'Lucky imaging' and speckle discrimination for the detection of faintcompanions with adaptive optics

摘要

We have analyzed the application of frame selection ("lucky imaging") to adaptive optics (AO), short-exposureobservations of faint companions. We have used the instantaneous Strehl ratio as an image quality metric. Theprobability density function (PDF) of this quantity can be used to determine the outcome of frame selection in terms ofoptimizing the Strehl ratio and the peak-signal-to-noise-ratio of the shift-and-add image. In the presence of staticspeckles, frame selection can lead to both: improvement in resolution – as quantified by the Strehl ratio, as well as faintsignal detectability – given by the peak-signal-to-noise-ratio. This theoretical prediction is confirmed with real data fromAO observations using Lick Observatory's 3m Shane telescope, and the Palomar Observatory's 5m Hale telescope. Inaddition, we propose a novel statistics-based technique for the detection of faint companions from a sequence of AO-corrected exposures. The algorithm, which we call stochastic speckle discrimination, utilizes the "statistical signature" ofthe centre of the point spread function (PSF) to discriminate between faint companions and static speckles. Thetechnique yields excellent results even for signals invisible in the shift-and-add images.