The next big thing in astronomy: the Extremely Large Telescope

The Extremely Large Telescope (ELT) will be the world’s largest visible and infrared telescope – when completed, this new eye on the sky will open up new windows onto the universe and see things we can’t yet imagine.

On the top of a mountain in Chile, construction of the largest visible and infrared telescope ever built – the Extremely Large Telescope, or ELT – is underway. When the telescope is operational, its 39-metre primary mirror will gather 217 times more light than the Hubble Telescope.

The ELT’s scale makes it a feat of engineering, and its ambition makes it a feat of imagination.

The science applications of the ELT are vast. It will allow us to image planets outside our solar system, tell us what they are made of and if they can support life, and maybe even help us understand more about the mysteries of dark matter. And that’s just the start.

As the ELT and its instruments evolve, it will generate discoveries that we can’t yet imagine.

Delivering this awe-inspiring project is beyond the reach of a single nation, but is within our grasp thanks to multinational collaboration and science-led innovation.

The UK is playing a key role in the ELT’s innovation.

At the Science and Technology Facilities Council (STFC), we fund the UK ELT Project Office, led by Dr Chris Evans. It co-ordinates activities across the UK’s principle partners for research and development for the project – the University of Cambridge, Durham University, University of Oxford, and STFC’s UK Astronomy Technology Centre and RAL Space – in close collaboration with the European Southern Observatory (ESO). As Dr Evans says: “It's exciting to be part of one of the biggest global science collaborations in history – and to see the UK helping to shape the project and drive it forwards.”

Let’s find out more about the ELT

The ELT will be the world’s largest optical telescope, meaning it will use mirrors to gather light in the visible and the infrared spectrum. The telescope is being built by the ESO and its 15 member states, of which the UK is a major partner.

When complete, the ELT will be a state-of-the-art facility, with capabilities far beyond any other ground-based optical telescope. It will have a footprint of 115 metres (to make room for the telescope, the top of the mountain has been levelled), and its dome will be 80 metres tall, making it taller than a football stadium.

The ELT telescope compared to Manchester's Etihad Stadium

The ELT will be taller than a football stadium.
(Credit: ESO)

This size is only possible because the ELT has driven an ‘industrial revolution’ in telescope construction. This has changed the way the telescope is made, and means that new types of businesses can be involved in its production, with greater emphasis on production speed, quality and logistics.

The ELT will be made up of lots of smaller components that fit together (like Lego), rather than a few one-off items.

The telescope’s primary mirror is a great example of this – it will be 39-metres across, and made up of 798 hexagonal segments. It’s the size of this primary mirror that determines how much light it can capture – and how much of the universe it will be able to see.

For astronomers, physicists and stargazers everywhere, developing a telescope this size with these capabilities is a major priority – it’s the one they have been waiting for.

Very large vs extremely large

Close up image of a bumblebee collecting nectar from a flower

The ELT could see a bumblebee at Land’s End from John O’Groats.
(Credit: Pixabay)

If you want to be precise, the difference between an extremely large telescope and a very large telescope is about 30.80 metres…that’s the difference in size between the ELT and the Very Large Telescope (VLT), currently the most advanced optical observatory in the world.

It’s this huge jump in class between the 8-10 metre telescopes and the ELT that’s getting astronomers so excited.

The ELT will be much more powerful than any other telescope currently in existence. If the telescope was placed at Land’s End, it could see a bumblebee at John O'Groats.

Right now, 8-10 metre telescopes are the best on the planet. With them, astronomers and physicists have made amazing discoveries, like producing the first image of a planet outside our solar system and tracking stars as they move around the black hole in the centre of our galaxy. The ELT won’t replace these telescopes; they will continue to power scientific discovery for years to come.

But they have also opened the door to new mysteries about our universe. To address the new questions raised by existing telescopes and make new discoveries, astronomers need a new class of telescope to complement them – one in the 30-60 metre diameter range.

Size isn’t the only thing that matters

4 lasers shining out from the ELT creating artificial stars

ELT deploying lasers to create artificial stars.
(Credit: ESO/L. Calçada/N. Risinger (

It’s not just the size of the telescope that makes the ELT so impressive. The engineers and scientists designing the telescope and its instruments are bringing expertise honed for other facilities to bear on every aspect of the ELT.

One of the most sophisticated pieces of technology underpinning the ELT’s operation is its adaptive optics system. Adaptive optics allow astronomers to take really clear images of the stars by stopping them from twinkling.

They can do this because the twinkling isn’t caused by the stars themselves – it’s caused by distortions in the earth’s atmosphere. By measuring the effect of the atmosphere on bright reference stars in the nearby sky (or on artificial laser guide stars), thousands of little tiny pistons (called actuators) under the surface of one of the mirrors push it gently to change its shape and correct for distortions in the atmosphere and create crisp images of the cosmos.

This technology also has applications for much smaller environments, like the environment within the human body. Biological and medical researchers have been working together with imaging experts to study the murky environment within cells.

An infographic showing various statistics about the ELT

Click to enlarge and find out more about the ELT’s design and its adaptive optics system.
(Credit: STFC/Ben Gilliland)

Exquisite instruments

Being able to capture light from the far reaches of the universe is one thing – but it’s the ELT’s instruments that will transform that light into scientific discoveries.

The ELT will have three key instruments in place at ‘first light’ or following soon after – MICADO (a camera), HARMONI (a spectrograph), and METIS (a mid-infrared spectrograph and imager).

HARMONI is the ELT’s ‘workhorse’ spectrograph. It will detect light in the visible and near-infrared parts of the spectrum, and produce 3D images of the sky with unparalleled sharpness and clarity. Because the ELT will have adaptive optics built in, the design of the telescope and HARMONI must stay closely coupled. This is an exciting challenge for the UK team leading the design of this critical instrument.

Leading things is Professor Niranjan Thatte from the University of Oxford, in collaboration with STFC’s UK Astronomy Technology Centre and Rutherford Appleton Laboratory, and experts at Durham University.

The group – along with contributions from international partners in Lyon, Marseille, Tenerife, and Madrid – will also be working together to ensure the subsystems for HARMONI operate seamlessly.

Read our interview with Professor Niranjan Thatte from the University of Oxford and lead investigator on the HARMONI instrument.

While HARMONI will be the first instrument to tackle the big questions ELT was built for, other instruments will be added to the telescope after first light. These include HIRES (a high-resolution spectrograph) and MOSAIC (a multi-object spectrograph).

MOSAIC will allow astronomers to observe large numbers of the most distant galaxies simultaneously, and build on scientific discoveries expected from the James Webb Space Telescope.

As part of an international consortium of 11 countries, UK science and engineering teams are leading aspects of the instrument design.

Find out why MOSAIC is the instrument Professor Simon Morris, ESO Council Member and Professor of Physics at Durham University, is most excited about.

The future of astronomy

The ELT will change the way we view the universe and open new avenues of exploration.

There are certain scientific questions about the universe we know ELT will be able to answer: it will let us observe atmospheres of planets inside and outside our own solar system (possibly detecting ‘bio-markers’ indicating that they could support life), look back in time at the most distant galaxies so we can understand their formation and evolution, and make direct measurements of the expanding Universe, which could tell us more about dark matter and how it is distributed.

But perhaps the most exciting questions the ELT will help us to answer are the ones we haven’t yet thought to ask, and the serendipitous discoveries that will take us by surprise.

It’s worth remembering that the ELT is not just being designed for today’s researchers.

This incredible telescope will inspire a new generation of astronomers to look to the sky, and fuel their discoveries for decades as they work to understand our place in the universe.

Find out more about the ELT on the ESO website
Discover more about big telescopes

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