Optical EngineeringSegmented mirrors for atmospheric compensation
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An optical wavefront propagating through the atmosphere will be perturbed by local variations in the refractive index ofthe atmospheric gases. When accumulated over long optical path distances they will impart a spatial and temporally random distortion to the wavefront. These distortions have a characteristic spatial coherence length r0 and an atmospheric decorrelation time ?0. In directed energy applications, atmospheric distortions can reduce the peak target energy densities of larger diameter laser beams by orders of magnitude. The problem is not solved through the use of larger apertures; once the aperture size increases beyond one or two r0, the far-field spot remains constant in size. Hence, for large aperture systems, the overall performance is set by the spatial coherence of the atmosphere and not by the system's exit pupil.