Phase diversity technique (PD) can jointly estimate the wavefront aberration and the target image of an optical imaging system. The PD technique reconstructs images by acquiring a focal plane image of optical system and one or more images with known aberrations (often selected defocus). Due to the simple construction of the optical system, the ability to detect discontinuous co-phase errors, and its applicability to both point sources and extended targets, The PD technique is uniquely suited for spatial target imaging applications, especially for the detection of multi-aperture piston errors. However, in a spatially low-illumination environment, Poisson noise as the main noise source of the imaging system seriously affects the accuracy of the reconstructed images. In this paper, we propose a method of phase diversity technique based on a fast Non-local Means (NLM) algorithm for reconstructing single-aperture images or multi-aperture images. For the two cases of single-aperture imaging and multi-aperture imaging with piston errors in spatial low illumination conditions, the method is used to solve the sensitivity problem of Poisson noise during image reconstruction. Numerical simulation results show that our method has significant improvement in structural similarity of the recovered images compared with the traditional phase diversity technique, and also is faster than the common non-local mean algorithm. The combination of this fast non-local means algorithm which using integral images and the phase diversity technique greatly reduce the computation time. The field experimental results and simulation results show good agreement. The new method would be useful in the AO system with active Poisson noise.
Coronagraph is a powerful instrumentation that can be used to realize direct exoplanet imaging. Because the stars are far brighter than the exoplanets, a very high requirement for extinction is raised to obtain a high contrast imaging. By optimizing the transmittance of the entrance pupil and the Lyot stop in Lyot Coronagraph, a sequential optimization procedure is proposed to obtain even higher contrast imaging. In this manuscript, the usually adopted joint optimization in the existing literatures is divided into two steps, which we call the sequential optimization procedure. First, the entrance pupil is optimized for maximum transmission while the contrast constraint is imposed on the focal plane of the lens behind it. Second, the Lyot stop is optimized for maximum transmission with the contrast constraint imposed on the imaging plane. Compared with the joint optimization procedure, the sequential one can provide additional advantages. In terms of performance, under the same conditions, an IWA(Inner working angle) of 1.53λ/D can be obtained while the IWA for joint one is 2.01λ/D. Moreover, the sequential optimization is much faster. Referring to practical applications, the optimum transmittance of Lyot stop in sequential case becomes binary and therefore easy to fabricate. However sequential optimization reduces the throughput approximately 36.81%, which is a drawback that should be compensated.
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