E-ELT mammoth telescope, Chile

Mega-telescopes sprouting in Chile and Hawaii

Bigger is better, at least when it comes to peering into the infinity of space. Now three teams of astronomers are in a race to build the three largest telescopes in the world.

The Atacama Desert in northern Chile is one of the most alien and hostile places on Earth. Some of its regions are so lifeless that when researchers did the same tests there as the Viking missions conducted on Mars, they failed to detect any DNA in its soil. But one life form that is greatly nourished by such barren conditions is the astronomer. The aridity and altitude make this otherworldly place perfect for looking at actual other worlds in space.

This is why in October 2003, telescope research scientist Warren Skidmore and his colleagues dared to enter the Atacama with few resources and a grand mission: Find the best site to build the Thirty Meter Telescope, or TMT. “We were living in shipping containers for weeks on end,” he recalls. But while he talks about how to set up astronomy equipment without electricity and how they struggled to buy the daily rations of water, he sounds as if he had the time of his life.

Presumably, the awareness of contributing to something so incredibly big sweetened the drudgery. Designed by an international consortium and expected to ‘see first light’ in the early 2020s, the TMT will dwarf any telescope currently in operation on Earth. With a diameter of 30m, its primary mirror will be three times as large as current cutting-edge telescope mirrors and 200 times as sensitive.

As if this wasn’t enough, giant telescopes, like London buses, seem to come in threes. The completion of two more telescopes of similar size, the Giant Magellan Telescope (GMT) and the European Extremely Large Telescope (E-ELT), should more or less coincide with the TMT’s. For all three colossi, most of the money is secured and construction has recently commenced.

However, the members of this trio of next-generation telescopes will considerably differ from each other in design. While everybody agrees that each has its strengths and weaknesses, the sweat and tears that the individual teams put into building these incredible machines, over a period of 15 years or more, naturally make them very proud of ‘their’ telescope.

Astronomers are on tenterhooks to get access to the biggest telescopes the world has ever seen, and each team is working hard to point them towards the sky as soon as they can.

Why size matters

Bigger is usually better in astronomy, especially when it comes to primary mirrors. These collect and bundle the incoming light, sending it off in an optical path that eventually ends up in the observer’s eye or the telescope’s instruments. Primary mirrors act like light buckets.

“The bigger your light-collecting area, the more light you can collect per unit time. And that means that you can just see fainter objects,” explains astronomer Joe Liske from the European Southern Observatory, which is building the E-ELT on top of Cerro Armazones in the Atacama desert in Chile.

Making telescopes bigger also means getting sharper images, crucial for separating objects in the sky that are very close together, says Liske. In short, with the increased sensitivity of larger telescopes, sharper images of fainter objects can be obtained faster than with smaller telescopes.

A bigger primary mirror alone just isn’t enough. In ground-based astronomy, the turbulence in the atmosphere distorts the view and blurs the images. “It’s a little like sitting at the bottom of a swimming pool and looking up into the outside world,” Liske explains. This is also why stars in the night sky seem to twinkle.
To deal with the problem, astronomers use so-called adaptive optics technology on telescopes that cancels out the deformations observers get from the atmosphere in real time by distorting mirrors in the optical path in just the right way.

Anybody out there?

Thanks to their size and adaptive optics, astronomers hope that the GMT, TMT and E-ELT will help them get a grip on a whole bouquet of scientific questions, from the composition of small objects in the Kuiper belt to supermassive black holes in distant galaxies.

To investigate objects close to Earth, telescopes must have good capabilities in the visible range of wavelengths. But this changes for distant objects. “The universe is expanding, if you want to look at the same type of object and do the same type of measurement for something much further away, you have to move to a longer wavelength,” says Skidmore. That means that the telescopes should be able to deal with infrared wavelengths, which is also the regime of thermal radiation.

Planets emit most of their light in the thermal infrared, so this regime is essential when it comes to exoplanets – planets beyond our solar system – says Andrew Skemer, Hubble fellow at the University of Arizona. The pie-in-the-sky goal is to find one with an atmosphere that would allow for extra-terrestrial life.

Exoplanets are the astronomical needle in a galactic haystack: small, faint and close to other, brighter objects. One way to spot an exoplanet and analyse its atmosphere is to observe its transit in front of the parent star, but this is a notoriously difficult task.

“The planet’s atmosphere does not remove a lot of light from the star’s spectrum, so you are looking for a small signal against a big signal,” says GMT Science Advisory Board member Anita Cochran of the University of Texas.

This is where the size of the GMT, TMT and E-ELT, along with first-class spectroscopic instruments, may make a real difference.

Design choices

While the three colossi have similar dimensions, each telescope is optimised for a specific set of scientific objectives, something telescope designers refer to as the ‘science case’. And although the science cases of all three of them overlap, their priorities differ. “It’s not so much a matter of ‘my design is better than yours.’ It’s more that ‘my design is better for this particular thing that my community of users wants to do’,” explains Liske.

Of the three, the E-ELT will have the largest primary mirror, 39m in diameter. The dimension wasn’t pulled out of thin air, though: “It’s exactly the science case to study the extrasolar planets,” explains Liske. In other words: we need a mirror of at least this size with a telescope set-up like the E-ELT to study planets revolving around stars other than our own sun.

Since making a single mirror of that size today is impossible, the mirror will be made of 798 hexagonal segments, each 1.45m in diameter. The E-ELT is also the most expensive: the estimated cost is $1.4bn.

The TMT team plans to have a similarly segmented primary mirror, but with a total diameter of 30m. While on the whole the TMT is not considerably cheaper than the E-ELT, the team believes that its choice of primary mirror hits a ‘sweet spot’ in terms of cost-effectiveness, especially at near-infrared wavelengths. “It’s a trade-off between the increase in scientific capability versus the cost,” says Skidmore.

The GMT is the odd one out: One central mirror will be surrounded by a ring of six, all of them 8.4m in diameter. Producing such huge mirrors is a daunting challenge.
“GMT’s technology gets hard building the mirrors in the first place,” says Cochran. But compared with the other designs, it’s much easier to keep them aligned, she adds. Plus, the GMT is the cheapest with an estimated cost of $1bn.

The optical path that the light takes before reaching the instruments is another aspect where the three designs diverge. The E-ELT has three curved mirrors and two folding mirrors, which house the adaptive optics system. “The E-ELT was chosen to be optimised for image quality and for really getting the best adaptive optics,” Liske says. He explains that this design gives superb image quality over the entire field-of-view, about a third of the full moon.

This also means that in the E-ELT the light is reflected five times before it reaches the instruments, which brings some disadvantages. “Every time you have a reflection, you lose a few per cent of light,” says Cochran.

This is something the GMT and the TMT aim to address – both went for a design with as few reflections as possible. In the TMT, the light will get reflected and then will reach a bungalow-sized adaptive optics system outside the telescope, while the GMT will incorporate the adaptive optics directly into its secondary mirror.

This will come in handy when analysing the thermal-infrared emissions of exoplanets. Skemer explains that telescope mirrors, because they are warm, glow like light bulbs at infrared wavelengths, which disturbs the measurement.

“There is nothing we can do about that other than have as few mirrors as possible,” he says. Hence the GMT’s adaptive optics system will be as good as it gets when it comes to correct blurring at thermal infrared wavelengths.

Location, location, location

A choice almost as crucial as the design is the site of the telescope. While the GMT and the E-ELT are going to be built in the Atacama Desert, the TMT team – after months of checking out various possible sites in Chile, Mexico and Hawaii – has opted instead for the top of Hawaii island’s Mauna Kea mountain.

From his former career as a site tester, Skidmore says he knows all the pros and cons of choosing a site. “For the TMT, there was a desire to get access to the very best conditions,” he says and explains that for about 10 per cent of the year Mauna Kea’s conditions are absolutely incredible while on average it is similar to the sites in Chile. The Chilean sites, on the other hand, are more consistent.

This choice has also meant controversy. Native Hawaiians see the telescope, which will be bigger than several already built on Mauna Kea, as the latest violation of a site they consider sacred, and have been protesting against its construction. Protesters have been camping on Mauna Kea for months, which led to a construction stop in April. The promise of the governor of Hawaii to dismantle three or four smaller telescopes on Mauna Kea over the next decade helped resolve the conflict, and construction resumed in late June.

Since Mauna Kea is a conservation district, the TMT team had already gone through a seven-year permissions process, working out the environmental impact in detail and in 2013 got the approval from Hawaii’s Board of Land and Natural Resources to proceed with the construction. In hindsight, this extra bit of thinking that the TMT had to frontload has led to a very efficient and well-rounded concept, says Skidmore, adding that it might give the TMT advantages in the long run. Comparing the TMT to the other two giants, he boldly states: “My expectation is that we are going to be the first telescope on the sky”.

Cochran disagrees. “The GMT is probably going to get there first, by a very small amount,” he asserts.

According to Liske, the relationship between the three teams could be termed ‘coopetition’ – somewhere between cooperation and competition. For example, the E-ELT’s and TMT’s teams cooperated on the primary mirror design. “The segment size is the same, and that was strategic,” says Skidmore. After all, making more of the same specialised mirror reduces the price.

When it comes to crossing the finish line, competition prevails, says Liske. “Whoever gets to be first on the sky will be able to pick off some low-hanging fruits, that’s for sure.”

Image credits: ESO, TMT International Observatory, GMT, Jarron Leisenring/LBTO

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