Issue |
A&A
Volume 461, Number 2, January II 2007
|
|
---|---|---|
Page(s) | 775 - 781 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361:20065788 | |
Published online | 04 October 2006 |
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
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.
Key words: atmospheric effects / instrumentation: interferometers / site testing
© ESO, 2006
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