Credit: Giant Magellan Telescope - Carnegie Observatories
ASK ASTRONOMERS WHERE the next frontier in space research will be and the answer is unanimous: a giant telescope due to be built on a remote mountain top in Chile.
With up to 30 times the resolving power of current telescopes, the Giant Magellan Telescope promises to answer some of astronomy's most fascinating questions.
The history of astronomy is marked by technological leaps that led to new discoveries. Galileo revolutionised astronomy with his 37-millimetre telescope, as did Edwin Hubble with his two-metre optical telescope, and his namesake, the Hubble Space Telescope, with its 4.2-metre telescope and view from space.
New telescopes are being built all the time. So why build another, and what's causing the buzz?
A world consortium of astronomy organisations plans to build the Giant Magellan Telescope (GMT) on a mountaintop near the village of Residencia in Chile's Atacama Desert. It will cost US$600 million and should be ready by 2016.
With a resolving power ten times sharper than Hubble and five times sharper than its replacement, the James Webb Space Telescope, the GMT is a big step up in terms of power. Composed of an array of seven mirrors, each 8.4 m in diameter (some of the largest ground-based telescopes currently have a diameter of 10 m), it will have the capacity of a telescope with a diameter of 24.5 m – far larger than any telescope built so far.
"THE BIGGER THE BUCKET, the more rain you catch, and the bigger the telescope, the more light you catch," says astronomer Matthew Colless, director of the Anglo-Australian Observatory in Sydney, who earlier this month gave a talk on the GMT to a bunch of astrophysicists at the Australian Institute of Physics Congress in Adelaide.
"The GMT can collect five to six times as much light as current telescopes. It will also produce sharper images, and be able to focus on objects two-and-a-half to three times smaller than current telescopes," he says.
The key is to use both capabilities – collecting lots of light and resolving very small objects – together. This will give the GMT 30 times the power of current telescopes. And that's a pretty big jump. It will, for example, be enough to allow the GMT to look the Universe's first stars, follow the formation of galaxies within a few million years of the Big Bang, and for the first time look directly at extrasolar planets, for a start.
"We are confident that it will have a great impact on our understanding of extrasolar planets, black holes and early star and galaxy formation," says astrophysicist Pat McCarthy, from the Observatories of the Carnegie Institute in Washington. "The great advantage offered by the next generation of telescopes is the combination of enormous collecting area and great angular resolution.
"Current infrared space telescopes, such as the Spitzer Telescope, are limited by the poor angular resolution in the infrared," he adds. With its large diameter, the GMT will have the high angular resolution needed to probe the structure of dense star-forming regions in the infrared and see directly into regions of massive star formation."
CURRENTLY, PLANS FOR the telescope, which views stars in visible, near and mid-infrared light, are at the design phase. The challenge in building big telescopes like the GMT is keeping them stiff – so they don't wobble in the wind. The increased mass also means that heat from the telescope can blur the images.
To solve these issues, most of the telescope will be made from steel, which is stiff, but the steel will be kept extremely thin to minimise heat build up. Carbon fibre and epoxy composite materials will help keep the weight low while retaining the stiffness essential to maintaining good image quality in high winds.
Like other top-notch, ground-based telescopes, the GMT will rely on adaptive optics to remove the twinkling effect you get from looking at stars through the Earth's atmosphere.
Adaptive optics works like this: the telescope beams a beacon of sodium light 90 km into the atmosphere, and the telescope focuses on this fixed (non-twinkling) point of light. Images of stars can then be corrected for blur by using this fixed-light reference.
Because the GMT has a very large aperture (which like widening a camera aperture reduces its depth of field, or the portion of a scene that appears sharp in an image), there may be discrepancies in how the telescope focuses on the nearby guide laser point and natural stars.
To combat this, the adaptive optics on the GMT uses six lasers to create a mini-constellation, widening the field of reference for the telescope to focus on and fine tuning its resolving capability.
SO GOOD WILL its resolving power be, that the GMT will for the first time look directly at the light from planets around other stars – which can be one billion times fainter than their parent stars. We currently know of more than 300 extrasolar planets, detected using various techniques.
In November, the GMT's older brother and sister, the twin 6.5-metre Magellan telescopes, zoomed in on the thermal emissions from one such planet. Hubble has detected methane, water vapour and carbon dioxide from the atmosphere of another large, hot planet – a so-called hot Jupiter, the easiest class of extrasolar planet to detect.
"Our hope is that GMT can detect planets using direct imaging and radial
velocity techniques and improve our understanding of systems already
known," says astrophysicist Scott Kenyon from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts. "As an example, suppose we know a system with a hot Jupiter.
A GMT deep image would allow us to detect cold Jupiters farther from the star. Or if we image a candidate molten Earth, we can look for radial velocity variations to estimate its mass. And if the planet is bright enough, get spectra to learn something about its atmosphere."
Kenyon says the GMT and other large telescopes will open up a new era of comparative planetology. "By looking at many planetary systems at many different ages, we can make a kind of 'movie' of planetary system formation and evolution. The GMT will make a huge contribution to this effort, and in this way, we will learn about our origins."
ANOTHER MAJOR GOAL for the GMT is the detection of the end of the 'dark ages', when the first light from stars and quasars reionised the universe, changing atoms into excited or reactive states. Then there's the question of dark energy.
"The GMT will allow us to look further than we have ever been able to before. This will allow us to track dark energy further back into time, and give us the opportunity to use methods and techniques that might not otherwise be practical in the nearby universe," says astrophysicist Brian Schmidt from the Research School of Astronomy and Astrophysics at the Australian National University (ANU) in Canberra.
The ANU is part of the consortium jumping on board to run the GMT and the rest of Australian astronomy is keen to follow. Australia plans to buy 10 per cent of the telescope, which would guarantee the Australian astronomy community 36.5 viewing nights each year.
"It would be the cherry on top of Australia's current science capacity," says Colless.
Heather Catchpole is a Sydney-based science writer and regular contributor to Cosmos.
