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MOSP: Monitor of Outer Scale Profile

Abstract 193

Submitted by Aziz ZIAD


A. Ziad, J. Maîre & J. Borgnino


Université de Nice Sophia-Antipolis, Laboratoire H. Fizeau


Wavefront outer scale is a relevant parameter for the experimental performance evaluation of large aperture telescopes. The actual size of the outer scale has long been controversial, with measured values ranging from less than 10 m to more than 2 km. What is not controversial is the conclusion that when the diameter of the telescope approaches or exceeds the size of the outer scale, the optical consequences of atmospheric turbulence are changed dramatically from their traditional Kolmogorov behavior. In particular, power in the lowest Zernike aberration modes, e.g., tip and tilt and the overall stroke required for an adaptive-optics system can be much reduced. A finite outer scale has implications for interferometry as well. With the current interest in the design of extremely large ground-based optical and infrared telescopes, reliable estimates of the outer scale profile have assumed considerable importance. A new instrument MOSP (Monitor of Outer Scale Profile) has been developed by our team for outer scale profile extraction. We retrieve the vertical distribution of wavefront outer scale by analysing angular correlation of wavefront Angle of Arrival fluctuations deduced from Moon’s limb images motion. We use simulated annealing algorithm to deduce the height dependence of the wavefront outer scale with given C_N^2 profiles simultaneously measured with the SCIDAR instrument. We present results obtained during two campaigns of observation at the Mauna Kea Observatory (Hawaii) and Observatoire de Haute Provence (France). Estimated outer scale profiles exhibit smaller values in the boundary layer than in the free atmosphere. Comparisons with GSM outer scale measurements are possible and give good agreement. Some implications for adaptive optics systems are considered.