Showing papers by "Éric Thiébaut published in 2013"

Principles of Image Reconstruction in Interferometry

Abstract: Image reconstruction from interferometric data is an inverse problem Owing to the sparse spatial frequency coverage of the data and to missing Fourier phase information, one has to take into account not only the data but also prior constraints Image reconstruction then amounts to minimizing a joint criterion which is the sum of a likelihood term to enforce fidelity to the data and a regularization term to impose the priors To implement strict constraints such as normalization and non-negativity, the minimization is performed on a feasible set When the complex visibilities are available, image reconstruction is relatively easy as the joint criterion is convex and finding the solution is similar to a deconvolution problem In optical interferometry, only the powerspectrum and the bispectrum can be measured and the joint criterion is highly multi-modal The success of an image reconstruction algorithm then depends on the choice of the priors and on the ability of the optimization strategy to find a good solution among all the local minima

Exploiting spatial sparsity for multiwavelength imaging in optical interferometry.

Abstract: Optical interferometers provide multiple wavelength measurements. In order to fully exploit the spectral and spatial resolution of these instruments, new algorithms for image reconstruction have to be developed. Early attempts to deal with multichromatic interferometric data have consisted in recovering a gray image of the object or independent monochromatic images in some spectral bandwidths. The main challenge is now to recover the full three-dimensional (spatiospectral) brightness distribution of the astronomical target given all the available data. We describe an approach to implement multiwavelength image reconstruction in the case where the observed scene is a collection of point-like sources. We show the gain in image quality (both spatially and spectrally) achieved by globally taking into account all the data instead of dealing with independent spectral slices. This is achieved thanks to a regularization that favors spatial sparsity and spectral grouping of the sources. Since the objective function is not differentiable, we had to develop a specialized optimization algorithm that also accounts for non-negativity of the brightness distribution.

Restoration of hyperspectral astronomical data with spectrally varying blur

Abstract: In this paper we present a method for hyper-spectral image restoration for integral field spectrographs (IFS) data. We specifically address two topics: (i) the design of a fast approximation of spectrally varying operators and (ii) the comparison between two kind of regularization functions: quadratic and spatial sparsity functions. We illustrate this method with simulations coming from the Multi Unit Spectroscopic Explorer (MUSE) instrument. It shows the clear increase of the spatial resolution provided by our method as well as its denoising capability.

An image reconstruction framework for polychromatic interferometry

Abstract: We describe a new approach to implement multi-wavelength image reconstruction in the case where the observed scene is a collection of point-like sources. We show the gain in image quality (both spatially and spectrally) achieved by globally taking into account all the data instead of dealing with independent spectral slices. This is achieved thanks to a regularization which favors spatial sparsity and spectral grouping of the sources. Since the objective function is not differentiable, we had to develop a specialized optimization algorithm.

First images from the PIONIER/VLTI optical interferometry imaging survey of Herbig Ae/Be stars

Abstract: The close environment of Herbig stars starts to be revealed step by step and it appears to be quite complex. Many physical phenomena interplay: the dust sublimation causing a puffed-up inner rim, a dusty halo, a dusty wind or an inner gaseous component. To investigate more deeply these regions, getting images at the first Astronomical Unit scale is necessary. This has become possible with near infrared instruments on the VLTI. We have developed a new imaging method adapted to young stellar objects where we process separately the stellar component from the rest of the image to reveal the environment by using the spectral differences between these two components. We present the result of this method on the first imaging survey of Herbig stars carried out by PIONIER on the VLTI.

Détection de sources en interférométrie optique hyperspectrale

Abstract: En faisant interferer la lumiere provenant de plusieurs telescopes, l'interferometrie optique fournit des mesures a tres haute resolution angulaire (de l'ordre de la milliseconde d'arc). Chaque mesure estime la valeur en une frequence spatiale de la transformee de Fourier de la distribution spatiale d'intensite emise par l'objet observe dans chacun des canaux spectraux. Le probleme traite ici est la detection, la localisation precise et l'extraction sans biais du spectre de chacune des etoiles d'un amas observe en interferometrie. C'est un verrou important pour l'etude des etoiles au voisinage du trou noir central de notre galaxie, but scientifique du futur instrument GRAVITY du VLTI. A la suite de nos precedent travaux, nous presentons ici une methode de reconstruction basee sur la methode de multiplicateur a directions alternees (ADMM). Cela permet d'utiliser dans le meme temps les donnees interferometriques et photometriques. L'introduction de variables auxiliaires permet de decouper le probleme de reconstruction en sous problemes plus faciles a traiter. Des tests sur des simulations montrent que la methode proposee permet de detecter toutes les etoiles d'un amas et de d'estimer leurs spectres avec un biais negligeable.

Déconvolution aveugle en microscopie de fluorescence 3D

Abstract: Dans ce papier, nous proposons une methode de deconvolution aveugle estimant la fonction de flou (PSF) et l'image nette en microscopie de fluorescence 3D. L'idee est d'utiliser un modele parametre par la fonction pupille pour la PSF en decomposant le module et la phase de la partie aberration sur la base de Zernike. Les resultats tant sur des simulations que sur des donnees experimentales montrent la superiorite de notre methode comparee a l'etat de l'art.

Approche inverse régularisée pour la reconstruction 4-D en tomographie dynamique sans compensation de mouvement

Abstract: La tomographie dynamique est la reconstruction, a partir de projections, d'objets induits d'un mouvement, le plus souvent periodique (e.g. le cycle respiratoire chez un patient). Le probleme de reconstruction devient alors 4-D (3-D spatiale + temps), a donnees parcimonieuses puisqu'une projection ne correspondra qu'a un instant specifique de la sequence 4-D d'un cycle (ou periode). Nous traitons la reconstruction dynamique comme un probleme inverse global avec un terme d'attache aux donnees utilisant la totalite des projections. Les parametres estimes sont l'image 4-D d'un cycle dynamique de l'objet. Le modele de reprojection est cale temporellement sur le cycle d'acquisition des projections grâce a un signal temporel 1-D decrivant l'evolution dynamique de l'objet, et sa periodicite. Une etape d'interpolation temporelle de la sequence 4-D sur les dates d'acquisition precede alors la projection standard a un instant donne. Nous injectons egalement une regularisation spatio-temporelle de l'objet sous forme d'une variation totale 4-D. La regularisation apporte alors la correlation temporelle entre les differentes tranches reconstruites, et permet ainsi d'extraire au mieux l'information fournie par les donnees, sans aucune estimation ni compensation de mouvement. Nous faisons la demonstration de notre approche sur des reconstructions 2-D+t d'un fantome mecanique acquises sur un scanner Cone-Beam. La regularisation spatio-temporelle apporte un gain sans equivoque sur la qualite des reconstructions dynamiques. Des premiers resultats 4-D (3-D+t) encourageants sont obtenus sur donnees cliniques d'un patient en respiration.