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Le Touquet – Paris-Plage, France

Riaud P.,60 rue des bergers | Riaud P.,University of Liege
Astronomy and Astrophysics | Year: 2012

This paper presents a new architecture for deformable mirrors, suitable for adaptive optics (AO) observations. The performance of these systems directly depends on the density of actuators, which correct for wavefront errors. Therefore, deformable mirrors of large sizes are difficult to build because of the required number of actuators. Moreover, these actuators use either piezoelectric, magnetic, or electrostatic systems, which all suffer from nonlinear behavior (hysteresis). To solve these problems, we propose to use addressable vertical-cavity surface-emitting laser (VCSELs) systems, which move, through radiation pressure, a CVD-diamond membrane coated with an aluminum/gold thin film. The advantage of such a system is the high actuator density (16 384 on 41 mm 2) without hysteresis problems. This technology is therefore well-suited for the next generation of AO systems. © 2012 ESO.

Riaud P.,60 rue des bergers | Riaud P.,University of Liege
European Physical Journal D | Year: 2012

In this paper we propose a new interferometric scheme using photon entanglement. The two main limitations of stellar interferometry are (a) the small sensitivity and (b) the need for long delay-lines to compensate the path difference between the telescopes during observing runs. Entangled-photon pairs, generated by spontaneous parametric down-conversion, open the way to measuring quantum states correlation in the near infrared between two spatially separated telescopes and at very high sensitivities (down to a few stellar photons), thanks to a new interferometric layout which does not make use of complex long delay-lines. A femtosecond laser coupled to a nonlinear crystal is used as a local oscillator to perform the double homodyne measurements. This new quantum interferometer allows to measure astronomical objet sizes with very high angular resolution down to μas level. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012.

Riaud P.,60 rue des bergers | Riaud P.,College de France
Monthly Notices of the Royal Astronomical Society | Year: 2014

The vector vortex coronagraph is a phase mask creating a phase ramp around its optical axis to cancel on-axis starlight in the focal plane of a telescope. This phase ramp also induces an orbital angular momentum (OAM) on the output stellar photons, equal to twice the topological charge of the vortex q. In this paper, we show that the residual starlight diffracted withinthe coronagraphic pupil by the central obscuration and by optical aberrations (including atmospheric turbulence) is also affected by a non-zero OAM, while the off-axis planetary signal remains mostly in a zero-OAM state. I propose an optical set-up, which we call the Photon OrbitalMomentum Interferometer , to separate the planetary signal from the remaining starlight. The proposed set-up is very simple, using rotated Dove prisms in the two arms of a Mach-Zehnder interferometer, and allows deep starlight cancellation to be reached. This new concept could be instrumental in the direct detection of terrestrial planets around nearby stars with future extremely large ground-based telescopes or with space-based systems. © 2014 The Authors.

Riaud P.,60 rue des bergers | Riaud P.,University of Liege | Mawet D.,European Southern Observatory | Mawet D.,Jet Propulsion Laboratory | Magette A.,University of Liege
Astronomy and Astrophysics | Year: 2012

We introduce a novel phase retrieval method for astronomical applications based on the Nijboer-Zernike (NZ) theory of diffraction. We present a generalized NZ phase retrieval process that is not limited to small and symmetric aberrations and can therefore be directly applied to astronomical imaging instruments. We describe a practical demonstration of this novel method that was recently performed using data taken on-sky with NAOS-CONICA, the adaptive optics system of the Very Large Telescope. This demonstration presents the first online on-sky phase retrieval results ever obtained, and allows us to plan subsequent refinements on a well-tested basis. Among the potential refinements, and within the framework of high-contrast imaging of extra-solar planetary systems (which requires exquisite wavefront quality), we introduce an extension of the generalized NZ to the high-dynamic range case, and particularly to its use with the vector vortex coronagraph. This induces conjugated phase ramps applied to the orthogonal circular polarizations, which can be used to instantaneously retrieve the complex amplitude of the field, yielding a real-time calibration of the wavefront that does not need any other modulation such as focus or other deformable mirror probe patterns. Paper II (Riaud et al. 2012, A&A, 545, A151) presents the mathematical and practical details of the new method. © 2012 ESO.

Nunez P.D.,College de France | Nunez P.D.,Laboratoire Lagrange | Labeyrie A.,College de France | Labeyrie A.,Laboratoire Lagrange | Riaud P.,60 rue des bergers
Monthly Notices of the Royal Astronomical Society | Year: 2014

Optical interferometry has been successful at achieving milliarcsecond resolution on bright stars.Imaging performance can improve greatly by increasing the number of baselines, which has motivated proposals to build large (̃100 m) optical interferometers with tens to hundreds of telescopes. It is also desirable to adaptively correct atmospheric turbulence to obtain direct phased images of astrophysical sources. When a natural guide star is not available, we investigate the feasibility of using a modified laser-guide-star technique that is suitable for large diluted apertures. The method consists of using subsets of apertures to create an array of artificial stars in the sodium layer and collecting back-scattered light with the same subapertures. We present some numerical and laboratory simulations that quantify the requirements and sensitivity of the technique. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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