Brief History of Stellar Interferometry

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The first measurement of a stellar angular diameter was made of the supergiant star Betelgeuse (alpha Orionis) with Michelson's stellar interferometer in 1920. Light collected by two small mirrors, carried on a steel beam mounted on the 100~inch Hooker telescope at Mount Wilson, was combined in the telescope and the resulting interference fringes were viewed by eye. However, for accurate measurements, optical interferometry requires extreme mechanical stability, sensitive detectors with good time resolution, and at least a simple adaptive optical system to reduce the effects of seeing. The lack of this technology stalled the development of the technique.

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Michelson's "20 foot" interferometer.

The field remained dormant until Hanbury Brown and Twiss developed the technique of intensity interferometry and demonstrated it at optical wavelengths by measuring the angular diameter of Sirius (alpha Canis Majoris), the brightest star in the sky. Compared with Michelson's interferometer this technique has the advantages of not requiring high mechanical stability and of being essentially unaffected by seeing. Following the successful demonstration of the technique the Narrabri Stellar Intensity Interferometer (NSII) was constructed. It was operated by the Chatterton Astronomy Department through the 1960s and early 1970s to measure the angular sizes of 32 hot stars with B magnitude < 2.5. These results were used to establish the temperature scale for stars hotter than the Sun.


The light collectors of the Narrabri Stellar Intensity Interferometer.

With the completion of the NSII program in 1972, the Chatterton Astronomy Department began designing a successor to the NSII. Initially this was conceived as a very large intensity interferometer but it became clear that a modern form of Michelson's instrument, an amplitude interferometer, would provide greater sensitivity and would be less expensive to build. Unlike intensity and speckle interferometry, amplitude interferometry can provide useful sensitivity (limiting magnitude ~7) with modest input aperture diameters (100 ~ 200mm). However, before a full-scale instrument could be built there were a number of technical problems to be investigated and it was decided to build a prototype interferometer which would incorporate most of the features of a major instrument but which would have a single, relatively short baseline of 11.4m to minimise the complexity and cost. The prototype interferometer was used to redetermine the angular diameter Sirius. This successful demonstration of the method was the basis of a proposal to build the Sydney University Stellar Interferometer (SUSI).

Initial funding for the detailed design and construction of SUSI was granted in 1985 and construction commenced in late 1987. The consulting engineering firm of Connell Wagner were the project managers for the site works and civil engineering during construction. SUSI was opened in 1991 and has been slowly increasing in capability since that time.

SUSI was originally designed to operate at the 'blue' end of the visual spectrum. Recent work has extended this to the 'red' end of the spectrum, which promises increased sensitivity. The SUSI group acquired first fringes with the new red-wavelengths beam combiner in November 2002.

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This aerial view of SUSI looking from the north along the 640m baseline array shows the individual siderostat stations and the system of evacuated pipes connecting the stations to the main laboratory. The T-shaped main laboratory area can be seen in the top centre of the photograph. The 80m long section of the building parallel to the baseline encloses the Optical Path Length Compensator (OPLC). The beam combining laboratory, the control room and other facilities are located in the western section of the building.

Astronomy with SUSI

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