Large Scale 3D Image Reconstruction in Optical Interferometry

Astronomical optical interferometers (OI) sample the Fourier transform of the intensity distribution of a source at the observation wavelength. Because of rapid atmospheric perturbations, the phases of the complex Fourier samples (visibilities) cannot be directly exploited , and instead linear relationships between the phases are used (phase closures and differential phases). Consequently, specific image reconstruction methods have been devised in the last few decades. Modern polychromatic OI instruments are now paving the way to multiwavelength imaging. This paper presents the derivation of a spatio-spectral ("3D") image reconstruction algorithm called PAINTER (Polychromatic opticAl INTErferometric Reconstruction software). The algorithm is able to solve large scale problems. It relies on an iterative process, which alternates estimation of polychromatic images and of complex visibilities. The complex visibilities are not only estimated from squared moduli and closure phases, but also from differential phases, which help to better constrain the polychromatic reconstruction. Simulations on synthetic data illustrate the efficiency of the algorithm.Comment: EUSIPCO, Aug 2015, NICE, Franc

Development of an ELT XAO testbed using a self referenced Mach-Zehnder wavefront sensor

Extreme adaptive optics (XAO) has severe difficulties meeting the high speed (>1kHz), accuracy and photon efficiency requirements for future extremely large telescopes. An innovative high order adaptive optics system using a self-referenced Mach-Zehnder wavefront sensor (MZWFS) allows counteracting these limitations. In addition to its very high accuracy, this WFS is the most robust alternative to segments gaps and telescope spiders which can result in strong wavefront artifacts. In particular in XAO systems when the size of these gaps in the wavefront measurement is comparable to the sub aperture size, loss in performance can be very high. The MZWFS estimates the wavefront phase by measuring intensity differences between two outputs, with a λ/4 path length difference between its two legs, but is limited in dynamic range. During the past few years, such an XAO system has been studied by our team in the framework of 8-meter class telescopes. In this paper, we report on our latest results with the XAO testbed recently installed in CRAL laboratory, and dedicated to high contrast imaging with 30m-class telescopes (such as the E-ELT or the TMT). A woofer-tweeter architecture is used in order to deliver the required high Strehl ratio (>95%). It consists of a 12x12 deformable mirror (DM) and a 512x512 Spatial Light Modulator (SLM) characterized both using monochromatic and polychromatic light. We present our latest experimental results, including components characterization, close loop performances and sensitivity to calibration errors. This work is carried out in synergy with the validation of fast iterative wavefront reconstruction algorithms and the optimal treatment of phase ambiguities in order to mitigate the dynamical range limitation of such a wavefront sensor

Fast and robust detection of a known pattern in an image

International audienceMany image processing applications require to detect a known pattern buried under noise. While maximum correlation can be implemented efficiently using fast Fourier transforms, detection criteria that are robust to the presence of outliers are typically slower by several orders of magnitude. We derive the general expression of a robust detection criterion based on the theory of locally optimal detectors. The expression of the criterion is attractive because it offers a fast implementation based on correlations. Application of this criterion to Cauchy likelihood gives good detection performance in the presence of outliers, as shown in our numerical experiments. Special attention is given to proper normalization of the criterion in order to account for truncation at the image borders and noise with a non-stationary dispersion

Regularized 4D-CT reconstruction from a single dataset with a spatio-temporal prior

X-ray Computerized Tomography (CT) reconstructions can be severely impaired by the patient’s respiratory motion and cardiac beating. Motion must thus be recovered in addition to the 3D reconstruction problem. The approach generally followed to reconstruct dynamic volumes consists of largely increasing the number of projections so that independent reconstructions be possible using only subsets of projections from the same phase of the cyclic movement. Apart from this major trend, motion compensation (MC) aims at recovering the object of interest and its motion by accurately modeling its deformation over time, allowing to use the whole dataset for 4D reconstruction in a coherent way.We consider a different approach for dynamic reconstruction based on inverse problems, without any additional measurements, nor explicit knowledge of the motion. The dynamic sequence is reconstructed out of a single data set, only assuming the motion’s continuity and periodicity. This inverse problem is solved by the minimization of the sum of a data-fidelity term, consistent with the dynamic nature of the data, and a regularization term which implements an efficient spatio-temporal version of the total variation (TV). We demonstrate the potential of this approach and its practical feasibility on 2D and 3D+t reconstructions of a mechanical phantom and patient data

Fast minimum variance wavefront reconstruction for extremely large telescopes

We present a new algorithm, FRiM (FRactal Iterative Method), aiming at the reconstruction of the optical wavefront from measurements provided by a wavefront sensor. As our application is adaptive optics on extremely large telescopes, our algorithm was designed with speed and best quality in mind. The latter is achieved thanks to a regularization which enforces prior statistics. To solve the regularized problem, we use the conjugate gradient method which takes advantage of the sparsity of the wavefront sensor model matrix and avoids the storage and inversion of a huge matrix. The prior covariance matrix is however non-sparse and we derive a fractal approximation to the Karhunen-Loeve basis thanks to which the regularization by Kolmogorov statistics can be computed in O(N) operations, N being the number of phase samples to estimate. Finally, we propose an effective preconditioning which also scales as O(N) and yields the solution in 5-10 conjugate gradient iterations for any N. The resulting algorithm is therefore O(N). As an example, for a 128 x 128 Shack-Hartmann wavefront sensor, FRiM appears to be more than 100 times faster than the classical vector-matrix multiplication method.Comment: to appear in the Journal of the Optical Society of America

VITRUV - Science Cases

VITRUV is a second generation spectro-imager for the PRIMA enabled Very Large Telescope Interferometer. By combining simultaneously up to 8 telescopes VITRUV makes the VLTI up to 6 times more efficient. This operational gain allows two novel scientific methodologies: 1) massive surveys of sizes; 2) routine interferometric imaging. The science cases presented concentrate on the qualitatively new routine interferometric imaging methodology. The science cases are not exhaustive but complementary to the PRIMA reference mission. The focus is on: a) the close environment of young stars probing for the initial conditions of planet formation and disk evolution; b) the surfaces of stars tackling dynamos, activity, pulsation, mass-loss and evolution; c) revealing the origin of the extraordinary morphologies of Planetary Nebulae and related stars; d) studying the accretion-ejection structures of stellar black-holes (microquasars) in our galaxy; e) unveiling the different interacting components (torus, jets, BLRs) of Active Galactic Nuclei; and f) probing the environment of nearby supermassive black-holes and relativistic effects in the Galactic Center black-hole.Comment: 15 pages. The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation, Allemagne (2005) in pres

Why Chromatic Imaging Matters

During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u-v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.Comment: Accepted for publication in Experimental Astronomy as part of the topical collection: Future of Optical-infrared Interferometry in Europ

VITRUV - Imaging close environments of stars and galaxies with the VLTI at milli-arcsec resolution

The VITRUV project has the objective to deliver milli-arcsecond spectro-images of the environment of compact sources like young stars, active galaxies and evolved stars to the community. This instrument of the VLTI second generation based on the integrated optics technology is able to combine from 4 to 8 beams from the VLT telescopes. Working primarily in the near infrared, it will provide intermediate to high spectral resolutions and eventually polarization analysis. This paper summarizes the result from the concept study led within the Joint Research Activity advanced instruments of the OPTICON program.Comment: In "The Power of Optical/IR Interferometry: Recent Scientific Results and 2nd Generation VLTI Instrumentation", Allemagne (2005) in pres

New insights on the population genetic structure of the great scallop (Pecten maximus) in the English Channel coupling microsatellite data and demogenetic simulations.

International audienceThe great scallop (Pecten maximus) is a commercially important bivalve in Europe, particularly in the English Channel, where fisheries are managed at regional and local scales through the regulation of fishing effort. In the long term, knowledge about larval dispersal and gene flow between populations is essential to ensure proper stock management. Yet, previous population genetic studies have reported contradictory results. In this study, scallop samples collected across the main fishing grounds along the French and English coasts of the English Channel (20 samples with temporal replicates for three sites,n= 1059 individuals), and the population genetic structure was analysed using 13 microsatellite loci. Coupling empirical genetic data with demogenetic modelling based on a biophysical model simulating larval exchanges among scallop beds revealed a subtle genetic differentiation between south-west English populations and the rest of the English Channel, which was consistent with larval dispersal simulations. The present study provides a step forward in the understanding of great scallop population biology in the English Channel, underlining the fact that even in a context of potentially high gene flow and recent divergence times since the end of the last glacial maximum, weak but significant spatial genetic structure can be identified at a regional scale

Food web structure and trophic interactions at the recently discovered deep-sea La Scala hydrothermal vent field (SW Pacific).

editorial reviewedHydrothermal vents are features of the seafloor where fluids (acidic, geothermally heated water enriched in chemical compounds such as hydrogen sulphide) discharge in the water column. These fluid emissions sustain unusual chemosynthesis-based ecosystems where abundant micro-organisms and animal communities can thrive under extreme conditions. In May 2019, the La Scala vent field was discovered in Woodlark Basin (Papua New Guinea, SW Pacific). Several active "black smokers" harbouring dense fauna were found at depths ranging from 3300 to 3400 m. The main engineer species were symbiont-bearing gastropods Ifremeria nautilei and Alviniconcha spp. in more active diffuse areas, and stalked barnacles Vulcanolepas sp. nov. in mildly active areas. At least 44 taxa were observed in these habitats. Here, we used trophic markers (stable isotope ratios of C, N and S) to identify energy fluxes supporting those communities, and understand how their feeding habits could influence interspecific interactions. Most sampled animals primarily depended (either directly or indirectly) on endogenous chemosynthetic vent production for their nutrition. This dependence spanned all sampled taxonomic and functional groups. It extended to organisms considered as peripheral fauna, or not strictly found at vents, such as Vulcanolepas sp. nov., anemones, or scavenging gastropods. Moreover, other peripheral fauna fed on a mix of both chemosynthesis- and photosynthesis-derived items. This emphasizes the importance of exported vent production for the surrounding deep-sea fauna. Animal communities showed considerable trophic diversity, and depended on several bacterial production mechanisms. Many taxa co-relied on two or more carbon sources, and inter- and intra-taxon differences in feeding habits could lead to a more even segregation of available food resources. While many questions about environmental and biological drivers of food web structure at La Scala vent field remain open, our results constitute a first glimpse at processes shaping those freshly discovered communities