Raphaël Galicher

Self-coherent camera: first results of a high-contrast imaging bench in visible light

Extreme adaptive optics and coronagraphy are mandatory for direct imaging of exoplanets. Quasi-static aberrations limit the instrument performance producing speckle noise in the focal plane. We propose a Self-Coherent Camera (SCC) to both control a deformable mirror that actively compensates wavefront error, and calibrate the speckle noise. We create a reference beam to spatially modulate the coronagraphic speckle pattern with Fizeau fringes. In a first step, we are able to extract wavefront aberrations from the science image and correct for them using a deformable mirror. In a second step, we apply a post-processing algorithm to discriminate the companion image from the residual speckle field. To validate the instrumental concept, we developed a high contrast imaging bench in visible light. We associated a SCC to a four quadrant phase mask coronagraph and a deformable mirror (DM) with a high number of actuators (32x32 Boston Michromachines MEMS). We will present this bench and show first experimental results of focal plane wavefront sensing and high contrast imaging. The measurements are compared to numerical simulations.

High contrast imaging on the THD bench: progress and upgrades

Direct imaging of exoplanets is very attractive but challenging and specific instruments like Sphere (VLT) or GPI (Gemini) are required to provide contrasts up to 16-17 magnitudes at a fraction of arcsec. To reach higher contrasts and detect fainter exoplanets, more-achromatic coronagraphs and a more-accurate wavefront control are needed. We already demontrated contrasts of ~10-8 at ~4 λ/D at 635nm using a four quadrant phase mask and a self-coherent camera on our THD bench in laboratory. In this paper, we list the different techniques that were tested on the THD bench in monochromatic and polychromatic lights. Then, we present the upgraded version of the THD bench that includes several deformable mirrors for correcting phase and amplitude simultaneously and obtain a field-of-view covering the complete 360 degrees arouns the star with contrasts down to ~10-8 -10-9.

A Measurement of the Systematic Astrometric Error in GeMS and the Short-Term Astrometric Precision in ShaneAO

We measure the long-term systematic component of the astrometric error in the GeMS MCAO system as a function of field radius and Ks magnitude. The experiment uses two epochs of observations of NGC 1851 separated by one month. The systematic component is estimated for each of three field of view cases (15 radius, 30 radius, and full field) and each of three distortion correction schemes: 8 DOF/chip + local distortion correction (LDC), 8 DOF/chip with no LDC, and 4 DOF/chip with no LDC. For bright, unsaturated stars with 13 < Ks < 16, the systematic component is < 0.2, 0.3, and 0.4 mas, respectively, for the 15 radius, 30 radius, and full field cases, provided that an 8 DOF/chip distortion correction with LDC (for the full-field case) is used to correct distortions. An 8 DOF/chip distortion-correction model always outperforms a 4 DOF/chip model, at all field positions and magnitudes and for all field-of-view cases, indicating the presence of high-order distortion changes. Given the order of the models needed to correct these distortions (~8 DOF/chip or 32 degrees of freedom total), it is expected that at least 25 stars per square arcminute would be needed to keep systematic errors at less than 0.3 milliarcseconds for multi-year programs. We also estimate the short-term astrometric precision of the newly upgraded Shane AO system with undithered M92 observations. Using a 6-parameter linear transformation to register images, the system delivers ~0.3 mas astrometric error over short-term observations of 2-3 minutes.

An aperture masking mode for the MICADO instrument

MICADO is a near-IR camera for the European 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.

Deformable mirror interferometric analysis for the direct imagery of exoplanets

Direct imaging of exoplanet systems requires the use of coronagraphs to reach high contrast levels (10-8 to 10-11) at small angular separations (0.100). However, the performance of these devices is drastically limited by aberrations (in phase or in amplitude, introduced either by atmosphere or by the optics). Coronagraphs must therefore be combined with extreme adaptive optic systems, composed of a focal plane wavefront sensor and of a high order deformable mirror. These adaptive optic systems must reach a residual error in the corrected wavefront of less than 0.1 nm (RMS) with a rate of 1 kHz. In addition, the surface defects of the deformable mirror, inherent from the fabrication process, must be limited in order to avoid the introduction of amplitude aberrations. An experimental high contrast bench has been developed at the Paris Observatory (LESIA). This bench includes a Boston Micromachine deformable mirror composed of 1024 actuators. For a precise analysis of its surface and performance, we characterized this mirror on the interferometric bench developed since 2004 at the Marseille Observatory (LAM). In this paper, we present this interferometric bench as well as the results of the analysis. This will include a precise surface characterization and a description of the behavior of the actuators, on a 10 by 10 actuator range (behavior of a single actuator, study of the cross-talk between neighbor actuators, influence of a stuck actuator) and on full mirror scale (general surface shape).

Experimental results of multi-stage four quadrant phase mask coronagraph

In the framework of exoplanet direct imaging, a few coronagraphs have been proposed to overcome the large flux ratio that exists between the star and its planet. However, there are very few solutions that gather in the same time broad band achromaticity, a small inner working angle (shortest angular distance for planet detection), a good throughput for the planet light, and a mature technical feasibility. Here, we propose to use a combination of chromatic Four Quadrant Phase Mask coronagraphs to achromatize the dephasing of this well-studied monochromatic coronagraph. After describing the principle of the technique, we present preliminary results for a compact prototype. Contrast larger than 10000 are reached with more than 250 nm of spectral bandwidth in the visible. Stability over time and effect of the filtering is also discussed.

Self-Coherent Camera: active correction and post-processing for Earth-like planet detection

Detecting light from faint companions or protoplanetary disks lying close to their host star is a demanding task since these objects are often hidden in the overwhelming star light. A lot of coronagraphs have been proposed to reduce that stellar light and thus, achieve very high contrast imaging, which would enable to take spectra of the faint objects and characterize them. However, coronagraph performance is limited by residual wavefront errors of the incoming beam which create residual speckles in the focal plane image of the central star. Correction or calibration of the wavefront are then necessary to overcome that limitation. We propose to use a Self-Coherent Camera (SCC, Baudoz et al. 2006). The SCC is one of the techniques proposed for EPICS, the futur planet finder of the European Extremely Large Telescope but can also be studied in a space telescope context. The instrument is based on the incoherence between stellar and companion lights. It works in two steps. We first estimate wavefront errors to be corrected by a deformable mirror and then, we apply a post-processing algorithm to achieve very high contrast imaging.

Speckle lifetime in XAO coronagraphic images: temporal evolution of SPHERE coronagraphic images

The major source of noise in high-contrast imaging is the presence of slowly evolving speckles that do not average with time. The temporal stability of the point-spread-function (PSF) is therefore critical to reach a high contrast with extreme adaptive optics (XAO) instruments. Understanding on which timescales the PSF evolves and what are the critical parameters driving the speckle variability allow to design an optimal observing strategy and data reduction technique to calibrate instrumental aberrations and reveal faint astrophysical sources. We have obtained a series of 52 min, AO-corrected, coronagraphically occulted, high-cadence (1.6Hz), H-band images of the star HR 3484 with the SPHERE (Spectro-Polarimeter High-contrast Exoplanet REsearch1) instrument on the VLT. This is a unique data set from an XAO instrument to study its stability on timescales as short as one second and as long as several tens of minutes. We find different temporal regimes of decorrelation. We show that residuals from the atmospheric turbulence induce a fast, partial decorrelation of the PSF over a few seconds, before a transition to a regime with a linear decorrelation with time, at a rate of several tens parts per million per second (ppm/s). We analyze the spatial dependence of this decorrelation within the well-corrected radius of the adaptive optics system and show that the linear decorrelation is faster at short separations. Last, we investigate the influence of the distance to the meridian on the decorrelation.

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%.

Development and characterization of Four-Quadrant Phase Mask coronagraph (FQPM)

The goal of a coronagraph is to reduce the flux of a bright object (e.g. a star) in order to distinguish its faint neighborhood (e.g. exoplanets and disks). In this context, we proposed one coronagraph that uses a four quadrant phase mask (FQPM). Since 2000, we fabricated several monochromatic FQPM working in visible and near-infrared light at the Paris Observatory. We have developed systematic procedures for fabrication and characterization of the phase masks. Visual inspections with an optical microscope are performed for every component and a coronagraphic performance measurement based on inclination of the component is done on a dedicated bench that is set up in a clean room. This procedure gives a quick feedback on the quality and performance of the component. Depending on the results, images of the central transition can be recorded with an electron microscope to understand the limitations of the fabrication process. This procedure allowed us to understand the influence of various parameters such as the width of the transitions between the quadrants, the alignment of the transitions or the step depth. Based on these results, we modified the mask design and the fabrication process to improve our success rate to nearly 100% when building a FQPM for any given optimal wavelength in visible or near-infrared. Moreover, we improved the performance of the components, reaching attenuations of more than 20,000 on the central peak in raw images for most coronagraphs. The best of these components are now used on the THD bench, an optical/NIR bench developed for the study of high contrast imaging techniques, reaching 10-8 contrast level routinely.