Atmospheric coherence times in interferometry: definition and measurement

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: akellere@eso.org
² Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile e-mail: atokovinin@ctio.noao.edu

Received: 9 June 2006
Accepted: 15 September 2006

Abstract

Aims.Current and future ground-based interferometers require knowledge of the atmospheric time constant t₀, but this parameter has diverse definitions. Moreover, adequate techniques for monitoring t₀ still have to be implemented.

Methods.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.

Results. 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.