Extremely large astronomical telescopes a step closer

Astronomers think big all the time: it’s their job. And on 13th December, at a meeting hosted by the Royal Astronomical Society in London, a group of them will juggle with some truly astounding large numbers. On this occasion, though, they won’t be discussing the distances to remote galaxies, but the phenomenal sizes of the telescopes they want to build so they can explore the universe to a level of detail previous generations of astronomers would never have dreamt possible. Announcing a significant development, Professor Gerry Gilmore of Cambridge University will tell the meeting that Europe’s astronomers have just agreed to join forces in a single project to design a new generation of ground-based optical/infrared telescopes, the Extremely Large Telescope.

The largest telescopes operating currently (the two Keck Telescopes in Hawaii) have segmented mirrors 10 metres across. Now, astronomers around the world are working towards a giant leap for astronomy – ’extremely large telescopes’ (ELTs) up to 100 metres across, 10 times bigger than the Kecks. According to Dr Adrian Russell, Director of the UK Astronomy Technology Centre (UK ATC) in Edinburgh, a telescope that large will take up more glass than has been used in all the telescopes built in the history of astronomy put together.

In Europe, several projects have been under study for some years, each aimed at identifying the key technological and organisational advances that must be met to achieve such a big step . From this month, the two main projects – Euro-50, led from Sweden, and OWL, led from the European Southern Observatory (ESO) – are joining forces with colleagues throughout Europe to create a single project, which will develop a proposal for substantial additional funding from the European Union.

“An ELT facility will revolutionise astronomy with its ability to collect light from faint objects and distinguish details in its images that have never been seen before”, says Eli Atad who is Head of the Applied Optics Group at the UK ATC and co-organiser of the meeting.

But ELTs are not just desirable, say astronomers: they are vital. The key to understanding a remote astronomical object is its spectrum. Collecting enough light to spread into a spectrum requires a much larger telescope than recording an ordinary image. “The largest telescopes we have today are struggling to obtain spectra of the faintest objects observable with the Hubble Space Telescope,” says Dr Tim Hawarden, Project Scientist for ELTs at the ATC and a speaker at the meeting. “Hubble’s successor, the James Webb Space Telescope, begins operation in less than 10 years. It will discover objects much fainter than Hubble can see and the problem of acquiring spectra will get ten times worse. To make the most of discoveries with the James Webb Space Telescope, it’s essential to have ELTs operating on the ground at the same time.”

As is the case with the Keck Telescopes, the mirrors of the Extremely Large Telescopes of the future will not be a single huge disc of glass, but will consist of thousands of hexagonal glass ’tiles’. “The technology exists”, says Eli Atad, “but the mass production of mirror segments is a challenge.”

“We have to prove that the key technologies are viable and affordable,” says Gerry Gilmore, who chairs the steering committee for the new combined European ELT project. “In particular, we have to demonstrate that the huge number of components needed for an ELT can be built taking advantage of industrial-scale efficiencies. The challenge is as much managerial and industrial as it is technical. But it must be met if Europe’s astronomers are to have the tools they need to keep abreast of international scientific developments.”

“The potential payoffs from ELTs can fairly be described as awesome” says Tim Hawarden. “For example, we may be able to see Earth-like planets, if there are any, in orbit around stars up to tens of light years away, and perhaps even find out what their atmospheres are made of. Just how large we can make the new giant telescopes is still a matter for debate, and that is part of what the meeting on 13th December is all about.”