Adaptive optics now routinely produces high dynamic range diffraction-limited images on ground-based telescopes using guide sources as far as 10 to 15 arcsec away from the object of interest. The same principles and techniques apply to telescope arrays. A dual-beam interferometer would allow the use of a bright guide source to compensate the wave-front and track the fringes, while observing much fainter nearby sources. Even a single beam interferometer should also have two channels, one for wave-front control, and another one for science. A dichroic beam splitter is then used to separate the two beams. Wave-front control is done with low-noise, fast parallel read-out detectors collecting photons over a very wide spectral band. Wave-front `curvature' sensing is particularly attractive to control an array of telescopes. This can be done by using a single membrane modulator as a common reference, but several detector arrays (one per telescope). In this case, closing the loop will automatically co-align and co-focus all the telescopes as if it were a single aperture. Co-phasing the array still requires an additional sensor (fringe tracker) to sense the wave-front `piston' modes. Because most of the scientific information is obtained under very low fringe visibilities, fringe tracking must be done either at a longer wavelength or - in case of an array - between adjacent apertures. The co-phasing of a large array then becomes the adaptive optics equivalent of reconstructing the wave-front phase from an array of phase differences. Once individual wave-fronts are compensated, co-focused, co-aligned, and co-phased, then it becomes possible to record long exposure interferograms for science applications.
Working on the Fringe: Optical and IR Interferometry from Ground and Space
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