Interferometry at optical wavelengths. It is technologically more challenging than radio interferometry because it detects individual photons and requires extreme mechanical precision because of the short wavelengths involved. Unlike radio interferometry, optical interferometry does not suffer from phase distortion caused by the atmosphere. However, it suffers from the difficulty that astronomical sources are not as bright in the visible as the radio.
Additional technological challenges include the lack of coherent broadband amplifiers the visible and the fact that, except using a CO2 laser in the infrared, signals cannot be mixed to an intermediate frequency. Finally, there are no linear detectors, so optical interferometry has to measure intensity (proportional to voltage squared ) rather than electric field (proportional to voltage V).
Three type of optical interferometry which do not make use of masks are speckle interferometry, Knox-Thompson interferometry, and bispectrum analysis.
In Sept. 1995, a University of Cambridge group became the first to combine the light of separate optical telescopes to create a single image. They used a set of four 40 cm telescopes (Baldwin et al. 1996, Schwarzschild 1996, Watson 1996) and were able to image the binary star Capella, whose components have an angular separation of 55 milliarcseconds.
Bispectrum Analysis, Interferometry, Knox-Thompson Interferometry, Speckle Interferometry
References
--. Sky & Telescope, 29, Jan. 1992.
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Haniff, C. A.; Mackay, C. D.; Titterington, D. J.; Sivia, D.; Baldwin, J. E. et al. "The 1st Images from Optical Aperture Synthesis." Nature 328, 694-696, 1987.
Nakajima, T.; Kulkarni, S. R.; Gorham, P. W.; Ghez, A. M.; Neugebauer, G. et al. "Diffraction-Limited Imaging. 2. Optical Aperture-Synthesis Imaging of 2 Binary Stars." Astron. J. 97, 1510-1521, 1989.
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Schwarzschild, B. "Telescope array Begins INterferometric Imaging of Stars and Optical Wavelengths." Phys. Today 49, 17-20, Apr. 1996.
Watson, A. "Star-Watchers Team Up Telescoped for a Sharper View." Science 271, 907-908, 1996.
