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Telescope to probe deepest space

E-ELT will open new windows on the cosmos, distant past

by Robin Mckie

The Observer

Cerro Armazones is a crumbling dome of rock that dominates the parched peaks of the Chilean coastal range north of Santiago.

Even the views look alien, with the surrounding boulder-strewn desert bearing a remarkable resemblance to the landscape of Mars.

Dramatic change is on its way to Cerro Armazones, however — for in a few weeks, the 10,000-foot-tall (3,000-meter) mountain is going to have its top knocked off. “We are going to blast it with dynamite and then carry off the rubble,” said engineer Gird Hudepohl.

“We will take about 80 feet (24 meters) off the top of the mountain to create a plateau. And when we have done that, we will build the world’s biggest telescope there.”

Hudepohl is one of the European Southern Observatory’s most experienced engineers and was involved in the decapitation of another nearby mountain, Cerro Paranal, on which his team erected one of the planet’s most sophisticated observatories.

The Paranal complex has been in operation for more than a decade and includes four giant instruments with 8-meter-wide mirrors — known as the Very Large Telescopes (VLTs) — as well as control rooms and a labyrinth of underground tunnels linking its instruments.

More than 100 astronomers, engineers and support staffers work and live there. A few dozen meters below the telescopes, they have a sports complex with a squash court, an indoor soccer pitch, and a luxurious 110-room residence that has a central swimming pool and a restaurant serving meals and drinks around the clock.

Built overlooking one of the world’s driest deserts, the place is an amazing oasis. Now the European Southern Observatory, of which Britain is a key member state, wants Hudepohl and his team to repeat this remarkable trick and take the top off Cerro Armazones, which is 20 km distant. This time, they will construct an instrument so huge it will dwarf all the telescopes on Paranal put together, and any other telescope on the planet.

The European Extremely Large Telescope (E-ELT) and its 39-meter mirror will allow astronomers to peer farther into space and look further back into the history of the universe than any other astronomical device in existence.

Its construction will push telescope-making to its limit, however. Its primary mirror will be made of almost 800 segments, each 1.4 meters in diameter but only a few centimeters thick, which will have to be aligned with microscopic precision.

Why go to a remote wilderness in northern Chile and chop down peaks to make homes for some of the planet’s most complex scientific hardware?

The answer is straightforward, says Cambridge University astronomer professor Gerry Gilmore. It is all about water.

“The atmosphere here is as dry as you can get, and that is critically important. Water molecules obscure the view from telescopes on the ground. It is like trying to peer through mist. . . . If you build your telescope where the atmosphere above you is completely dry, you will get the best possible views of the stars — and there is nowhere on Earth that has air drier than this place. For good measure, the high-altitude winds blow in a smooth, laminar manner above Paranal — like slabs of glass — so images of stars remain remarkably steady as well.”

The view of the heavens there is close to perfect, in other words — as an evening stroll around the viewing platform on Paranal demonstrates vividly.

During my visit, the Milky Way hung over the observatory like a single white sheet. I could see the four main stars of the Southern Cross; Alpha Centauri, whose unseen companion Proxima Centauri is the closest star to our solar system; the two Magellanic Clouds, satellite galaxies of our own Milky Way; and the Coalsack Nebula — an interstellar dust cloud that forms a striking silhouette against the starry Milky Way.

None are visible in northern skies, and none appear with such brilliance anywhere else on the planet. Hence the decision to build this extraordinary complex of VLTs.

At sunset, each one’s housing is opened and the four great telescopes are brought slowly into operation. Each machine is made to rotate and swivel, like soccer players stretching muscles before a match. Each housing is the size of an apartment block. Yet they move in complete silence, so precise is their engineering.

Building the four VLTs, which have been named Antu (sun), Kueyen (moon), Melipal (Southern Cross) and Yepun (Venus) in the language of the Mapuche people of Chile, was a formidable challenge. Each has a giant mirror that is 8.2 meters in diameter but only 17 cm thick: any thicker, and the mirror would be too heavy to move and point.

Such thinness leaves the mirrors liable to deform as temperatures and air pressure fluctuate, however, and so each has 150 actuators fitted to its unpolished side. These push the mirrors to keep them within a few billionths of a centimeter of their proper shape. In addition, ESO astronomers use a laser-based system known as adaptive optics to measure turbulence in the upper atmosphere and to change each telescope’s internal mirror configuration to compensate for any disturbance they measure.

The result is a cluster of astronomical devices of incredible power and flexibility, one that has been involved in an astonishing number of critically important discoveries and observations over the past decade, as ESO astronomer Olivier Hainaut explains.

“Perhaps the VLT’s most spectacular achievement was its tracking of stars at the center of the Milky Way. Astronomers followed them as they revolved around . . . nothing. Eventually they were able to show that something incredibly small and dark and massive lay at the center of this interstellar waltz. This was the first time, we now know, that scientists had directly observed the effect of the supermassive black hole that lies at the heart of our galaxy.”

The VLTs also played a key role in providing observations that showed, from the behavior of distant supernovas, that the expansion of the universe was actually accelerating, thanks to the action of a force now known as dark energy.

This discovery won Saul Perlmutter, Brian Schmidt and Adam Riess the 2011 Nobel Prize in physics. And in 2004, the telescopes were used to make a direct observation of an exoplanet — a planet that orbits around a star other than our sun. It was another astronomical first. Until then, scientists had only been able to infer the existence of exoplanets from the way they affected the movement of their parent star or its light output.

These discoveries have only whetted astronomers’ appetites for more, however. Hence the decision to build the E-ELT.

Engineers have now completed a road to the mountain from Paranal, and on June 16 are set to begin blasting to remove the top from Cerro Armazones. Then they will start to build the E-ELT using 798 hexagonal pieces of mirror to create a mammoth device that will be able to collect 100 million times more light than the human eye.

When completed in around 2025, the 2,700-ton telescope will be housed in a 74-meter- high dome and operated by astronomers working 20 km away in Paranal. It will be the world’s biggest eye on the sky.

An indication of the E-ELT’s potential is provided by ESO astronomer Linda Schmidtobreick. “There are fundamental issues that only a telescope the size of the E-ELT can resolve,” she said. “Its mirror will have a surface area 10 times bigger than any other telescope, which means it will take a tenth of the time to collect the same amount of light — i.e., the same number of photons — from an object compared with these other instruments.”

For Schmidtobreick, this ability to collect light quickly is crucial to her research. She studies stars known as cataclysmic variables: pairs of stars in which one is pulling vast amounts of gas — mainly hydrogen — from its companion, a process that can trigger gigantic thermonuclear eruptions, sometimes within 30 seconds or so.

“With current instruments, it can take minutes or hours to collect light from these objects, which is too long to resolve what is happening,” said Schmidtobreick. “But with the E-ELT, we will be able to study many, many more cataclysmic variables because we will be able to collect significant amounts of light from them in seconds rather than minutes or hours and so will be to resolve their behavior.”

Simone Zaggia, of the Inaf Observatory of Padua, Italy, is another frequent visitor to Paranal, and has a very different reason for backing the E-ELT. He believes it will play a vital role in the hunt for exoplanets — in particular, exoplanets that are Earth-like and could support life. “At present, our biggest telescopes can only spot really big exoplanets, giants that are as big as Jupiter and Saturn,” he said.

“But we really want to know about the smaller worlds that make up the solar systems in our galaxy. In other words, we want to find out if there are many Earth-like planets in our part of the universe. More importantly, we want to find out if their atmospheres contain levels of oxygen or carbon dioxide or methane or other substances that suggest there is life there. To do that, we need a giant telescope like the E-ELT.”

This point is backed by Gilmore. “We can see exoplanets but we cannot study them in detail because — from our distant perspective — they appear so close to their parent stars. However, the magnification that the E-ELT will provide will mean we will be able to look at them directly and clearly. In 15 years, we should have a picture of a planet around another star, and that picture could show its surface changing color just as Earth does as the seasons change — indicating that vegetation exists on that world. We will then have found alien life.”