The Statistics of Isoplanatic Angle and Adaptive Optics Time Constant derived from DIMM Data

Abstract

The performance of Adaptive Optics and Interferometry depends on several parameters of the atmospheric turbulence: seeing, time constant τ0, and isoplanatic angle θ0. It is very challenging to design reliable instruments capable of monitoring them from remote sites. We propose here new simplified methods to extract τ0 and θ0 from the data of standard portable seeing monitors (DIMMs). They rely on the analyses and forecasts of synoptic atmospheric parameters produced daily by global meteorological models. Using the results of balloon borne microthermal soundings performed at the VLT and Gemini sites, it is shown that the wind velocity at 12 km above sea level is strongly correlated to the average wavefront velocity, permitting to compute τ0. Similarly, this velocity can be used to remove the finite-exposure-time bias in the DIMM scintillation data, leading to an estimate of θ0. The statistics of these parameters at Cerro Paranal is given for the year 2000. Global wind pattern suggests that longer time constants are expected at equatorial and polar sites.