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ScienceWise - May/Jun 2008

New window on the universe

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The Giant Magellan Telescope will be built by a consortium of The Australian National University, Carnegie Institution of Washington, Harvard University, Smithsonian Astrophysical Observatory, Texas A&M University, University of Arizona, The University of Texas at Austin. (Giant Magellan Telescope - Carnegie Observatories.)
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The location for the GMT at Las Campanas Observatory in Chile.
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Conventional observatory domes tend to trap warm air which creates turbulent eddy currents in their apertures. This warm turbulent air degrades a telescopes ability to resolve fine detail. Open structures take less time to come to equilibrium and often offer superior views. It’s a bit like sleeping with your bedroom window open on a hot night verses sleeping on the open verandah.
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Imagine a structure as big as the Sydney Opera House, but one that can rotate and point a thousand ton telescope to any point in the sky with a precision of a millionth of a degree.
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Before an adaptive optics system can be designed, it’s important to know what the local atmospheric conditions are like. Dr Charles Jenkins from the Mt Stromlo Observatory has developed a system for measuring the extent of turbulence at different heights. The image of a closely separated double star such as Alpha Centauri is monitored in real time whilst an aperture mask blocks pert of the incoming light. By the use of trigonometry, and monitoring which parts of the image of the two stars move in sync and which do not, it is possible to determine the altitude of the turbulence. The plot above shows data from the Las Campanas site. There is a narrow band of ground turbulence then for most of the time, very little distortion from the remained of the atmosphere.

Creating the world’s most powerful telescope

Following the destruction of the Mt Stromlo observatory in the 2003 bushfires the then Director, Professor Penny Sackett was faced with a difficult decision; what was the best way forward?

Her solution was bold and innovative. Rather than rebuild facilities on the increasingly light polluted site at Stromlo, she opted for two major new telescopes. The Skymapper, to be built under the pristine skys of University’s Siding Spring Observatory – this instrument will complete a survey of the entire southern sky in unprecedented detail. And second to enter into a partnership with a consortium of other leading institutions to create what will be the most powerful telescope the world has yet seen; the GMT or Giant Magellan Telescope.

The advanced capabilities of the GMT will come from its enormous aperture. Seven 8.4 metre diameter mirrors will be used in combination to create a single gigantic 24.5 metre optic. Large size is important because the power of a telescope is dictated by the aperture of its primary mirror. The bigger the mirror the more light it collects and therefore, the fainter the objects it can see. But a large mirror also has another less obvious advantage. The laws of physics limit the maximum resolution of any optical device. Diffraction from the edge of the aperture defines the smallest spot that can be focussed at any given wavelength – the so called Airy Disc. For a typical amateur telescope with a 200mm aperture this limit is roughly half an arcsecond, or 1/7200th of a degree. This means that the telescope can just resolve two stars separated by this amount (or see planetary features of this size). The Hubble Space telescope has ten times this aperture and hence ten times this resolution. That might not sound like a huge improvement, but because the finer resolution is along both x and y axis, the result is squared. In fact it’s the same difference in imaging capability as that between a 5 mega pixel digital camera and a 50k web cam. The GMT has the potential to achieve ten times more resolution than Hubble, but to do this, the problems introduced by the earth’s atmosphere have to be overcome.

Air is almost totally transparent at optical wavelengths but its refractive index varies with temperature and pressure. Since the atmosphere is a seething mass of turbulence and winds, the effect is to distort incoming starlight. It’s as though we are looking up from the bottom of a swimming pool. We see outside but the image is rippling and moving constantly. Astronomers call this natural seeing, and for a typical location, it limits the resolving power of any telescope to about one arcsecond. If left unchecked this would mean that the largest professional telescope would have no better resolution that the average amateur scope. However in the past couple of decades, scientists have been developing adaptive optics that cancel out these atmospheric distortions.

The basic principle of adaptive optics is to monitor the light coming from a star close to the object one is observing. In the absence of the atmosphere, the image should be an almost stationary, infinitesimally small point of light. However turbulence and distortion smears this out into a dancing blob. By using multiple actuators to distort the surface of a flexible optical element in the telescope and intelligent computer control, it is possible to cancel out most of the distortion. Of course as with all corrective systems, the better the material you begin with, the better the outcome. For this reason the GMT will be sited at Las Campanas Observatory in Chile. This site has high elevation, and very steady skies with a natural seeing often approaching 0.4 arcseconds – some of the best in the world.

Professor Sackett explains, “The GMT offers some very exciting research possibilities because of both its enormous light grasp and its resolution”.

Although galaxies can be seen at enormous distances, individual stars can only be detected relatively close by with existing telescopes. The GMT’s greatly increased light collecting area will enable it to observe individual stars up to almost twice the range presently possible which equates to an eight fold increase in the volume of space and hence number of stars observable. This is important in refining our models of the initial mass function – the proportion of stars formed with each particular mass. Being able to probe further and deeper into the universe is also valuable to study the process of galaxy assembly and better understand the phenomena of dark matter and dark energy (the force that appears to be causing the expansion of the universe to accelerate).

Once the adaptive optics are installed and working properly, the phenomenal resolution of the GMT should also enable it to resolve some planets beyond the solar system. Although astronomers have detected the presence of many such planets, to date it has not been possible to see them directly.

Application of the GMT won’t be restricted to extra-solar planets. With ten times the resolution of Hubble, it will be able to make contributions to solar system science too, producing images comparable to some of those gathered by space probes.

New generation telescopes like the GMT make the early twenty first century an exciting time to be involved in astronomy.

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