Founding on a physical transformation process described by aFredholm integral equation of the first kind, we first recall themain difficulties appearing in linear inverse problems in thecontinuous case as well as in the discrete case. We describe severalsituations corresponding to various properties of the kernel of theintegral equation. The need to take into account the properties of the solution notcontained in the model is then put in evidence. This leads to theregularization principles for which the classical point of view aswell as the Bayesian interpretation are briefly reminded. We then focus on the problem of deconvolution specially applied toastronomical images. A complete model of image formation isdescribed in Section 4, and a general method allowing to deriveimage restoration algorithms, the Split Gradient Method (SGM), is detailedin Section 5. We show in Section 6, that when this method is applied to thelikelihood maximization problems with positivity constraint, theISRA algorithm can be recovered in the case of the pure Gaussianadditive noise case, while in the case of pure Poisson noise, thewell known EM, Richardson-Lucy algorithm is easily obtained. Themethod is then applied to the more realistic situation typical ofCCD detectors: Poisson photo-conversion noise plus Gaussian readoutnoise, and to a new particular situation corresponding to dataacquired with Low Light Level CCD. Some numerical results areexhibited in Section 7 for these two last cases. Finally, we showhow all these algorithms can be regularized in the context of theSGM and we give a general conclusion.

In this paper we give a survey of the methods we have developed for multiple image deconvolution, with application to the reconstruction of the images of the Large Binocular Telescope (LBT). We first describe the main features of LBT and of the Fizeau interferometer, denoted LINC-NIRVANA, that will be one of the basic instruments of the telescope. It will allow to reach the resolution of a 22.8 m mirror by combining different images taken with different orientations of the baseline. Next we discuss the problem of multiple image deconvolution, that is crucial for obtaining a unique high-resolution image from the multiple images provided by LINC-NIRVANA. We present the state-of-the art of the methods based on the Richardson-Lucy (RL) approach and we discuss topics such as computational efficiency, correction of boundary effects and super-resolution. Then, in the perspective of going beyond RL, we extend to the problem ofmultiple image deconvolution the split gradient method (SGM) that isa general approach to the design of iterative methods for the constrained minimization of regularized functionals. Finally we present an application of SGM to the regularized reconstruction of objects with high-dynamic range. The different methods are illustrated with examples taken from the many numerical experiments we performed on this problem.

This paper first gives an introduction to the broad features of the Darwin mission of ESA and then describes in some detail the data processing that is necessary to detect planets and spectrally characterize them. The proposed data processing method is validated by means of simulated data.

A specificity of direct imaging systems for extra solar planetdetection is that neither the residuals of the turbulence, nor the ideal coronagraphed point spread function can be neglected with respect to the planet.This has a direct consequence on the nature of the statistical distribution of the images,which plays a central part in the derivation of a powerful detection algorithm.The purpose of this paper is to study these specific effects: the responseof the coronagraph is first analyzed then the statistical distribution of the images ischaracterized.

IRAN is a method of beam-combination in thehypertelescope imaging technique recently introduced byLabeyrie in optical interferometry. We propose to observe the interferometric image in the pupil plane, performing multi-axial pupil plane interferometry. Imaging is performed in a combined pupil-plane where the point-source intensity distribution (PSID) tends towards apseudo Airy disc for a sufficiently large number of telescopes. The image is concentrated into the limited support of the output pupil of the individual telescopes, in which the object-image convolution relation is conserved. Specific deconvolution algorithms have been developped for IRAN hypertelescope imagery, based upon Lucy-like iterative techniques. We show that the classical (image plane) and IRAN (pupil plane) hypertelescope imaging techniques are equivalent if one uses optical fibers for beam transportation. An application to the VLT/VIDA concept is presented.

This paper deals with adaptive optics in the framework of very high-contrast astronomical imaging. Starting from a brief global overview of classical adaptive optics (AO) systems, the specificities and main characteristics of extreme AO (XAO) systems are exposed. Along the way from AO to XAO, some relevant points (relevant with respect to the goals claimed by high-contrast imaging techniques) are emphasized.

After a brief historical summary of the control strategies for Adaptive Optics, we present a new optimal control law for AO, based on a Kalman filter on a state space formalism and we apply it to Multiconjugate AO and eXtreme AO. We discuss its interest with respect to the previous existing control laws and its applicability to real systems.

In this paper, we briefly describe the principle of analytical modelization of the adaptive optics (AO) corrected point spread function in the spatialfrequency domain, both for the turbulent and the telescope static aberrations,and demonstrate how this can be used for simulating AO corrected phase screenson extremely large hexagonal segmented telescopes, required for performance analysis of diffraction limited coronagraphs.

This paper provides an overview of wavefront sensing technologies. A brief background to the field of wavefront sensing is given, and several wavefront sensors are studied in detail. Particular attention is given to the Shack-Hartmann, Phase Diversity and Curvature wavefront sensors. Interferometric and Pyramid wavefront sensors are discussed as well as Schlieren imaging, algorithmic approaches (such as image sharpening), Shack-Hartmann Curvature hybrid wavefront sensing and modal wavefront sensors.

The extreme cleaning of stellar images, needed for distinguishing their planets and especially those analogous to the Earth, requires coronagraphy and apodization. These techniques suppress the diffraction rings, but not the residual speckles resulting from the bumpiness of the optical surfaces.Several methods, coherent and incoherent, can in principle remove these speckles, but some of them are affected if the star is slightly resolved. With the future large hypertelescopes, spanning 100 kilometers or more to resolve the details of exo-Earths, this could require cleaning procedures applied separately for each sub-aperture before recombining the beams.

In this communication we illustrate the main steps required for the preprocessing of the Lyot Project Coronagraph data, starting from the raw data to the reduced data. We then discuss the estimation of the performance on direct, unocculted data, by measuring the Strehl Ratio on these images. We show that Strehl Ratios of the order of 80% can be obtained for the best images in the H Band, using the AEOS telescope adaptive optics, and the Kermit infrared camera. We then present a few methods to estimate the dynamic range in coronagraphic images, and their results are discussed.

Direct detection of Earth-like planets is one of the major objectivesfor the next generation of ground based telescopes (Extremely LargeTelescopes, ELT). ELT instruments will be conditionned by suchscientific programs. Here, we consider here a system made with anadaptive optics device, a coronagraph and a simultaneous calibrationsystem. We refined this simple model by including static aberrationsupstream the coronagraph and differential aberrations downstream. Usingnumerical simulations, we evaluate how these static aberrations limitthe detectability of telluric planets.

The existing high contrast imaging techniques cannot be applieddirectly to the telescope of the diameter 30–100 m without takinginto account the specific characteristics of a segmented surface.While the increase in telescope diameter is an advantage for thehigh contrast range science, the segmentation sets a limit on theperformance of the coronagraph. In particular, diffraction fromintersegment gaps sets a floor to the achievable extinction of thestarlight. Masking out the gaps in the Lyot plane although helpsincreasing the contrast, does not solve completely the problem: thehigh spatial frequency component of the diffractive light remains.In the paper I suggest using a Lyot stop which produces aphase-amplitude modulation in order to reduce the effect ofsegmentation.

Recent developments in coronagraphy have increased the reachable dynamic range limit of telescopes and thus opened the possibility for the imaging of fainter and fainter companions. They rely on a series of modifications of the original concept involving amplitude and phase masks in either the pupil or the image plane. They are designed to operate optimally at one wavelength. In this paper we first study the chromatic leakage in Pupil Apodised Lyot Coronagraphs and then present a theoretical solution that create a polychromatic null while maximizing the photon flux from the companion. Finally we discuss a promising experimental implementation of this solution using interferometric apodisations.

We present the principle and performances of a special kind of interferometric arrays that yields images of high dynamic range and large field. This array consists of a punched foil, in which numerous individual apertures form a pattern related to a Fresnel zone plate. We present the concept, theoretical results and simulations for exoplanets detection.

Pupil Replication is a technique which could be added to many existing high contrast imaging systems to improve the inner working angle or the contrast when using Pupil Replication. The output pupil of the telescope is replicated and mapped onto the input pupil of the high contrast imaging system. This decreases the size of the image of the star on axis and thereby increases the performance of the high contrast imaging system. In this paper the viability of such a system is assessed through mathematical analysis and simulations.

This paper exposes a method for evaluating the performances of a systemdedicated to exoplanet direct imaging. It is based on the use of a statisticaldescription of a somewhat ideal star/planet couple in terms of contrast and angular separation.Among many already available techniques, the Phase Induced Amplitude ApodizationCoronagraph (PIAAC) seems to gather all the necessary qualities to performdirect imaging of hypothetical Earth-like exoplanets in the close Solarneighbourhood (30 pc).After a quick description of its concept, the proposed evaluation method is used and provides an original set of criteria to classify concepts.

This paper essentially is a reminder of basic principles and notions found in Long Baseline Interferometry. Namely, general comments and vocabulary, science motivations and goals, conceptual toolbox and operating principles are recalled. For this latter item, working in ideal world and in real life are considered.


We describe in this communication the principle of imaging withdiluted apertures from the theoretical point of view of Fourieroptics, for a noiseless experiment using perfect telescopes inspace. Two extreme cases are considered, corresponding to denseand sparse arrays. Dense arrays make it possible to obtain acomplete spatial frequency coverage; after signal processing, theimages are comparable to those obtained with a very largemonolithic aperture. Such a perfect frequency coverage cannot beobtained with parse arrays for which the distances betweentelescopes are very large compared to their size. These arrays canonly sample the Fourier plane at point-like individualfrequencies, with the drawback of a field limitation. Sparsearrays have however the most promising perspectives for imagery ofstars and exoplanets surfaces.