has reached this milestone.”

This image, taken in late June 2023, shows a drone shot of the construction site of ESO’s Extremely Large Telescope at Cerro Armazones, in Chile’s Atacama Desert. There, engineers and construction workers are currently assembling the structure of the telescope dome at a staggering pace. Visibly changing each day, the steel structure will soon acquire the familiar round shape typical of telescope domes.  People visible at the bottom of the frame give a sense of scale to the photo, showcasing just how big the dome of ELT will be. Behind the telescope, we see the shadow of Cerro Armazones, projected onto the desert landscape. Credit: ESO

The observatory aims to gather 100 million times more light than the human eye, 13 times more light than the largest optical telescopes, and be able to correct for atmospheric distortion with adaptive optics, and eight laser guide star units, and most notably an adaptive, flexible mirror that will adjust its shape a thousand times a second to correct for distortions caused by air turbulence. The telescope also has a 4.2 m (14 ft) diameter secondary mirror and four science instruments.

The telescope mirrors and other components are being built by companies in Europe. ESO said that all other systems needed to complete the ELT, including the control system and the equipment needed to assemble and commission the telescope, are also progressing well in their development or production.

Additionally, all four of the first scientific instruments are in their final design phase with some about to start manufacturing, and most of the support infrastructure for the ELT is now in place at or near Cerro Armazones.

ESO expects that the finishing the remaining 50% of the project will take much less time than building the first half of the ELT. The design was still being finalized when construction began and other parts of the telescope needed to undergo testing. Also, construction was affected by the COVID-19 pandemic, with the site closing for several months and production of many of the telescope components suffering delays.