Exo-planets much fainter than their parent star are likely imageable atvisible wavelengths with a 2 to 8 meter space telescope equipped forcoronagraphy. The dynamic range can be further increased by nulling thespeckled star residue, before detection, with an adaptive hologram. Like theMach-Zehnder interferometer of Codona & Angel (2004), the hologramsubtracts from the stellar residue. a copy of it, made with the light rejectedby the Lyot mask. The final residue can in principle be as low as one photonper speckle, on average. It makes exo-Earths detectable if their Airy peakcontains a few photons. The method can also relax the difficult figuringtolerances for the pre-focal optics. With Lippmann-Bragg holograms, it canbe achromatized.

Telescope pupils are usually not well suited for high dynamical range imaging. Central obstruction, telescope spiders, hard edges and sparseness (for interferometers) can make it very challenging to build a coronagraph. Fortunately, it is often possible to remap a telescope pupil to achieve high contrast imaging without loosing light. In the case of a monolithic telescope, pupil remapping can produce an apodized pupil with 2 aspheric mirrors which redistribute the light without introducing phase aberrations. Similarly, an interferometer pupil can be remapped into a tight configuration compatible with coronagraphic techniques.

The difficult goal of directly detecting a planet around a star requiresto cancel as much as possible the stellar light.Since the first proposal by Bracewell of a nulling interferometer, where thestar is put on a central dark fringe, several interferometricconfigurations have been presented in order to improve the quality ofthe rejection, especially to avoid the leaks due to the finite angular dimensionof the stellar disk, resolved by the interferometer. In the Bracewell interferometer, the behaviour of the nulling efficiency vs the angular distance θ to the star is as (1-cosθ)∝ θ2. One goal is to increase the exponent of the term θn whichgives the cancellation efficiency. I present one method to define configurations of telescopes positions, sizes and phase-shift that canachieve any given power of θ. The principle is based on a peculiar property foundby Prouhet of a partition into two sets of the integers, done according to the Thué-Morse sequence.2L telescopes regularly spaced on a line, are distributed into two groups, following their rankin the Thué-Morse sequence and, to the telescopes of one of the groups, is applied a π phase shift. The result is a fractal-like distribution of the telescopes where redundancy is minimum andwhose interferometric combination produces a very efficient nulling in θ2L.I first examine 1-D patterns of identical telescopes, then extend the method to 2-D configurations, then show that the lattercan be used to define 1-D arrays of non identical telescopes, according to some algebra of interferometers. The generalizationto arrays where the phase shift between n groups of telescopes is 2kπ/n is finally proposed.

Phase masks have numerous applications in astronomical optics, in particular related to two themes: coronagraphy for detection and analysis of extrasolar planets or circumstellar disks, and wavefront analysis for extremely precise adaptive optics systems or cophasing of segmented mirrors. I review some of the literature concerning phase masks and attempt to bridge the gap between two instrumental systems in which they are often found: the Mach-Zehnder interferometer and the coronagraph.

High contrast imaging opportunties abound in astronomy, and some carry such intrinsic interest that they justify considerable effort to master the technical challenges. The field has a rich history dating back to the beginnings of electromagnetic theory, and ten years ago one might have been forgiven for thinking that the topic was passé. But just within the last decade the study has fluorished with major advances on multiple fronts - scientific as well as technical. The analytical and conceptual generalization of classical coronagraphy to include complex masks and apodizations has opened a fertile field of study. Nulling in interferometry and direct imaging has developed in several promising directions, with empirical and theoretical understanding sometimes passing via different routes to arrive at firm conclusions. Critical supporting technology is developing apace, often for reasons having little to do with high contrast imaging. Thus very large telescopes, space telescopes, adaptive optics, and ultra-precision optics are all steadily advancing, and thus building a priceless technological foundation for future high contrast imaging experiments and facilities. This chapter offers a brief, simple overview, intended primarily to give a context for the many detailed technical papers in this volume.

In this communication, we analyze the possibility of detectingexoplanets using shaped and apodized apertures alone (withoutcoronagraphy) for the case of a perfect telescope operated inspace. In a first section, after a brief introduction to thetheory of diffraction and the determination of several usefulparameters, we show that the effects of shaped and apodizedaperture can be presented in an unified manner, making use of theRadon transform of the aperture of the telescope. This permits asimple comparison of the techniques of apodization and apertureshaping between them. An illustration of this approach is madefor a few apertures. In a second section, we introduce thesemi-classical theory of photodetection. Using simplificativeassumptions, we derive signal-to-noise ratio (SNR) expressions anddraw the corresponding curves for an experiment using a circulartelescope apodized by a prolate spheroidal function. We found thatfor given condition there is an apodization that can optimize theSNR. This apodization is generally a very strong one.

In this communication we study the statistics of Adaptive Optics remnant speckles, and we discuss how a coronagraph can defeat the noise associated with these speckles. At high Strehl Ratio regimes, residual speckles are pinned on the diffraction rings of the airy pattern. It can be shown that these speckles are due to small defaults of the wavefront, amplified by the coherent part of the wave and that the statistics of their intensity can be described by a modified Rice distribution. At low flux levels, a Poisson-Mandel transformation provides an analytical expression of the Probablility Density Function. We show the results of a numerical simulation and compare the results to the theoretical model. Simple analytical expressions can be derived for the variance of the noise.We discuss the efficiency of a coronagraph in terms of Signal to Noise Ratio, based on the analysis of the noise contributions which can be reduced by a coronagraph.

The structure function (or the power spectrum)of phasefluctuations induced by atmospheric turbulence is the mainquantity which must be known to optimize the High AngularResolution techniques used for astronomical observations. Itallows to evaluate the state of coherence of the perturbedwavefronts. In this paper are firstly recalled classical resultsconcerning the propagation of the luminous waves through theterrestrial atmosphere and consequently the image formation. Then,different models generally used to describe the optical effects ofatmospheric turbulence, due to numerous thin layers, are presentedand their influence discussed. A second order statistical analysisis performed and the coherence parameters deduced. The necessityof measuring these parameters appears evident and dedicatedmonitors must be built.

On the Antarctica plateau, a joint project of french and italianpolar programmes in just near completion: the Concordia stationwill be open for winterover operation in 2005. The high altitudeand high latitude of this site, the exceptionally cold, clear andstable atmosphere, the almost indefinitely flat snow surface andthe not so difficult access make this site the most promising onEarth for future ground based astronomical projects in variousfields, including High Angular Resolution and High ContrastImaging.

Due to its capacity in retrieving the angular resolution of ground-based optical telescopestogether with their intrinsic sensitivity, adaptive optics (AO) is well known to be of fundamentalimportance for a wide range of astronomical applications. In the case of veryhigh-contrast imaging it is even more critical given the limitation due to the atmosphericresiduals. The ability of an AO system in "cleaning" the obtained point-spread functionbefore a coronagraphic stage is therefore of central importance - and so the robustnessof a given coronagraphic stage with respect to the post-AO atmospheric residuals.Within this framework, thepresent paper describes the physical ingredients that are necessary to be modelized whendesigning a detailed numerical simulation study of this kind, together with a typical numericaltool used in this context. An example involving a very high-order AO system is given, asimple implementation of different coronagraphic concepts is exposed, and an illustrativepreliminary result involving the high-order AO-correction stage coupled with the different coronagraphicdevices implemented is shown.

This paper describes a general methodology for simulating varioustypes of coronagraphs, especially in the context of their relativecomparison in terms of performance. Some critical points arediscussed such as sampling problems, numerical dynamic range andcomputation errors which could hamper the results. Moreover,comparing the performance of these High Dynamic Range Imaging(HDRI) instruments is not straightforward, so I will define anddiscuss some criteria and their significance.

This paper gives a description of some procedures to performnumerical simulations for coronagraphic instruments in realisticconditions.

SOAR, located on Cerro Pachón in Chile, is a recently completed 4.1 m telescope, dedicated to high angular resolution. To complement the other two larger telescopes, Gemini on Cerro Pachón and the Blanco telescope on Cerro Tololo, SOAR's niche is high resolution, imaging in the visible and spectroscopy, which calls for the development of an adaptive optics (AO) module. This instrument will provide improved seeing over a wide field, for visible wavelengths. In this paper, I briefly describe which kind of science can be carried out with this type of AO and compare its performance with other ground-based and space-based telescopes. Then, the ground layer compensation principle and its application for Pachón atmospheric conditions are presented. Most of the nights, the perturbations are located close to the ground; thus a partial correction of the first kilometer of the atmosphere will be efficient to improve the seeing significantly (by a factor of 2 or more). Finally, technical details on the instrument itself including the deformable mirror, the wavefront sensor, the laser and the turbulence simulator are given.

We report on the facility we developed at the Observatoire de Meudon to test the Four Quadrant Phase Mask (4QPM) coronagraph in the Infrared (IR) and at low temperature. This facility has been developed in the context of the mid-IR instrument MIRI for the James Webb Space Telescope. Three 4QPM will take place inside MIRI and need to be qualified for space environment. The performance of the coronagraph is in agreement with our expectations from numerical simulations. The peak intensity of the diffraction pattern is reduced by a factor 380, a performance being limited by the chromatic effects and the size of the bench source.

MIRI, the mid-IR instrument of JWST (James Webb Space Telescope),will include a coronagraphic device for which the ultimate goal isthe detection and caracterization of extrasolar planets aroundnearby stars. In this paper, we describe the optical concept ofMIRI and the instrumental choices that were made in phase A and Bregarding the coronagraphic system. Finally, we will presentnumerical simulations that we carried out to evaluate thecapability for detecting extrasolar giant gazeous planets.

In order to optimize the occulting process of a Lyot coronagraph and to provide a high dynamic range imaging, a new kind of occulting disk has been developed at the National Observatory ofRio de Janeiro. A mercury (Hg) drop glued onto an optical window by molecular cohesion and compressed by a pellicle film is used as the occulting disk. The minimum of the superficial tension potential function provides an optical precision (lambda/100) of the toric free surface of the mercury. This process provides a size control for the adaptation to the seeing conditions and to the apparent diameter of a resolved object, and in the case of adaptive optics, to the Airy diameter fraction needed. The occultation is a three dimensional process near the focal plane on the toric free surface that provides an apodization of the occultation. The Hg-Mask coronagraph has been projected for astrometric observations of faint satellites near to Jovian planets and works since 2000 at the 1.6 m telescope of the Pico dos Dias Observatory (OPD - Brazil).

This paper aims at describing (broad lines) the various instrumental concepts for nulling interferometry and how some observational constraints have driven the evolution from the initial Bracewell configuration to the ESA-Darwin mission configuration. Before this short overview, basic principles of interferometry and of nulling interferometry are recalled. Since a key point in the nulling mode is to insert achromatic phase shifts between interfering waves, a short description of several Achromatic Phase Shifters concepts is given as well as the associated constraints.

As simple it appears to be on the paper, nulling interferometry is a difficult technique when experimentally implemented to reach high rejection factors. In that case, each defect of the system is a possibility to bring back on the detector, the photons we want to eliminate. The loss in terms of S/N ratio is quickly important. In this paper, I develop present state of the art in nulling interferometry field and experimental realisations. I describe their performance and limitations, and present technological developments all arounf the world. Particularly, I emphasize present development regarding to optical functions in the infrared spectral range (delay lines, achromatic phase shifters, beam combiners...).

Two competitive design studies for the Ground-based European Nulling Interferometer Experiment(GENIE) have recently been initiated by the European Space Agency and the European SouthernObservatory. A major issue in these studies is the influence of atmospheric turbulence on theperformance of the instrument, and how atmospheric effects can be compensated in order to reach thegoal performance (detection of faint exozodiacal clouds). In this paper, we review the mainatmospheric processes affecting a nulling interferometer and discuss possible ways to reduce themby means of real-time control systems. Preliminary performance estimates of GENIE are thenpresented. The effects of the thermal background and its fluctuations (Absil & Bakker 2004) are notconsidered here.