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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.

The author(s) of this abstract have provided an email address for comments about the abstract: romana@galaxy.ps.uci.edu

**Program
listing for Monday**