Showing papers by "Éric Thiébaut published in 2011"

Image reconstruction in optical interferometry: benchmarking the regularization

Abstract: Context. With the advent of visible and infrared long-baseline interferometers with more than two telescopes, both the size and the completeness of interferometric data sets have significantly increased, allowing images based on models with no a priori assumptions to be reconstructed with an aperture synthesis technique.Aims. Our main objective is to analyze the multiple parameters of the image reconstruction process with particular attention to the regularization term and the study of their behavior in different situations (types of astrophysical objects, telescope array configurations, level of noise, etc.). The secondary goal is to derive practical rules for the users.Methods. Using the Multi-aperture image Reconstruction Algorithm (MiRA), we performed multiple systematic tests, analyzing 11 regularization terms commonly used. The tests are made on different astrophysical objects, different (u,v ) plane coverages and several signal-to-noise ratios to determine the minimal configuration needed to reconstruct an image. We establish a methodology and we introduce the mean-square errors (MSE) to discuss the results.Results. From the ~24 000 simulations performed for the benchmarking of image reconstruction with MiRA, we are able to classify the different regularizations in the context of the observations. We find typical values of the regularization weight. A minimal (u,v ) coverage is required to reconstruct an acceptable image, whereas no limits are found for the studied values of the signal-to-noise ratio. We also show that super-resolution can be achieved with increasing performance with the (u,v ) coverage filling. Conclusions. Using image reconstruction with a sufficient (u,v ) coverage is shown to be reliable. The choice of the main parameters of the reconstruction is tightly constrained. We recommend that efforts to develop interferometric infrastructures should first concentrate on the number of telescopes to combine, and secondly on improving the accuracy and sensitivity of the arrays.

3D deconvolution of hyper-spectral astronomical data

Abstract: In this paper we present a general method for multichannel image restoration based on regularized 2 . We introduce separable regularizations that account for the dynamic of the model and take advantage of the continuities present in the data, leaving only two hyper-parameters to tune. We illustrate a practical implementation of this method in the context of host galaxy subtraction for the Nearby SuperNova factory. We show that the image restoration obtained fulfills the stringent requirements on bias and photometricity needed by this program. The reconstruction yields sub-percent integrated residuals in all the synthetic filters considered both on real and simulated data. Even though our implementation is tied to the SNfactory data, the method translates to any hyper-spectral data. As such, it is of direct relevance to several new generation instruments like MUSE. Also, this technique could be applied to multiband astronomical imaging for which image reconstruction is important, for example to increase image resolution for weak lensing surveys.

Image reconstruction in optical interferometry: Benchmarking the regularization

Abstract: With the advent of infrared long-baseline interferometers with more than two telescopes, both the size and the completeness of interferometric data sets have significantly increased, allowing images based on models with no a priori assumptions to be reconstructed. Our main objective is to analyze the multiple parameters of the image reconstruction process with particular attention to the regularization term and the study of their behavior in different situations. The secondary goal is to derive practical rules for the users. Using the Multi-aperture image Reconstruction Algorithm (MiRA), we performed multiple systematic tests, analyzing 11 regularization terms commonly used. The tests are made on different astrophysical objects, different (u,v) plane coverages and several signal-to-noise ratios to determine the minimal configuration needed to reconstruct an image. We establish a methodology and we introduce the mean-square errors (MSE) to discuss the results. From the ~24000 simulations performed for the benchmarking of image reconstruction with MiRA, we are able to classify the different regularizations in the context of the observations. We find typical values of the regularization weight. A minimal (u,v) coverage is required to reconstruct an acceptable image, whereas no limits are found for the studied values of the signal-to-noise ratio. We also show that super-resolution can be achieved with increasing performance with the (u,v) coverage filling. Using image reconstruction with a sufficient (u,v) coverage is shown to be reliable. The choice of the main parameters of the reconstruction is tightly constrained. We recommend that efforts to develop interferometric infrastructures should first concentrate on the number of telescopes to combine, and secondly on improving the accuracy and sensitivity of the arrays.

Fast model of space-variant blurring and its application to deconvolution in astronomy

Abstract: Image deblurring is essential to high resolution imaging and is therefore widely used in astronomy, microscopy or computational photography. While shift-invariant blur is modeled by convolution and leads to fast FFT-based algorithms, shift-variant blurring requires models both accurate and fast. When the point spread function (PSF) varies smoothly across the field, these two opposite objectives can be reached by interpolating from a grid of PSF samples. Several models for smoothly varying PSF co-exist in the literature. We advocate that one of them is both physically-grounded and fast. Moreover, we show that the approximation can be largely improved by tuning the PSF samples and interpolation weights with respect to a given continuous model. This improvement comes without increasing the computational cost of the blurring operator. We illustrate the developed blurring model on a deconvo-lution application in astronomy. Regularized reconstruction with our model leads to large improvements over existing results.

3-D deconvolution of hyper-spectral astronomical data

Abstract: In this paper we present a general forward fitting method for multichannel image restoration based on regularized chi2. We introduce separable regularizations that account for the dynamic of the model and take advantage of the continuities present in the data, leaving only two hyper-parameters to tune. We illustrate a practical implementation of this method in the context of host galaxy subtraction for the Nearby SuperNova factory. We show that the image restoration obtained fulfills the stringent requirements on bias and photometricity needed by this program. The reconstruction yields sub-percent integrated residuals in all the synthetic filters considered both on real and simulated data. Even though our implementation is tied to the SNfactory data, the method translates to any hyper-spectral data. As such, it is of direct relevance to several new generation instruments like MUSE. Also, this technique could be applied to multi-band astronomical imaging for which image reconstruction is important, for example to increase image resolution for weak lensing surveys.

A low optical depth region in the inner disk of the Herbig Ae star HR 5999

Abstract: Context. Circumstellar disks surrounding young stars are known to be the birthplaces of planetary systems, and the innermost astronomical unit is of particular interest. Near-infrared interferometric studies have revealed a complex morphology for the close environment surrounding Herbig Ae stars. Aims. We present new long-baseline spectro-interferometric observations of the Herbig Ae star, HR 5999, obtained in the H and K bands with the AMBER instrument at the VLTI, and aim to produce near-infrared images at the sub-AU spatial scale. Methods. We spatially resolve the circumstellar material and reconstruct images in the H and K bands using the MiRA algorithm. In addition, we interpret the interferometric observations using models that assume that the near-infrared excess is dominated by the emission of a circumstellar disk. We compare the images reconstructed from the VLTI measurements to images obtained using simulated model data. Results. The K-band image reveals three main elements: a ring-like feature located at ~0.65 AU, a low surface brightness region inside 0.65 AU, and a central spot. At the maximum angular resolution of our observations (B/λ ~ 1.3 mas), the ring is resolved while the central spot is only marginally resolved, preventing us from revealing the exact morphology of the circumstellar environment. We suggest that the ring traces silicate condensation, i.e., an opacity change, in a circumstellar disk around HR 5999. We build a model that includes a ring at the silicate sublimation radius and an inner disk of low surface brightness responsible for a large amount of the near-infrared continuum emission. The model successfully fits the SED, visibilities, and closure phases in the H and K bands, and provides evidence of a low surface brightness region inside the silicate sublimation radius. Conclusions. This study provides milli-arcsecond resolution images of the environment of HR 5999 and additional evidence that in Herbig Ae stars, there is material in a low surface brightness region, probably a low optical depth region, located inside the silicate sublimation radius and of unknown nature. The possibility that the formation of such a region in a thick disk is related to disk evolution should be investigated.

First astronomical unit scale image of the GW Orionis triple system - Direct detection of a new stellar companion

Abstract: Context. Young and close multiple systems are unique laboratories to probe the initial dynamical interactions between forming stellar systems and their dust and gas environment. Their study is a key building block to understanding the high frequency of main-sequence multiple systems. However, the number of detected spectroscopic young multiple systems that allow dynamical studies is limited. GW Orionis is one such system. It is one of the brightest young T Tauri stars and is surrounded by a massive disk. Aims. Our goal is to probe the GW Orionis multiplicity at angular scales at which we can spatially resolve the orbit. Methods. We used the IOTA/IONIC3 interferometer to probe the environment of GW Orionis with an astronomical unit resolution in 2003, 2004, and 2005. By measuring squared visibilities and closure phases with a good UV coverage we carry out the first image reconstruction of GW Ori from infrared long-baseline interferometry. Results. We obtained the first infrared image of a T Tauri multiple system with astronomical unit resolution. We show that GW Orionis is a triple system, resolve for the first time the previously known inner pair (separation ρ ~ 1.4 AU) and reveal a new more distant component (GW Ori C) with a projected separation of ~ 8 AU with direct evidence of motion. Furthermore, the nearly equal (2:1) H-band flux ratio of the inner components suggests that either GW Ori B is undergoing a preferential accretion event that increases its disk luminosity or that the estimate of the masses has to be revisited in favour of a more equal mass-ratio system that is seen at lower inclination. Conclusions. Accretion disk models of GW Ori will need to be completely reconsidered because of this outer companion C and the unexpected brightness of companion B.

Restoration of hyperspectral astronomical data from integral field spectrograph

Abstract: In this paper we present a method for hyper-spectral image restoration for integral field spectrographs (IFS) data. It takes advantage of all the spectral and spatial correlations in the observed scene to enhance the spatial resolution. 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.

A new representation and projection model for tomography, based on separable B-splines

Abstract: Data modelization in tomography is a key point for iterative reconstruction The design of the projector, ie the numerical model of projection, is mostly influenced by the representation of the object of interest, decomposed on a discrete basis of functions

Inverse problem approach to the detection of exoplanets in multi-wavelength data

Abstract: Images obtained at different wavelengths may be used to discriminate faint exoplanets from residual speckle in the stellar PSF. We have developed an inverse problem approach to fit multi-wavelength data which shows improved detection limits.

Modèle direct pour la tomographie 3D : apport d'une approximation par B-splines séparables

Abstract: En tomographie, les methodes de reconstruction iteratives necessitent une modelisation discrete du processus d'obtention des mesures. La representation de l'objet d'interet est le point de depart a l'elaboration d'un modele de projection tomographique sur le detecteur, precis et rapide. Les modeles conventionnels ray driven et distance driven, construits a partir d'indicatrices de voxels, ont l'inconvenient d'etre fortement anisotropes. Nous proposons un modele utilisant des fonctions de bases B-splines de degre adequat, nous fournissant un projecteur quasiment isotrope. Une approximation de la projection par une B-spline separable sur le detecteur conduit a un modele efficace en geometrie parallele et conique, de qualite superieure aux modeles conventionnels. Nous montrons notamment que l'erreur de modelisation est amelioree d'un facteur 10 par rapport au modele distance driven. Nous illustrons l'amelioration de la qualite de reconstruction apportee par notre modele sur des simulations du fantome de Shepp-Logan, en utilisant une methode iterative de reconstruction regularisee.

Image Reconstruction in Optical Interferometry

Abstract: Inverse problem approach is a suitable framework to analyze the challenging issues in image reconstruction from interferometric data. It can be exploited to describe and formally compare the new methods specifically developed for optical interferometry.

Déconvolution de données hyper-spectrales en astronomie

Abstract: Ce papier presente une methode de deconvolution de donnees hyper-spectrales prenant en compte les correlations spatiales et spectrales de la scene observee. Pour cela nous introduisons une regularization separable appliquee a l'objet renormalise pour tenir compte de la dynamique des objets observe. Il ne reste ainsi que deux hyper-parametres qui restent a pres constant d'une observation a l'autre. Cette methode a ete appliquee avec succes tant a des donnees simulees qu'a des donnees reelles provenant du spectrographe SNIFS de la Nearby SuperNova factory.