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Radio Astronomy

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Author Topic: Radio Astronomy  (Read 104 times)
Jennie McGrath
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Posts: 4348

« Reply #15 on: August 16, 2007, 11:18:21 pm »

A simple two-element optical interferometer. Light from two small telescopes (shown as lenses) is combined using beam splitters at detectors 1, 2, 3 and 4. The elements creating a 1/4 wave delay in the light allow the phase and amplitude of the interference visibility to be measured, which give information about the shape of the light source. A single large telescope with an aperture mask over it (labelled Mask), only allowing light through two small holes. The optical paths to detectors 1, 2, 3 and 4 are the same as in the left-hand figure, so this setup will give identical results. By moving the holes in the aperture mask and taking repeated measurements, images can be created using aperture synthesis which would have the same quality as would have been given by the right-hand telescope without the aperture mask. In an analogous way, the same image quality can be achieved by moving the small telescopes around in the left-hand figure - this is the basis of aperture synthesis, using widely separated small telescopes to simulate a giant telescope.

Once these technical considerations had been addressed, all of the principles used at radio wavelengths could be taken over with almost no modifi­cation. This included the use of imaging software developed for VLBI at radio wavelengths, which was used to reconstruct the ­first image from an array of optical telescopes, that of the 50 milliarcsecond binary star Capella (Baldwin et al . 1996). The only signi­cant differences between the two wavelength regimes is the increased importance of photon shot noise and the small temporal and spatial scales of the atmospheric fluctuations at optical wavelengths. For example, the characteristic time-scale for these fluctuations is measured in milliseconds at optical wavelengths rather than minutes, and the spatial scale is typically smaller than the telescope mirror diameter, whereas at centimetric radio wavelengths this scale can be as large as 20 km. An important consequence of this small spatial scale is that the area of sky over which the atmospheric phase path is constant, the isoplanatic patch, is at most a few arcseconds at visual wavelengths.

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