60 rue des bergers

Paris, France

60 rue des bergers

Paris, France
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Riaud P.,60 rue des bergers | Riaud P.,University of Liège
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 Liège
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.,University of Liège | Mawet D.,European Southern Observatory | Mawet D.,Jet Propulsion Laboratory | Magette A.,University of Liège
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.


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

Coronagraphs are used as high-contrast imaging tools. However, it is well-known that the achievable contrast is primarily limited by wavefront aberrations in the optical train. Various kinds of devices and methods have been proposed to correct and calibrate these errors and, hence, improve the efficiency of coronagraphs. Here, we present an innovative idea that allows instantaneous measuring of the phase and the amplitude of residual stellar speckles in coronagraphic images. The technique is based on the unique polarization properties of the vector vortex coronagraph, which serves as a new type of phase diversity (POAM diversity), as well an extension of the Nijboer-Zernike theory of aberrations. We also propose and discuss a simple practical optical implementation of the technique, which only requires polarization splitting at the back-end of any existing vector vortex coronagraph systems. © 2012 ESO.


Sluse D.,University of Liège | Hutsemekers D.,University of Liège | Anguita T.,Andrés Bello University | Anguita T.,Millennium Institute of Astrophysics | And 2 more authors.
Astronomy and Astrophysics | Year: 2015

Testing the standard Shakura-Sunyaev model of accretion is a challenging task because the central region of quasars where accretion takes place is unresolved with telescopes. The analysis of microlensing in gravitationally lensed quasars is one of the few techniques that can test this model, yielding to the measurement of the size and of temperature profile of the accretion disc. We present spectroscopic observations of the gravitationally lensed broad absorption line quasar H1413+117, which reveal partial microlensing of the continuum emission that appears to originate from two separated regions: a microlensed region, corresponding the compact accretion disc; and a non-microlensed region, more extended and contributing to at least 30% of the total UV-continuum flux. Because this extended continuum is occulted by the broad absorption line clouds, it is not associated with the host galaxy, but rather with light scattered in the neighbourhood of the central engine. We measure the amplitude of microlensing of the compact continuum over the rest-frame wavelength range 1000-7000 Å. Following a Bayesian scheme, we confront our measurements to microlensing simulations of an accretion disc with a temperature varying as T ∝ R-1/ν. We find a most likely source half-light radius of R1/2 = 0.61 × 1016cm (i.e., 0.002 pc) at 0.18μm, and a most-likely index of ν = 0.4. The standard disc (ν = 4/3) model is not ruled out by our data, and is found within the 95% confidence interval associated with our measurements. We demonstrate that, for H1413+117, the existence of an extended continuum in addition to the disc emission only has a small impact on the inferred disc parameters, and is unlikely to solve the tension between the microlensing source size and standard disc sizes, as previously reported in the literature. © 2015 ESO.


Riaud P.,60 rue des Bergers | Riaud P.,Collège 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.


Nunez P.D.,Collège de France | Nunez P.D.,Laboratoire Lagrange | Labeyrie A.,Collège 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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