The sky's the limit: construction approved for the Giant Magellan Telescope

June 2015 marked a major milestone in the field of astronomy, with construction approval announced for the highly anticipated Giant Magellan Telescope (GMT). Unanimously approved by 11 collaborating institutions, including the Australian National University (ANU) and Astronomy Australia Limited (AAL), the GMT will be the biggest optical telescope in the world.

Wendy Freedman, the chair of the Giant Magellan Telescope Organization (GMTO) board of directors, said the project was originally announced in 2003 as the brainchild of a small group of US institutions. Since then it has grown to a US$1 billion international collaboration, with partners in Australia (following a $93 million contribution from the Australian Government), Brazil, the US, the Republic of Korea and Chile, where the telescope will be hosted.

The GMT aims to be the first of the new generation of extremely large telescopes. With an optical surface of around 25 metres, it will focus more than six times the amount of light of the current largest telescopes into images up to 10 times sharper than those of the Hubble Space Telescope (HST). This will enable astronomers to observe extremely distant and ancient galaxies whose light has been travelling to Earth since just after the Big Bang, 13.8 billion years ago.

“Every photon is precious, particularly so when you’re trying to look at the very distant reaches of the universe,” explained Dr Kim-Vy Tran from Texas A&M University. “You have to remember that these photons have been travelling for billions of years, so by the time they get to your telescope, there are precious few of them. The larger the telescope, the more photons you can collect. The more photons you can collect, the more information you can get.”

Professor Dennis Zaritsky, from the University of Arizona, said the current largest telescope corresponds to a piece of glass that’s about 11 metres. The GMT, on the other hand, will comprise seven mirrors — each of which contains 17 tonnes of glass — that will be the equivalent of one 25-metre-sized mirror.

The creation of these mirrors is being carried out at the University of Arizona’s Steward Observatory Mirror Lab. In a year-long process, each mirror is melted in a giant oven and spun into the rough shape it needs to be. Once it is cool enough to take out of the oven, it is ready for three years of surface generation and polishing.

The process has been particularly challenging for the GMT mirrors, explained Professor Zaritsky, because the team usually creates telescope mirrors which are “centred in the telescope, so they’re symmetrical”. But the GMT mirrors are “all over the place”, he said, so they have “a very off-centre shape”.

“Basically, anywhere you’re at, polishing the mirror has a different shape to anywhere else, and so the polisher has to keep adjusting and know exactly where it is on the mirror.”

Australia will also be making a contribution to the telescope’s construction, with the ANU Research School of Astronomy and Astrophysics (RSAA) charged with designing and building the GMT Integral Field Spectograph. This will record spectra from each point across the field of view simultaneously, taking advantage of the telescope’s light-collecting power and high resolution.

“If we have a spectrograph on it, we can spread out the light of an object into a rainbow and analyse the velocities and the composition,” said Dr Kevin Krisciunas from Texas A&M University.

ANU instrument scientists will also develop and build key elements of the crucial adaptive optics system for the GMT. Adaptive optics removes distortions in images, such as twinkling stars, caused by turbulence in the Earth’s atmosphere.

“We have to make this optic precise enough so that when the light travels 5, 10 billion light years, and comes in and hits our telescope, we don’t scramble and lose that information that’s travelled so long,” said GMT Director Dr Patrick McCarthy. “We have to make these large optics to a 20th of a wavelength of light, even though it’s 25 metres across. It’s a challenge of about one part in 10 billion in terms of precision manufacturing, so it’s an extraordinarily challenging process.”

The telescope will be based at the Carnegie Las Campanas Observatory in Chile’s Atacama Desert — one of the highest and driest locations on Earth — where it will experience clear skies for more than 300 nights a year. There, it will be housed in a 22-storey building which “has to rotate to allow you to move to different parts of the sky as you’re looking out with the telescope”, said Freedman.

Additionally, noted Dr Robert Kirschner from the Harvard-Smithsonian Center for Astrophysics, the ground-based telescope has the advantage of being able to have its instruments easily upgraded. “So I think that even when we build the telescope, that won’t be its final form,” he said. “Those instruments will eventually be replaced by better ones that use the technology that’s developed over the period from now to then.”

Once fully operational in 2024, the GMT will be used for everything from understanding how the first stars and galaxies formed, to searching for new planets, to measuring the masses of distant black holes, to discovering the nature of dark matter. According to Freedman, “We will witness, directly, the first galaxies forming, the first supernovae forming, the first black holes forming, and see how the universe that we’re living in now… came to be.”

“The GMT will play a leading role in the international race to identify planets orbiting stars near the Sun that could host life and potentially reveal the signatures of biological processes,” said AAL representative Professor Chris Tinney. Dr Kirschner added that the massive size of the GMT would be particularly advantageous for the characterisation of other planets, with its large collecting area meaning “you gather information very rapidly”.

But perhaps the most exciting aspect of the GMT is the possibility of new discoveries of which we haven’t yet conceived, some of which will be enabled by the development of new instrumentation in the future. According to Dr McCarthy, “As we try to track down the mystery objects, the gamma ray bursters, the neutron stars, things that we haven’t discovered yet… that’s where we’ll stumble upon these new discoveries.

“Neutron stars were discovered almost by accident,” he said. “A whole host of phenomenon were discovered when we could first look into the infrared. So I expect that, as we sleuth along trying to solve the mysteries that we can see, we’ll bump into new things.”