The Extremely Large Telescope is halfway complete

The largest visible and infrared light telescope in the world is on target to see ‘first light’ by 2028.

 This telescope – aptly named the Extremely Large Telescope (ETL) – is part of the European Southern Observatory (ESO) and will be more than 39 metres in diameter once finished.

“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,” says ESO Director General Xavier Barcons.

“Reaching 50% completion is no small feat, given the challenges inherent to large, complex projects … I am extremely proud that the ELT has reached this milestone.”

The total cost is now over 1.3 billion euros ($2.1 billion AUD).

Currently the telescope in Chile’s Atacama Desert is just the steel structure, but it  will eventually house five separate mirrors. The largest is about 70% complete and is made of 798 individual hexagonal segments.

Like the James Webb Space Telescope (JWST), each mirror piece needs to be perfectly cast, polished and placed to be effective. But while the JWST is able to provide wonderful images from outside of our atmosphere, it is only 6.5 metres in diameter.

ELT under construction in July 2023. Credit: ESO

In comparison, the ELT’s largest mirror is 39.3 metres (120 feet) wide which will allow the telescope to take in more light – 100 million times more than a human eye.

Scientists want these bigger and bigger optical telescopes because it makes the objects sharper. The team suggest that it could provide images 16 times sharper than the Hubble Space Telescope.

However the largest optical telescope ever planned – named the “Overwhelmingly Large Telescope” and which would have been 100 metres in diameter, was cancelled due to the huge cost and complexity. Radio telescopes – even single dish ones or the sprawling SKA in outback WA – can be much bigger.  This is because they don’t need carefully designed mirrors, and radio waves are significantly bigger than optical waves.

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