chapter 6, Aperture Averaging

Excerpt

6.1 Introduction

The decrease in scintillation with increasing telescope collecting area, known as aperture averaging, had been recognized in early astronomical measurements made in the 1950s [1]. These same measurements revealed that aperture averaging causes a shift of the relative frequency content of the irradiance power spectrum toward lower frequencies—in essence, averaging out the fastest fluctuations. More recently, aperture averaging effects have been studied in the context of laser beam propagation through atmospheric turbulence [2–8].

The reduction in scintillation due to aperture averaging can be deduced from the ratio of power fluctuations by a finite-size collecting aperture to that obtained by a “point” aperture. Let I(r,L) be the irradiance of an optical wave on the surface of a circular collecting lens of diameter D of a receiver system. Here, L is propagation distance and r is a vector in the transverse plane at the collecting lens. The detector responds to the entire light flux or power P = ∫ I(r,L)dr through the collecting aperture. For the special case of an unbounded plane wave or spherical wave, the mean power in the receiver plane is given by [9]