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

Focal plane wavefront sensor achromatization : The multireference self-coherent camera

High contrast imaging and spectroscopy provide unique constraints for exoplanet formation models as well as for planetary atmosphere models. But this can be challenging because of the planet-to-star small angular separation and high flux ratio. Recently, optimized instruments like SPHERE and GPI were installed on 8m-class telescopes. These will probe young gazeous exoplanets at large separations (~1au) but, because of uncalibrated aberrations that induce speckles in the coronagraphic images, they are not able to detect older and fainter planets. There are always aberrations that are slowly evolving in time. They create quasi-static speckles that cannot be calibrated a posteriori with sufficient accuracy. An active correction of these speckles is thus needed to reach very high contrast levels (>1e7). This requires a focal plane wavefront sensor. Our team proposed the SCC, the performance of which was demonstrated in the laboratory. As for all focal plane wavefront sensors, these are sensitive to chromatism and we propose an upgrade that mitigates the chromatism effects. First, we recall the principle of the SCC and we explain its limitations in polychromatic light. Then, we present and numerically study two upgrades to mitigate chromatism effects: the optical path difference method and the multireference self-coherent camera. Finally, we present laboratory tests of the latter solution. We demonstrate in the laboratory that the MRSCC camera can be used as a focal plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640 nm. We reach a performance that is close to the chromatic limitations of our bench: contrast of 4.5e-8 between 5 and 17 lambda/D. The performance of the MRSCC is promising for future high-contrast imaging instruments that aim to actively minimize the speckle intensity so as to detect and spectrally characterize faint old or light gaseous planets.Comment: 14 pages, 20 figure

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&

Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed

Specific high contrast imaging instruments are mandatory to characterize circumstellar disks and exoplanets around nearby stars. Coronagraphs are commonly used in these facilities to reject the diffracted light of an observed star and enable the direct imaging and spectroscopy of its circumstellar environment. One important property of the coronagraph is to be able to work in broadband light. Among several proposed coronagraphs, the dual-zone phase mask coronagraph is a promising solution for starlight rejection in broadband light. In this paper, we perform the first validation of this concept in laboratory. First, we recall the principle of the dual-zone phase mask coronagraph. Then, we describe the high-contrast imaging THD testbed, the manufacturing of the components and the quality-control procedures. Finally, we study the sensitivity of our coronagraph to low-order aberrations (inner working angle and defocus) and estimate its contrast performance. Our experimental broadband light results are compared with numerical simulations to check agreement with the performance predictions. With the manufactured prototype and using a dark hole technique based on the self-coherent camera, we obtain contrast levels down to $2\,10^{-8}$ between 5 and 17$\,\lambda_0/D$ in monochromatic light (640 nm). We also reach contrast levels of $4\,10^{-8}$ between 7 and 17$\lambda_0/D$ in broadband ($\lambda_0=675$ nm, $\Delta\lambda=250$ nm and $\Delta\lambda / \lambda_0 = 40$ %), which demonstrates the excellent chromatic performance of the dual-zone phase mask coronagraph. The performance reached by the dual-zone phase mask coronagraph is promising for future high-contrast imaging instruments that aim at detecting and spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure

Interpreting the photometry and spectroscopy of directly imaged planets: a new atmospheric model applied to beta Pictoris b and SPHERE observations

We aim to interpret future photometric and spectral measurements from these instruments, in terms of physical parameters of the planets, with an atmospheric model using a minimal number of assumptions and parameters. We developed Exoplanet Radiative-convective Equilibrium Model (Exo-REM) to analyze the photometric and spectro- scopic data of directly imaged planets. The input parameters are a planet's surface gravity (g), effective temperature (Teff ), and elemental composition. The model predicts the equilibrium temperature profile and mixing ratio profiles of the most important gases. Opacity sources include the H2-He collision-induced absorption and molecular lines from eight compounds (including CH4 updated with the Exomol line list). Absorption by iron and silicate cloud particles is added above the expected condensation levels with a fixed scale height and a given optical depth at some reference wavelength. Scattering was not included at this stage. We applied Exo-REM to photometric and spectral observations of the planet beta Pictoris b obtained in a series of near-IR filters. We derived Teff = 1550 +- 150 K, log(g) = 3.5 +- 1, and radius R = 1.76 +- 0.24 RJup (2-{\sigma} error bars from photometric measurements). These values are comparable to those found in the literature, although with more conservative error bars, consistent with the model accuracy. We were able to reproduce, within error bars, the J- and H-band spectra of beta Pictoris b. We finally investigated the precision to which the above parameterComment: 15 pages, 14 figures, accepted by A&

An aperture masking mode for the MICADO instrument

MICADO is a near-IR camera for the Europea ELT, featuring an extended field (75" diameter) for imaging, and also spectrographic and high contrast imaging capabilities. It has been chosen by ESO as one of the two first-light instruments. Although it is ultimately aimed at being fed by the MCAO module called MAORY, MICADO will come with an internal SCAO system that will be complementary to it and will deliver a high performance on axis correction, suitable for coronagraphic and pupil masking applications. The basis of the pupil masking approach is to ensure the stability of the optical transfer function, even in the case of residual errors after AO correction (due to non common path errors and quasi-static aberrations). Preliminary designs of pupil masks are presented. Trade-offs and technical choices, especially regarding redundancy and pupil tracking, are explained.Comment: SPIE 2014 Proceeding -- Montrea

Exoplanet direct imaging in ground-based conditions on THD2 bench

The next generation of ground-based instruments aims to break through the knowledge we have on exoplanets by imaging circumstellar environments always closer to the stars. However, direct imaging requires an AO system and high-contrast techniques like a coronagraph to reject the diffracted light of an observed star and an additional wavefront sensor to control quasi-static aberrations, including the non common path aberrations. To observe faint objects, a focal plane wavefront sensor with a sub-nanometric wavefront control capability is required. In the past few years, we developed the THD2 bench which is a testbed for high-contrast imaging techniques, working in visible and near infrared wavelengths and currently reaching contrast levels lower than 1e-8 under space-like simulated conditions. We recently added a turbulence wheel on the optical path which simulates the residuals given by a typical extreme adaptive optics system and we tested several ways to remove quasi-statics speckles. One way to estimate the aberrations is a method called pair-wise probing where we record few images with known-shapes we apply on the adaptive optics deformable mirror. Once estimated, we seek to minimize the focal-plane electric field by an algorithm called Electric Field Conjugation. In this paper, we present the first results obtained on the THD2 bench using these two techniques together in turbulent conditions. We then compare the achieved performance with the one expected when all the quasi-static speckles are corrected.Comment: 9 pages, 3 figures, AO4ELT6 Qu\'ebec cit

Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements

We present new astrometric measurements from our ongoing monitoring campaign of the HR 8799 directly imaged planetary system. These new data points were obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and 2014. In addition, we present updated astrometry from previously published observations in 2007 and 2008. All data were reduced using the SOSIE algorithm, which accounts for systematic biases present in previously published observations. This allows us to construct a self-consistent data set derived entirely from NIRC2 data alone. From this dataset, we detect acceleration for two of the planets (HR 8799b and e) at $>$3$\sigma$. We also assess possible orbital parameters for each of the four planets independently. We find no statistically significant difference in the allowed inclinations of the planets. Fitting the astrometry while forcing coplanarity also returns $\chi^2$ consistent to within 1$\sigma$ of the best fit values, suggesting that if inclination offsets of $\lesssim$20$^{o}$ are present, they are not detectable with current data. Our orbital fits also favor low eccentricities, consistent with predictions from dynamical modeling. We also find period distributions consistent to within 1$\sigma$ with a 1:2:4:8 resonance between all planets. This analysis demonstrates the importance of minimizing astrometric systematics when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A

SEE COAST, a spectro-polarimetric imaging mission to characterize exoplanets

SEE COAST is a space mission concept submitted to Cosmic Vision in 2007. It is designed for the characterization of giant gazeous planets and possibly Super Earths both in spectroscopy and polarimetry at visible wavelengths (0.4-1.2μm). The SEE COAST concept relies on a series of high contrast imaging techniques like coronagraphy, wavefront control and differential imaging. The strategy of the mission is presented and the instrumental concept briefly introduced