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Authors: M. Prato, A. La Camera, S. Bonettini and M. Bertero
Abstract: In this paper, we propose a blind deconvolution method which applies to data perturbed by Poisson noise. The objective function is a generalized Kullback–Leibler (KL) divergence, depending on both the unknown object and unknown point spread function (PSF), without the addition of regularization terms; constrained minimization, with suitable convex constraints on both unknowns, is considered. The problem is non-convex and we propose to solve it by means of an inexact alternating minimization method, whose global convergence to stationary points of the objective function has been recently proved in a general setting. The method is iterative and each iteration, also called outer iteration, consists of alternating an update of the object and the PSF by means of a fixed number of iterations, also called inner iterations, of the scaled gradient projection (SGP) method. Therefore, the method is similar to other proposed methods based on the Richardson–Lucy (RL) algorithm, with SGP replacing RL. The use of SGP has two advantages: first, it allows one to prove global convergence of the blind method; secondly, it allows the introduction of different constraints on the object and the PSF. The specific constraint on the PSF, besides non-negativity and normalization, is an upper bound derived from the so-called Strehl ratio (SR), which is the ratio between the peak value of an aberrated versus a perfect wavefront. Therefore, a typical application, but not a unique one, is to the imaging of modern telescopes equipped with adaptive optics systems for the partial correction of the aberrations due to atmospheric turbulence. In the paper, we describe in detail the algorithm and we recall the results leading to its convergence. Moreover, we illustrate its effectiveness by means of numerical experiments whose results indicate that the method, pushed to convergence, is very promising in the reconstruction of non-dense stellar clusters. The case of more complex astronomical targets is also considered, but in this case regularization by early stopping of the outer iterations is required. However, the proposed method, based on SGP, allows generalization to the case of differentiable regularization terms added to the KL divergence, even if this generalization is outside the scope of this paper.

Rating: 18 user(s) have rated this article Average rating: 5.0
Posted by: andrea, on 5/31/2013, in category "2013"
Views: this article has been read 6206 times
Authors: A. R. Conrad, W. J. Merline, A. La Camera, P. Boccacci, M. Bertero, T. M. Herbst, M. Kuerster, B. Carry, J. Drummond, M. Norris, J. C. Christou
Abstract: Asteroid satellites help determine mass, density, and composition. LINC-NIRVANA will yield a factor of 3 improvement in the limiting separation for detections.

Rating: 10 user(s) have rated this article Average rating: 5.0
Posted by: andrea, on 5/30/2013, in category "2013"
Views: this article has been read 6279 times
Authors: Ralph Hofferbert; and other 67 co-authors (LINC-NIRVANA Team)
Abstract: LINC-NIRVANA (LN) is the near-infrared, Fizeau-type imaging interferometer for the large binocular telescope (LBT) on Mt. Graham, Arizona (elevation of 3267 m). The instrument is currently being built by a consortium of German and Italian institutes under the leadership of the Max Planck Institute for Astronomy in Heidelberg, Germany. It will combine the radiation from both 8.4 m primary mirrors of LBT in such a way that the sensitivity of a 11.9 m telescope and the spatial resolution of a 22.8 m telescope will be obtained within a 10.5×10.5 arcsec scientific field of view. Interferometric fringes of the combined beams are tracked in an oval field with diameters of 1 and 1.5 arcmin. In addition, both incoming beams are individually corrected by LN's multiconjugate adaptive optics system to reduce atmospheric image distortion over a circular field of up to 6 arcmin in diameter. A comprehensive technical overview of the instrument is presented, comprising the detailed design of LN's four major systems for interferometric imaging and fringe tracking, both in the near infrared range of 1 to 2.4 μm, as well as atmospheric turbulence correction at two altitudes, both in the visible range of 0.6 to 0.9 μm. The resulting performance capabilities and a short outlook of some of the major science goals will be presented. In addition, the roadmap for the related assembly, integration, and verification process are discussed. To avoid late interface-related risks, strategies for early hardware as well as software interactions with the telescope have been elaborated. The goal is to ship LN to the LBT in 2014.

Rating: 928 user(s) have rated this article Average rating: 5.0
Posted by: andrea, on 11/30/2013, in category "2013"
Views: this article has been read 7504 times