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Session 22 - Disk Galaxies.
Oral session, Monday, January 13
Harbour C,

[22.04] Measurements of the Outer Scale of Turbulence at the Keck Telescope and its Impact on Inferred Seeing Parameters and Adaptive Optics Performance

R. A. Crnkovic (UCI)

Atmospheric characterization studies were performed at the Keck Telescope in order to quantify the atmospheric conditions at the site and to determine the requirements for and predicted performance of an adaptive optics system.

High speed measurements of image motion at the Keck are consistently smaller by a factor of several than the values predicted by the atmospheric turbulence model with an infinite outer scale. We calculated aperture-to-aperture correlations in the instantaneous tilt of a wavefront distorted by atmospheric turbulence for three different empirical models with finite outer scales: Von Karman, Greenwood-Tarzano and exponential models. By comparing the measured correlations to the theoretical values calculated with different outer scales we determined that the outer scale L_0 is actually of order 100 meters or less.

Properly accounting for the outer scale of turbulence alters the estimates of the atmospheric parameters and adaptive optics performance predictions that are sensitive to the low frequency end of the image motion power spectrum. Tip/tilt Zernike coefficients calculated for 9 m aperture and 50 m outer scale are half those calculated for an infinite outer scale. The coherence length calculated using tilt variance on a segment aperture of the Keck Telescope D = 1.6 m would be overestimated by a factor of 1.4 if a model with infinite outer scale is used in the atmosphere with outer scale of turbulence of 50 m, leading to underestimating the number of actuators needed in a deformable mirror of the adaptive optics system.

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Program listing for Monday