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ScienceWise - Autumn 2011

Taming the jitters

Article Illustration
Dr Rod Conan
Article Illustration
Adaptive optics allows large telescopes to achieve their theoretical resolution (right) in spite of the blurring effects of the atmosphere (left).
Article Illustration
Adaptive optics is not just about achieving the diffraction limited resolution. This double star is resolved in both cases however the faint exoplanet is lost in the halo of the left image because a significant amount of the light fails to be concentrated into the Airy disc.

Developing the adaptive optics for the world’s most powerful telescope

Although it’s perhaps not widely known, Australia enjoys an excellent reputation in the field of astronomy, rating very highly on the world stage in terms of quantity and quality of scientific papers in this area. Fortunately, the future of astronomy here also looks promising as a major grant through the Government’s Education Investment Fund in 2010 has effectively secured Australian partnership in what will be the worlds most powerful telescope, the Giant Magellan Telescope (GMT). Professor Harvey Butcher, Director of the ANU Mt Stromlo Observatory says “Securing our place in the GMT partnership is a really important step in Australian astronomy. It guarantees that the next generation of astronomers will have good access to the most powerful instruments that are likely to be generating the cutting edge science of the 21st century.”

With the partnerships and finances on a solid footing, work is now progressing with the construction of the seven massive 13 ton segments that will create the GMT’s 24 metre primary mirror. This will give it twice the resolution and four times the light gathering power of the largest existing telescopes enabling astronomers to peer deeper into the universe than ever before. GMT will be sited in the thin clear air of  Las Campanas Observatory high in the mountains of Chile. However even at excellent observing sites such as Las Campanas, the Earth’s atmosphere with its turbulence and convective flows still distorts the light of stars considerably.

To get around this, modern telescopes use adaptive optics; deformable mirrors that precisely cancel out the wavefront distortion enabling a telescope to resolve the maximum detail that it’s aperture will allow.  Dr Rodolphe Conan of the ANU is an acknowledged expert in the field of Adaptive optics and is currently the lead scientist working on the design of an adaptive optics package for the GMT.

There are two major parts to the adaptive optics systems of large modern telescopes. Firstly as the telescope moves to different points in the sky the huge primary mirrors that can weigh several tons each, tend to bend and sag under their own weight so they have to be mounted on actuators that gently push and pull the back of the mirror to compensate for this. The movements are slow so the compensation can also be slow.

The second more difficult problem for the adaptive optics to solve is that of the distortions created by the Earth’s turbulent atmosphere. Even at the best observing sites the seeing is limited to a fraction of an arc second. If left uncorrected, this would limit the resolving power of even the largest telescopes to that of the better backyard amateur telescopes.

Because the atmospheric structure changes thousands of times each second, it would be almost impossible to deform the massive primary mirrors quickly enough to compensate. So on the GMT, it will be the adaptive secondary mirrors that will accomplish this task. “These are still large mirrors, “Dr Conan explains, “but by mounting a fairly thin reflective surface over a backing plate with 672 individual actuators, we can deform them at very high speeds.”
But how do you know which way to bend the mirrors to compensate for the atmospheric distortions? One way is to use a natural guide star – a star that happens to be in the field of view of the object you’re looking at. You know the star is supposed to be an infinitesimally small point so you can work out from its actual shape and position what the atmosphere is doing and apply a correction. However, in most cases there isn’t a convenient natural guide star in just the right position so scientists use lasers to create artificial guide stars by exciting atoms such as sodium high in the upper atmosphere.

This isn’t as straight forward as it sounds though. The same distortions that deform the starlight coming down deform the laser on the way up causing the guide star to wobble about from side to side. “You need to know which way your artificial guide star is being moved so that it can become a useful reference,” Dr Conan says, “ and we generally use a natural guide star to calculate this. In this role the brightness of the natural guide star isn’t nearly so critical so we can generally find one that will work for us.”

Once you know how your laser guide star is moving you can compensate for that and begin using it to iron out the ripples in the incoming starlight. However on GMT there will be six laser generated artificial guide stars. There are also seven secondary mirrors each with 672 movable actuators each one of which needs to move independently a thousand times every second. “Developing a suitable algorithm to correct such a large wavefront and harnessing enough computing power to drive all those actuators is massive challenge.” Dr Conan says.

However this is really only the beginning of the problem. “Achieving a first order correction to the wave front so we can achieve the telescope’s diffraction limited resolving power on something like a double star is the relatively easy part. “ He says, “It’s getting the higher order terms right that’s really hard and it’s these which really concentrate all the light into that central point and that’s what really gives the image it’s contrast, quality and scientific usefulness.”

This level of contrast in the image is of vital importance in areas like exoplanet research where the star around which the planet orbits is many thousands of times brighter than the planet. Even though the telescope can theoretically resolve the two, the planet will be invisible if it lies within the general halo of stray light surrounding the star. This is exactly the situation with SiriusB, even though any small telescope can theoretically separate the two, the hugely bright Sirius A swamps its faint companion making it invisible in all but the best telescopes.

Dr Conan and his team expect the design phase of the GMT adaptive optics module to be complete by the end of 2011, at which point there will be a scientific review before proceeding to build the components of the system. The completed telescope is expected to see first light in around 2020.

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