Credit: Giant Magellan Telescope - Carnegie Observatories

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."

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