Focus anisoplanatism in the limit of infinitely many artificial-guide-star reference spots
Fried, David L.
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The concept that the focus anisoplanatism effect, which limits the useful diameter of an adaptive-optics system that relies on an artificial guide star [(AGS), a laser atmospheric backscatter spot] as a reference source for determining the turbulence-induced wave-front distortion, can be eliminated (or greatly reduced) by use of a multiplicity of AGS spots is evaluated. The case of an infinite density of such spots with an infinite density of wave-front sensor subapertures (each infinitely small) is analyzed assuming that performance is limited only by the fact that turbulence is distributed along the propagation path rather than being contained in a single plane. It is found that even in this case focus anisoplanatism limits performance. Relative to what can be achieved with a single AGS spot, it is found that at most approximately a factor-of-2.5 increase in the useful aperture diameter can be obtained by use of infinitely many AGS spots and that this increase is available only for a laser backscatter altitude as high as the 90-km mesospheric sodium layer.
J. Opt. Soc. Am. A, Volume 12, No. 5, pp. 939-949 (May 1995)