High-contrast imaging in polychromatic light with the self-coherent camera

Context. In the context of direct imaging of exoplanets, coronagraphs are commonly proposed to reach the required very high contrast levels. However, wavefront aberrations induce speckles in their focal plane and limit their performance. Aims. An active correction of these wavefront aberrations using a deformable mirror upstream of the coronagraph is mandatory. These aberrations need to be calibrated and focal-plane wavefront-sensing techniques in the science channel are being developed. One of these, the self-coherent camera, of which we present the latest laboratory results. Methods. We present here an enhancement of the method: we directly minimized the complex amplitude of the speckle field in the focal plane. Laboratory tests using a four-quadrant phase-mask coronagraph and a 32x32 actuator deformable mirror were conducted in monochromatic light and in polychromatic light for different bandwidths. Results. We obtain contrast levels in the focal plane in monochromatic light better than 3.10^-8 (RMS) in the 5 - 12 {\lambda}/D region for a correction of both phase and amplitude aberrations. In narrow bands (10 nm) the contrast level is 4.10^-8 (RMS) in the same region. Conclusions. The contrast level is currently limited by the amplitude aberrations on the bench. We identified several improvements that can be implemented to enhance the performance of our optical bench in monochromatic as well as in polychromatic light.Comment: 4 pages, 3 figures, accepted in Astronomy & Astrophysics (02/2014

Theory and laboratory tests of the multi-stage phase mask coronagraph

A large number of coronagraphs have been proposed to overcome the ratio that exists between the star and its planet. The planet finder of the Extremely Large Telescope, which is called EPICS, will certainly need a more efficient coronagraph than the ones that have been developed so far. We propose to use a combination of chromatic Four Quadrant Phase Mask coronagraph to achromatize the dephasing of the device while maintaining a high rejection performance. After describing this multi-stage FQPM coronagraph, we show preliminary results of a study on its capabilities in the framework of the EPICS instrument, the planet finder of the European Extremely Large Telescope. Eventually, we present laboratory tests of a rough prototype of a multi-stage four-quadrant phase mask. On one hand, we deduce from our laboratory data that a detection at the 10^-10 level is feasible in monochromatic light. On the other hand, we show the detection of a laboratory companion fainter than 10^-8 with a spectral bandwidth larger than 20%.Comment: 9 pages, 9 figures, To appear in SPIE proceeding- conference 7015 held in Marseille in June 200

Fomalhaut b: Independent Analysis of the Hubble Space Telescope Public Archive Data

The nature and even the existence of a putative planet-mass companion ("Fomalhaut b") to Fomalhaut has been debated since 2008. In the present paper we reanalyze the multi-epoch ACS/STIS/WFC3 Hubble Space Telescope (HST) optical/near infrared images on which the discovery and some other claims were based. We confirm that the HST images do reveal an object in orbit around Fomalhaut but the detailed results from our analysis differ in some ways from previous discussions. In particular, we do not confirm flux variability over a two-year interval at 0.6 microns wavelength and we detect Fomalhaut b for the first time at the short wavelength of 0.43 microns. We find that the HST image of Fomalhaut b at $0.8\,\mu$m may be extended beyond the PSF. We cannot determine from our astrometry if Fomalhaut b will cross or not the dust ring. The optical through mid-infrared spectral energy distribution (SED) of Fomalhaut b cannot be explained as due to direct or scattered radiation from a massive planet. We consider two models to explain the SED: (1) a large circumplanetary disk around an unseen planet and (2) the aftermath of a collision during the past 50-150 years of two Kuiper Belt-like objects of radii 50 km.Comment: 24 pages, 9 figures, 5 tables, accepted on April, 3rd, 201

Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes

Less than 3% of the known exoplanets were directly imaged for two main reasons. They are angularly very close to their parent star, which is several magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated instrumentation with large telescopes and accurate wavefront control devices for high-angular resolution and coronagraphs for attenuating the stellar light. Coronagraphs are usually chromatic and they cannot perform high-contrast imaging over a wide spectral bandwidth. That chromaticity will be critical for future instruments. Enlarging the coronagraph spectral range is a challenge for future exoplanet imaging instruments on both space-based and ground-based telescopes. We propose the multi-stage four-quadrant phase mask that associates several monochromatic four-quadrant phase mask coronagraphs in series. Monochromatic device performance has already been demonstrated and the manufacturing procedures are well-under control since their development for previous instruments on VLT and JWST. The multi-stage implementation simplicity is thus appealing. We present the instrument principle and we describe the laboratory performance for large spectral bandwidths and for both pupil shapes for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The multi-stage four-quadrant phase mask reduces the stellar flux over a wide spectral range (30%) and it is a very good candidate to be associated with a spectrometer for future exoplanet imaging instruments in ground- and space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&

Self-coherent camera as a focal plane wavefront sensor: simulations

Direct detection of exoplanets requires high dynamic range imaging. Coronagraphs could be the solution, but their performance in space is limited by wavefront errors (manufacturing errors on optics, temperature variations, etc.), which create quasi-static stellar speckles in the final image. Several solutions have been suggested for tackling this speckle noise. Differential imaging techniques substract a reference image to the coronagraphic residue in a post-processing imaging. Other techniques attempt to actively correct wavefront errors using a deformable mirror. In that case, wavefront aberrations have to be measured in the science image to extremely high accuracy. We propose the self-coherent camera sequentially used as a focal-plane wavefront sensor for active correction and differential imaging. For both uses, stellar speckles are spatially encoded in the science image so that differential aberrations are strongly minimized. The encoding is based on the principle of light incoherence between the hosting star and its environment. In this paper, we first discuss one intrinsic limitation of deformable mirrors. Then, several parameters of the self-coherent camera are studied in detail. We also propose an easy and robust design to associate the self-coherent camera with a coronagraph that uses a Lyot stop. Finally, we discuss the case of the association with a four-quadrant phase mask and numerically demonstrate that such a device enables the detection of Earth-like planets under realistic conditions. The parametric study of the technique lets us believe it can be implemented quite easily in future instruments dedicated to direct imaging of exoplanets.Comment: 15 pages, 14 figures, accepted in A&A (here is the final version

Independent confirmation of {\beta} Pictoris b imaging with NICI

Context. {\beta} Pictoris b is one of the most studied objects nowadays since it was identified with VLT/NaCo as a bona-fide exoplanet with a mass of about 9 times that of Jupiter at an orbital separation of 8-9 AU. The link between the planet and the dusty disk is unambiguously attested and this system provides an opportunity to study the disk/planet interactions and to constrain formation and evolutionary models of gas giant planets. Still, {\beta} Pictoris b had never been confirmed with other telescopes so far. Aims. We aimed at an independent confirmation using a different instrument. Methods. We retrieved archive images from Gemini South obtained with the instrument NICI, which is designed for high contrast imaging. The observations combine coronagraphy and angular differential imaging and were obtained at three epochs in Nov. 2008, Dec. 2009 and Dec. 2010. Results. We report the detection with NICI of the planet {\beta} Pictoris b in Dec. 2010 images at a separation of 404 \pm 10 mas and P A = 212.1 \pm 0.7{\deg} . It is the first time this planet is observed with a telescope different than the VLT.Comment: Letter accepted for publication in Astronomy and Astrophysics on Feb. 21, 2013. 4 pages, 2 figure