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arXiv · astro-ph/0610207

Atmospheric coherence times in interferometry: definition and measurement

A. Kellerer·A. Tokovinin
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Abstract

Current and future ground-based interferometers require knowledge of the atmospheric time constant t_0, but this parameter has diverse definitions. Moreover, adequate techniques for monitoring t_0 still have to be implemented. We derive a new formula for the structure function of the fringe phase (piston) in a long-baseline interferometer, and review available techniques for measuring the atmospheric time constant and the shortcomings. It is shown that the standard adaptive-optics atmospheric time constant is sufficient for quantifying the piston coherence time, with only minor modifications. The residual error of a fast fringe tracker and the loss of fringe visibility in a finite exposure time are calculated in terms of the same parameter. A new method based on the fast variations of defocus is proposed. The formula for relating the defocus speed to the time constant is derived. Simulations of a 35-cm telescope demonstrate the feasibility of this new technique for site testing.

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A. Kellerer, A. Tokovinin. 2006-10-06. Atmospheric coherence times in interferometry: definition and measurement. https://doi.org/10.1051/0004-6361%3A20065788

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