What is Stellar Interferometry?

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With very few exceptions, the diameters of stars subtend angles that are too small to be resolved - even with the latest generation of 8 to 10m aperture optical telescopes employing modern Adaptive Optics techniques to produce the sharpest possible images. Despite the advantage of being in space, the smaller 2.3m Hubble Space Telescope can do only a little better than ground-based optical telescopes, with a resolution of about 0.06 arc seconds.

Sirius, for example, is the brightest tstar in our sky, largely because it is amongst the closest stars. Its apparent size is 0.05 arc seconds. This is the same size as an Australian $1 coin (or a UK 1 pound coin or a US quarter) seen from a distance of about 100 km. For comparison the Sun and Moon are around 1800 arc seconds in size - about half a degree.

To measure these angles it is generally necessary to employ interferometric techniques in which starlight collected at two or more widely separated apertures is combined "coherently". Optical astronomers first attempted this around 80 years ago with only modest success. The longer wavelengths of radio waves allowed radio astronomers to begin using this technique 50 years ago and it now forms the basis of modern radio astronomical imaging.

AT Antennae of the Australia Telescope Compact Array (ATCA) at the Paul Wild Observatory near Narrabri - on the same site as SUSI.

Optical astronomers have applied the general principles of interferometry to techniques such as speckle interferometers and non-redundant mask interferometers (such as the IoA's MAPPIT) which are attached to existing telescopes. SUSI is in the vanguard of a new generation of Optical Stellar Interferometers which are independent of conventional astronomical telescopes.

A popular level article on stellar interferometery is 'A sharper view of the stars' which appeared in the March 2001 issue of Scientific American. A longer discussion of Stellar Interferometry can be found at Astronomical Optical Interferometry described by Bob Tubbs, Cambridge University.

A list of Interferometer Projects can be found at the JPL Optical Long Baseline Interferometry News.

Brief History of Stellar Interferometry
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