Cn2 profilometry from Shack-Hartmann data: model and experiment


J. Voyez, C. Robert, N. VĂ©drenne, B. Fleury, V. Michau, T. Fusco




The design phase for the Wide Field Adaptive Optics (WFAO) systems for the ELTs has started. LTAO (ATLAS), MCAO (MAORY) and MOAO (EAGLE) approaches have been studied for the E-ELT. All these systems have in common a need for a precise tomographic reconstruction of the turbulent volume. In that frame, the Cn2 structure, representing the turbulence strength, becomes a critical parameter for the final tomographic reconstruction performance. Getting of high-resolution Cn2 profiles is then a crucial point for the design of E-ELT AO systems. In this context, we have proposed a new profilometry method using Shack-Hartmann (SH) data. Slopes and scintillation indexes being recorded simultaneously with a Shack-Hartmann wavefront sensor (SHWFS), their correlation is exploited in order to retrieve the Cn2 profile. This method, named CO-SLIDAR (COupled Slope and scIntillation Detection And Ranging), uses correlation of SHWFS data from two separated stars. CO-SLIDAR has been validated in numerical simulations in a precedent work. The next step is an on-sky validation with a full-dedicated SHWFS, to measure a Cn2 profile with 40 layers, up to 20 km of altitude, with a 500m altitude resolution. The system will be set up on a 1.5m telescope. Here, we first expound the theoretical background of SH data and CO-SLIDAR processing. The Cn2 profile is estimated by minimizing a maximum likelihood criterion under positivity constraint. Then, we describe the experimental system and the future on-sky experiment. Determination of outer scale L0 with CO-SLIDAR, taking advantage of the large telescope diameter, is investigated.

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