The need for a profiler

The performance of adaptive optics (AO) systems depends on the turbulence vertical profile ${\rm C^2_n}(h)$ in many ways. The size of compensated field of view is defined by the isoplanatic angle $\theta_0$ (more specifically, PSF variation over the field depends on $\theta_0$). Strehl ratio when using off-axis reference stars is a very sensitive function of $\theta_0$. Consequently, sky coverage depends on $\theta_0$. The performance of laser guide stars depends on ${\rm C^2_n}(h)$ through cone effect and tilt anisoplanatism, and the knowledge of ${\rm C^2_n}(h)$ is required to determine the sky coverage and to schedule the LGS-assisted operation of the future AO systems. Finally, the time constant (hence the required speed of correction and the limiting magnitude) is an additional important parameter for AO.

Existing techniques for ${\rm C^2_n}(h)$ measurements include a direct in situ balloon micro-thermal sounding and a remote optical sounding with double stars, known as SCIDAR. Both methods are not adapted for continuous profile monitoring. It is then not surprising that no statistical database of ${\rm C^2_n}(h)$ exists for any site. Recognizing this need, we and some other researchers turn to the study of single-star turbulence profilers that would be suitable for monitoring. The idea is to analyze the spatial structure of the scintillations from single stars and to extract from this information a low-resolution turbulence profile.

The principle of turbulence profile estimation from multi-aperture scintillation data is described in the accompanying documents: the paper presented at the Marrakesh IAU workshop ``Site 2000'' [1], and the Part 1 of the Proposal prepared for ESO [2]. Details of the instrument design and data reduction are given in the Part 2 [3].