For adaptive optic systems, the use of aperture filter functions calculated using various Zernike modes can be
useful in describing lower-order aberrations caused by atmospheric turbulence. Traditionally, these filter functions
are calculated using the step function depicting a hard aperture that introduces integrals that are sometimes difficult
to integrate and must be done numerically. The Gaussian method, equivalent to the ABCD matrix method, can be
used in place of the conventional method for calculating the aperture filter functions. Evaluation of the Gaussian
approximation for modeling a finite receiver aperture can be made by comparison of reduction in phase variance
with results achieved using the conventional method. The validity of Gaussian approximation in this application is
demonstrated by the consistency of results between the two methodologies. Comparison of reduction in scintillation
by the two methodologies reveals several benefits derived from utilization of Gaussian approximation. The
Gaussian approximation produces data that can be interpreted analytically. This paper will first examine the use of
statistical models for predicting atmospheric turbulence and then the use of Zernike polynomials in adaptive optics.
Next, this paper compares the reduction of phase variance and scintillation using the conventional method with the
Gaussian approximation to evaluate the effectiveness of the new filter functions. The results of these comparisons
are presented both as mathematical expressions and graphically.
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