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23 August 2005 Introduction to laser guide star theory versus experiment
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Abstract
We discuss the reliance on backscatter by a laser guide star to generate a propagative probe and we show that spatial reciprocity can be accomplished by compensating phase alone. An analytic plane to plane propagation framework is introduced in which the spatial reciprocity of Maxwell's equations Is utilized to demonstrate that the adaptive optics compensation can do no better than the beacon initial conditions (I.e. cannot correct for the beacon too.) It is shown analytically that use of point to point reciprocity reasoning fails. While the laser guide star itself may be compensated to optimize uplink spatial coherence at altitude, the backscattering process is completely incoherent and the backscattering volume constitutes a very bad mirror or diffuse source. While diffraction restores some coherency as described by the van Cittert-Zernike theorem[2,5], the consequences of the incoherency of the beacon, lead to problems for the adaptive optics system which do not affect natural guide stars. The main consequence for the laser guide star system is that the wave sensor of the adaptive optics cannot distinguish between the phase aberrations from the backscattering process and those phase aberrations induced by turbulence. The question of the beacon and propagative path being different is weighed within the context of correlated versus uncorrelated ensemble members of turbulence.
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S. Enguehard and B. Hatfield "Introduction to laser guide star theory versus experiment", Proc. SPIE 5895, Target-in-the-Loop: Atmospheric Tracking, Imaging, and Compensation II, 589507 (23 August 2005); https://doi.org/10.1117/12.621111
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