OSA-ESO ADAPTIVE OPTICS TOPICAL MEETING

MONITORING THE OUTER SCALE OF TURBULENCE

The outer scale of turbulence is a key parameter to estimate the relative energy in the wavefront modes, in particular the first mode (tilt) on large telescopes.

The scientists disagree on the order of magnitude of the outer scale. The results are very dependent on the method used. The current estimates run from 10-100 m with the Mark III interferometer (Busher et Al., 1995), down to 50 m with COME-ON La Silla (Rigaut et Al. 1991), 5-8 m with a Hartmann-Shack at CFHT (Tallon, 1989) and 1-4 m with balloon borne microthermal sensors at Paranal during PARSCA92 (Fuchs, 1995).

Thanks to microthermal measurements, is is now demonstrated that thermal turbulence peaks inside very thin layers (Coulman et Al., 1995) but most wavefront calculations still use a formulation based on monotonic vertical distributions.

Ballon borne microthermal measurements show the layered structure of thermal turbulence (a flight above Paranal during PARSCA92)