Focusing on a Turbulent Layer: Principle of the “Generalized SCIDAR”

Abstract

During the last two decades, much effort has been put into the remote sensing of atmospheric turbulent layers, using optical devices. The main scientific interest concerns optical path fluctuations relevant to communication and astronomy and the widening of our understanding of atmospheric physics. Here we describe an original technique that involves moving a virtual plane of analysis in the atmosphere. Using this technique, a simple optical combination allows us to explore the atmosphere, wiping out a particular layer while distant layers located beneath or above are reinforced. When combined with the SCIDAR technique, this phenomenon leads to the "generalized SCIDAR" concept, allowing turbulence profiles to be remotely assessed throughout the whole atmosphere, including the boundary layer. The potential of this new concept is shown in a laboratory experiment using a vein to simulate an atmospheric turbulent layer.

Automatic Determination of Wind Profiles with Generalized SCIDAR

“…To allow such a determination, Fuchs et al (1998) proposed shifting the (virtual) plane of analysis a few kilometers below the pupil plane. Avila et al (1997) implemented this method on a telescope.…”

First evidence of the finite horizontal extent of the optical turbulence layers. Implications for new adaptive optics techniques

“…(1) We compare measured and simulated C 2 N profiles along the same lines of sight. The measured profiles are provided by a Generalized Scidar (Fuchs et al 1998;Avila et al 1997;Avila et al 1998) at the focus of a 2.1 m telescope during a site testing campaign on 16−21 May 2000 in the Observatorio Astronómico Nacional de San Pedro Mártir (Mexico) (31.0441 N, 115.4569 W) and the simulations are provided by the Meso-Nh model (Masciadri et al 1999a;Masciadri et al 1999b In order to give a reliable comparison between measurements and simulations we modified the code so as to give averaged estimations of the optical turbulence calculated over simulations of some hours. We recall that the previous results Masciadri 2001a) were not the result of an average.…”

Using stellar scintillation for studies of turbulence in the Earth’s atmosphere

“…In its classical version, the Scidar is still insensitive to low-layer turbulence, which can largely affect the astronomical seeing (or image degradation) within a 50-80% proportion. Fuchs et al [80] first proposed analysing the scintillation in a virtual plane, below the pupil plane by a distance h gs , as sketched in figure 2a. This plane disposition allows a layer at ground altitude to have enough Fresnel propagation to give scintillation, because h gs = 0.…”