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Abstract Review
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| Authors |
| Name | | Affiliation |
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Julien Borgnino |
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Laboratoire Universitaire d'Astrophysique de Nice, UMR 6525, Université de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice cedex 02, FRANCE
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| Abstract |
| Session | | 3 (Approaches for modeling atmospheric optical turbulence.) |
| Title | | 'How to Monte-Carlo Simulate the Optical Turbulence Boiling Beyond the Frozen Flow Hypothesis' |
| Abstract | | High Angular Resolution through turbulence requires an accurate description of the optical turbulence time-variations. As for spatial fluctuations, time-variations of the optical turbulence observables are random. Their statistical and spectral properties are however reproducible. This property allows performing Monte-Carlo simulations of the spatial distributions and time-variations of the optical turbulence observables such as phase fluctuations or scintillation. The optical turbulence time-variation is usually described by the so-called Frozen Flow Hypothesis in which turbulence is supposed to be equivalent to a series of solid phase-screens that slide horizontally across the observation field-of-view. Experimental evidence shows however that an additional physical process must be taken into account. In fact, while translating above the observer, turbulence undergoes a proper time-variation which affects differently the astronomical observations. The proper time-variation of the optical turbulence observables as described here will be called the optical turbulence boiling in the following. The central topic of our discussion concerns the optical turbulence time-variation beyond the paradigm of the frozen flow hypothesis, and how to Monte-Carlo simulate the optical turbulence boiling effect. The usual simulation consists in generating a two-dimensional spatially distributed phase-screen and to move it schematically over the telescope entrance aperture. The spatiotemporal statistical properties depend upon the spatial power spectrum of the phase-screen and upon the wind velocity. In our approach to model the optical turbulence boiling, we will first discuss how to establish a spectral description of the boiling process. This description leads formally to a redistribution of the optical turbulence energy over time-frequencies. It also make it possible to Monte-Carlo simulate the time-variation of an optical turbulence spatial distribution such as phase fluctuations at the entrance pupil of telescope through the optical turbulence boiling process. The spatiotemporal statistical properties depend then upon the spatial power density of the optical turbulence observable and upon a boiling constant we introduce while discussing the theoretical approach. We present then a Monte-Carlo simulation method of a time-varying spatial distribution of phase fluctuations through the optical turbulence boiling, which is based upon variation of the usual FFT-based method for phase-screen generation. This method can be easily generalized to the case in which both the optical turbulence boiling and horizontal transportation by the wind are involved in a multilayered-turbulence configuration. In this case the boiling optical turbulence must be moved over the telescope so that the spatiotemporal statistical properties of the time-varying observable depend upon the spatial spectral densities, boiling constants and wind velocities across the turbulence vertical profile above the telescope.
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