Cn2 profilometry from Shack-Hartmann data: model and experiment
J. Voyez, C. Robert, N. Védrenne, 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.