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New vision for AAT

An infrared camera that provides new views of astronomical objects has now completed its testing runs on the Anglo-Australian Telescope. Already, it is unveiling to astronomers previously unsuspected features in the sky.

The new instrument, named IRIS, relies on a special sensor that detects radiation beyond the red end of the visible spectrum, in the region known as the infrared.

Although infrared detectors have been in use for two decades, astronomers have only been able to measure the total amount of radiation falling on a single detector, much as the exposure meter in a camera sums all the light it receives but has no knowledge of the scene it is viewing. This new detector resembles the light sensors in video cameras, and produces pictures. At the focus of the Anglo-Australian Telescope it shows the infrared radiation from distant stars, nebulae and galaxies.

The detector was manufactured by the Rockwell Science Center in California, and because of its advanced technology, export of these detectors is strictly controlled. It was made possible in this case through COCOM, the co-ordinating committee on multi-lateral export controls. The detector used in IRIS is the first of its kind to be exported from the USA.

Although the infrared sensor is central to the working of IRIS, a complex and sophisticated instrument has been built around it. This instrument provides the working environment for the detector, including cooling to -196°C. It allows the astronomer close control of the observations, such as the colour of filter used and the field of view covered. IRIS can also take spectra of the stars and galaxies being observed, which will improve our understanding of their physical and chemical state.

Celestial objects look very different in the infrared. In visible light, clouds of dust grains are often so dark that they obscure what lies behind them, like smoke from a chimney. These dust clouds become transparent to infrared radiation, and allow scientists to view objects that are otherwise hidden. Often this dust is warmed by nearby stars so that it gives off heat, and actually appears bright to the infrared detector.

Objects can also take on a different appearance because of their particular chemical compositions. For example, IRIS sees the planet Jupiter very differently to optical sensors because regions of methane gas in its atmosphere absorb the infrared radiation. Another example is cool hydrogen, the most abundant gas in the universe, which appears bright in the infrared whereas it is quite invisible to normal optical detectors. New insights are being gained into how and where this hydrogen exists.

IRIS will also be used to search for objects which have no obvious visible counterparts. These include extremely distant galaxies and the so-called brown dwarf stars predicted to exist in large numbers, but so small and cool that they scarcely give out any light.

IRIS has been tested three times, and has already produced some unexpected views that have excited astronomers working with the data. On March 1 it will become available for the first time to all scientists who use the Anglo-Australian Telescope. Astronomers have been waiting a long time for this latest infrared camera, and demand for time on IRIS has already proved much higher than anticipated. In the first week of March IRIS will be used by four different groups of astronomers from Australia and the UK.

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© Anglo-Australian Observatory 2004, PO Box 296, Epping NSW 1710 Australia

Roger Bell rb@aaoepp.aao.gov.au
01 Jan 1998