The Extremely Large Telescope will transform astronomy

It will be the world’s biggest optical telescope by far—and a powerful time machine

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ESO’s Extremely Large Telescope construction site

Video: G. Hüdepohl/ESO

It is the colours of a sunset that inspire Joseph Anderson, an astronomer at the European Southern Observatory (ESO) in the high Atacama desert, in northern Chile. “They start off very blue and turquoise. And gradually, as it gets more purple, then we’re getting closer to observing the universe.” Once night falls the sky is dominated by the star-spangled curve of the Milky Way. If there were any extra left to give, more than two kilometres above sea level, your correspondent’s breath would have been taken away.

The Atacama is home to more than a dozen different astronomical observatories, and for good reason. It is far from the light pollution of civilisation. The air is thin and dry, which improves what astronomers call the “seeing”. And the desert averages 325 cloudless nights each year. Dr Anderson is standing on top of a mountain called Cerro Paranal, showing off the Very Large Telescope (VLT). The VLT is made up of four individual telescopes, each individually one of the largest in the world, alongside another four much smaller ones. It is Earth’s most productive astronomical facility, yielding more than one scientific paper each day. In 2004 it took the first picture of an extrasolar planet—one that orbits a star other than the Sun—and was the first instrument to track individual stars whipping around the enormous black hole at the centre of the Milky Way.

But it may not hold that title for much longer. An hour’s drive from the VLT, atop Cerro Armazones, a 3,046-metre peak, sits the half-finished bulk of the ELT, or Extremely Large Telescope. (ESO is a fan of quotidian names.) Like so many big projects, the ELT is behind schedule. But when it is finished—in 2028, on current plans, at a cost of €1.5bn ($1.6bn)—it will be, by far, the biggest optical telescope in the known universe. The result, says Robert de Rosa, an astronomer at ESO, will be “a step change in what we can do in terms of observational astronomy”.

Optical telescopes use a series of mirrors to capture light from space and redirect it to their various instruments. A bigger mirror can collect more light, which means it can both see dimmer things and resolve them in finer detail than a smaller one. The ELT’s main mirror will have a diameter of 39.3 metres, more than four times that of the VLT’s big telescopes (8.2 metres) and over three times that of the present record-holder, the 10.4-metre Gran Telescopio Canarias (GTC), in the Canary Islands. Since a telescope’s power depends on the area of its mirror, looking only at the diameter understates the difference. The GTC has a collecting area of around 75 square metres. The ELT will boast 978 square metres, a little smaller than four tennis courts (see diagram 1).

That size will be a boon for many branches of astronomy. The ELT will shed light on everything from the role of black holes in shaping the large-scale structure of the universe to how dark matter and dark energy affect the rate at which it is expanding, and even whether the supposed constants of physics really are constant over vast intergalactic distances.

It should also provide a big boost to the study of planets outside the solar system. These days, the existence of most exoplan­ets is inferred from the effects they have on the light from their parent stars. Taking pictures of them—so-called direct imaging—is rare. Of the roughly 5,500 known exoplanets, scientists have pictures of only around 1% of them.

Time and relative dimension in space

The ELT’s enormous mirror will allow astronomers to separate the faint light of a planet from the overwhelming glare of its star from dozens of light-years away. The result should be a direct-imaging bonanza. And direct imaging will also help reveal the chemical composition of exoplanet atmospheres, and whether any show signs of potential alien life.

And because telescopes are also time machines, the ELT will allow scientists more insight into what happened shortly after the universe began. Since the speed of light is finite, astronomers see distant objects not as they are today, but as they were when the light that arrives in their instruments first set out. Astronomers are keen to use the ELT to investigate how stars and galaxies formed when the universe was young. Early results from the James Webb Space Telescope, launched in 2021, have already turned up an array of stars and galaxies that seem too old to fit easily into existing theories of universal evolution. The ELT could help resolve that mystery.

Assuming, that is, that everything works. When your correspondent visited, the 80-metre-tall steel dome that will shelter the telescope was still being built. Each segment takes around 20 minutes to lift and slot into place; a further six hours are needed to tighten each of the 200 bolts. Work must finish before night, lest gusts of wind blow a panel loose. Once finished, the entire 5,500-tonne dome will be able to rotate so that the telescope can follow the stars as they move across the sky.

One of the few downsides of doing astronomy in Chile is that the country is prone to earthquakes. The telescope will therefore float atop a thin layer of oil. The oil, in turn, will sit atop hundreds of rubber shock absorbers, with the whole lot built on a 3-metre concrete foundation. That will isolate the dome both from earthquakes and any vibrations from the offices and laboratories next door.

The most impressive parts are the mirrors, of which the ELT will have five. Astronomical mirrors are precise, delicate things. Even the comparatively small mirrors of the big VLT telescopes are so heavy that, if not supported properly, they would shatter under their own weight. Engineers must lift them with a special harness with 15 hooks when they need cleaning.

The ELT’s main mirror is so big that it cannot be made as a single piece. Instead Schott, a German optics firm, will make 798 separate pieces that will act as a single mirror. Each is a slightly curved, 1.5-metre-wide hexagonal slice of high-tech glass ceramic that undergoes almost no thermal expansion. The segments are cast in Germany, polished in France, and then mated with supports produced in the Netherlands before being transported to Chile.

Each is checked to ensure it has survived the trip unscathed. Ricardo Parra, an ELT engineer, likens the process to ringing a bell. Vibrations are induced in the glass, and measurements made by accelerometers in strategic locations. The segments are finished by coating them with several further layers of chemicals, including a 100-nanometre layer of silver that provides the reflectivity. (A nanometre is a billionth of a metre) That silver is protected from tarnishing by a layer of hard silicon nitride glass. Even so, the ESO thinks each segment will need re-coating every two years.

Getting all 798 segments to work together presents another set of difficulties. To produce a usable image each segment must be held in a precise position, with an accuracy of just tens of nanometres. Each is backed by a system of sensors and motors that can subtly deform the surface of the glass in order to correct for warping due to everything from small temperature variations to the changing angle of gravity as the mirror moves and tilts.

The primary mirror is just the first stop (see diagram 2). Light hitting it will be redirected towards a secondary and tertiary mirror, which are designed to correct various subtle optical defects. At around 4 metres across, each could be an impressive telescope main mirror in its own right.

The job of the fourth mirror is to counteract the vagaries of Earth’s atmosphere. The reason stars appear to twinkle when seen from the ground is that the atmosphere is constantly churning. Frédéric Gonté, an instrumentation engineer at ESO, compares the effect to peering into water. “Try to see the ground of the swimming pool, you can see it is moving,” he says. “The atmosphere is doing that to us.”

Space telescopes avoid this problem by flying above the atmosphere. Ground-based ones can rely instead on a technology called adaptive optics. This involves deforming the surface of a mirror to cancel out the distortions imposed by the air. The technology is not unique to the ELT. Many modern telescopes sport it, including one of the VLT’s big telescopes (it is being added to the other three). But the ELT’s sheer size makes it more susceptible to atmospheric distortion than smaller telescopes. More than 5,000 actuators behind the ELT’s fourth mirror will make tiny, rippling adjustments to its shape a thousand times each second. Without the adjustments, the ELT’s images would be hopelessly blurred.

Working out exactly how the mirror must be deformed, millisecond by millisecond, requires the presence in the sky of something whose shape is known in advance. Comparing what the telescope actually sees with what it should see reveals the state of the atmosphere at that particular moment, allowing the system to counter­act it. Often the object in question is a bright star near the object being studied. If no convenient star is available, though, astronomers can create an artificial one. “Laser guide stars” are made by firing four bright orange laser beams upwards so that they converge in a single point around 90 kilometres up, above the atmosphere’s thickest layers. Because the system knows exactly what the ersatz star should look like, it can make whatever mirror-twisting adjustments are needed.

The construction site of ESO’s Extremely Large Telescope at Cerro Armazones, in Chile’s Atacama Desert

Image: Eso

You might think that once the ELT is up and running, all other telescopes will be rendered obsolete. That is not really true, for even a machine such as the ELT cannot do everything. The twin Keck telescopes in Hawaii, for example, once the world’s largest, have mirrors that are a comparatively puny ten metres across. But they have the advantage of sitting on a substantially taller mountain, where the seeing is even better than it is in Chile. And the fact that there are two of them means they can serve twice as many astronomers at once.

The VLT, and other multi-mirror telescopes, can also use a technique called interferometry, a clever way of combining signals such that resolving power depends not on the size of individual mirrors, but on the distance between them. For the VLT that is more than 100 metres. On the other hand, that resolving power comes at the cost of a narrower field of view. The ELT is not competing with telescopes like the VLT, says Dr Gonté. “It’s completing.”

Ain’t no replacement for displacement

But when it comes to detecting the dimmest and most distant objects, there is no substitute for sheer light-gathering size. On that front the ELT looks like being the final word for the foreseeable future. A planned successor, the “Overwhelmingly Large Telescope”, would have sported a 100-metre mirror. But it was shelved in the 2000s on grounds of complexity and cost. The Giant Magellan Telescope is currently being built several hundred kilometres south of the ELT on land owned by the Carnegie Institution for Science, an American non-profit, and is due to see its first light some time in the 2030s. It will combine seven big mirrors into one giant one with an effective diameter of 25.4 metres. Even so, it will have only around a third the light-gathering capacity of the ELT.

A consortium of scientists from Amer­ica, Canada, India and Japan, meanwhile, has been trying to build a mega-telescope on Hawaii. The Thirty Metre Telescope would, as its name suggests, be a giant—though still smaller than the ELT. But it is unclear when, or even if, it will be finished. Construction has been halted by arguments about Mauna Kea, the mountain on which it is to be built, which is seen as sacred by some. For the next several decades, it seems, anyone wanting access to the biggest telescope money can buy will have to make their way to northern Chile.

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