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Dark speckle lifetime

Because the number of photon-events detected per pixel in each exposure is critical, these exposures should be as long as possible without degrading too much the darkness of the dark speckles. The optimal value is obviously shorter than the usual speckle lifetime considered by Roddier, Gilli & Lund (1982). A tentative lower limit can be estimated by linearly scaling the speckle lifetime in proportion to the dark speckle size defined by Eq.(7). It leads to impossibly short exposures for large R values. However, since the adaptive optics decreases the wave disturbance, it increases markedly the speckle lifetime at positions close to the Airy peak, depending upon the factor $n{\lambda\over D}$, where n is the number of actuators across the pupil containing n2 of them ([Ryan 1996]). The optimal exposure time is therefore dependant upon the adaptive optics performance.