The useful dynamic range of an image in the diffraction limited regime isusually limited by speckles caused by residual phase errors in the opticalsystem forming the image. The technique of speckle decorrelation involvesintroducing many independent realizations of additional phase error into awavefront during one speckle lifetime, changing the instantaneous specklepattern. A commonly held assumption is that this results in the speckles being`moved around' at the rate at which the additional phase screens are applied.The intention of this exercise is to smooth the speckles out into a moreuniform background distribution during their persistence time, thereby enablingcompanion detection around bright stars to be photon noise limited rather thanspeckle-limited. We demonstrate analytically why this does not occur, andconfirm this result with numerical simulations. We show that the originalspeckles must persist, and that the technique of speckle decorrelation merelyadds more noise to the original speckle noise, thereby degrading the dynamicrange of the image.
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