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D. Hope, S. Prasad (University of New Mexico)
To image an object at optical wavelengths with a ground-based interferometer, we need measurements and calibration of both the fringe power and the bispectrum. The phase of the bispectrum, the closure phase, being the sum of phases along any three baselines of the interferometer that form a closed triangle is insensitive to atmospheric turbulence for vanishingly small apertures. For weak atmospheric turbulence (Fried parameter r0 comparable to or larger than the diameter D of an aperture) one expects only small variations in the measured bispectrum during an observation. For strong turbulence, r0 << D, the aperture contains multiple r0 patches, which result in a noisy bispectrum measurement with reduced fringe visibility. This turbulence induced noise in the bispectrum measurement degrades the phase reconstruction process and hence the final image of the object. We present a means of quantifying this effect on the image reconstruction process using Shannon Information. In the language of Shannon, the fringe power and bispectrum contain information about an object, information that is compromised by the calibration process and turbulence induced noise in the bispectrum. Simulations are presented for an aperture mask on a telescope for varying degrees of atmospheric turbulence.
Acknowledgment: This work was performed under contract with the Jet Propulsion Laboratory (JPL) through the Michelson Fellowship Program funded by NASA as an element of the Planet Finder Program. JPL is managed for NASA by the California Institute of Technology.