Pierre Riaud

The Four-Quadrant Phase-Mask Coronagraph. I. Principle

We describe a new type of coronagraph, based on the principle of a phase mask as proposed by Roddier and Roddier a few years ago but using an original mask design found by one of us (D. R.), a four-quadrant binary phase mask (0, n) covering the full Ðeld of view at the focal plane. The mutually destructive interferences of the coherent light from the main source produce a very efficient nulling. The computed rejection rate of this coronagraph appears to be very high since, when perfectly aligned and phase-error free, it could in principle reduce the total amount of light from the bright source by a factor of 108, corresponding to a gain of 20 mag in brightness at the location of the Ðrst Airy ring, relative to the Airy peak. In the real world the gain is of course reduced by a strong factor, but nulling is still performing quite well, provided that the perturbation of the phase, for instance, due to the EarthIs atmosphere, is efficiently corrected by adaptive optics. We show from simulations that a detection at a contrast of 10 mag between a star and a faint companion is achievable in excellent conditions, while 8 mag appears routinely feasible. This coronagraph appears less sensitive to atmospheric turbulence and has a larger dynamic range than other recently proposed nulling techniques : the phase-mask coronagraph (by Roddier and Roddier) or the Achro- matic Interfero-Coronagraph (by Gay and Rabbia). We present the principle of the four-quadrant corona- graph and results of a Ðrst series of simulations. We compare those results with theoretical performances of other devices. We brieNy analyze the di†erent limitations in space or ground-based observations, as well as the issue of manufacturing the device. We also discuss several ways to improve the detection of a faint companion around a bright object. We conclude that, with respect to previous techniques, an instrument equipped with this coronagraph should have better performance and even enable the imaging of extrasolar giant planets at a young stage, when coupled with additional cleaning techniques.

The Four‐Quadrant Phase Mask Coronagraph. IV. First Light at the Very Large Telescope

We present the first high-contrast images obtained at the ESO Very Large Telescope using a four-quadrant phase mask coronagraph. The two-night commissioning was carried out in 2004 January on NACO, the near-IR camera with adaptive optics at UT4. We evaluated the behavior of the coronagraph on a variety of astrophysical targets: binary stars, circumstellar disks, and active galactic nuclei. The performance of the coronagraph is in agreement with our expectations based on numerical simulations. The phase mask provides a stellar peak attenuation of a factor of about 10 on average (for long exposure), and its performance is limited only by the phase residuals (mainly low-order aberrations) that are left uncorrected by the adaptive optics system.

The Four‐Quadrant Phase‐Mask Coronagraph. II. Simulations

In the first paper in this series, we described the principle of a coronagraph utilizing a four-quadrant phase mask and the results of numerical simulations obtained in the perfect case. In this second paper, we performed additional numerical simulations to assess in more detail the performances and limitations of this coronagraph under real conditions. The effect of geometrical parameters such as shape and size of both the phase mask and the Lyot stop is studied. We also analyze the effect of low- and high-order aberrations generated, for instance, by the atmospheric turbulence. An important issue is the wavelength dependence of the phase mask. We show that the performance decreases rapidly as the spectral bandwidth is increased, and as a consequence, we discuss the manufacturing of achromatized masks using multiple thin films. An optical concept is proposed.

The four-quadrant phase-mask coronagraph: white light laboratory results with an achromatic device

Achromatic coronagraphs are the subject of intensive research since they will be mandatory for many programs which aim at detecting and characterizing exoplanets. We report a laboratory experiment assessing the performance of the Four-Quadrant Phase-Mask coronagraph (FQPM) over a broadband wavelength range (R≈2). The achromatization of the FQPM is provided by achromatic halfwave plates (HWP). These phase shifters combine birefringent plates made of different materials with properly chosen thicknesses. The HWP thickness control is relaxed by two orders of magnitudes with respect to the classical (non-birefringent) dispersive plate approach. In our experiment we used a two stage stack of Quartz and MgF_2. This combination allows to cover a large spectral range in the visible (500-900nm) with a small phase error residual around π(≈0.12 rad rms). With this achromatization, we obtained an attenuation of 755 on the white light PSF peak. This solution is directly applicable to ground-based telescopes using high order adaptive optics such as the ESOs VLT-Planet Finder project and could easily be transposed in the mid-infrared domain for future space-based missions like DARWIN/TPF.

Subwavelength surface-relief gratings for stellar coronagraphy

We present a new design of a phase mask coronagraph implemented with subwavelength diffractive optical elements consisting of optimized surface-relief gratings. Phase mask coronagraphy is a recent technique that seeks to accommodate both high dynamic and high angular resolution imaging of faint sources around bright astrophysical objects such as exoplanets orbiting their host stars. The original design we propose is a new, integrated, and flexible solution to the pi phase-shift chromaticity of the phase mask coronagraphs. It will allow broadband observations, i.e., shorter integration times and object characterizations, by means of spectroscopic analysis. The feasibility of the component manufacturing is also considered through a tolerance study.

Multi wavelength study of the gravitational lens system RXS J1131-1231. II. Lens model and source reconstruction

Aims. High angular resolution images of the complex gravitational lens system RXS J1131-1231 (a quadruply imaged AGN with a bright Einstein ring) obtained with the Advanced Camera for Surveys and NICMOS instruments onboard the Hubble Space Telescope are analysed to determine the lens model and to reconstruct the host galaxy. Methods. The lens model is constrained by the relative astrometric positions of the lens and point-like images, and by the extended lensed structures. The non-parametric light distribution is recovered in the source plane by means of back ray-tracing. Results. 1- Precise astrometry and photometry of the four QSO lensed images (A-D) and of the lensing galaxy (G) are obtained. They are found in agreement with an independent study presented in a companion paper. The position and colours of the X object seen in projection close to the lens are found to be only compatible with a satellite galaxy associated with the lens. 
2- The Singular Isothermal Ellipsoid plus external shear provides a good fit of the astrometry of images A-D. The positions of extended substructures are also well reproduced. However an octupole (

IMPROVING INTERFEROMETRIC NULL DEPTH MEASUREMENTS USING STATISTICAL DISTRIBUTIONS: THEORY AND FIRST RESULTS WITH THE PALOMAR FIBER NULLER

m=4

Fresnel rhombs as achromatic phase shifters for infrared nulling interferometry

) must be added to the lens potential in order to reproduce the observed lens position, as well as the

Speckle Symmetry with High-Contrast Coronagraphs

I_{\rm B}/I_{\rm C}

Microlensing in H1413+117: disentangling line profile emission and absorption in a broad absorption line quasar

point-like image flux ratio. The ellipticity and orientation of the mass quadrupole are found similar to those of the light distribution, fitted by a Sersic profile. The lens (