arXiv subjects

Raphaël Galicher

Publications and source records attributed to Raphaël Galicher.

31 records · Page 2Linked to original sources

arXiv2020

A Family of Phase Masks For Broadband Coronagraphy Example of the Wrapped Vortex Phase Mask Theory and Laboratory demonstration

Future instruments need efficient coronagraphs over large spectral ranges to enable broadband imaging or spectral characterization of exoplanets 1e8 fainter than their star. Several solutions were proposed. Pupil apodizers can attenuate the star intensity by a 1e10 factor but they transmit a few percents of the planet light only. Cascades of phase/amplitude masks can both attenuate the starlight and transmit most of the planet light but the number of optics to align is not a practical solution for an instrument. Finally, vector phase masks can be used to detect faint sources close to bright stars but they require the use of high quality circular polarizers and as for the previous solution, this leads to a complex instrument with numerous optics to align and stabilize. We propose simple coronagraphs that need one scalar phase mask and one binary Lyot stop only providing high transmission for the planet light (>50%) and high attenuation of the starlight over a large spectral bandpass (~30%) and a 360 degree field-of-view. From mathematical considerations, we find a family of 2D-phase masks optimized for an unobscured pupil. One mask is an azimuthal wrapped vortex phase ramp. We probe its coronagraphic performance using numerical simulations and laboratory tests. From numerical simulations, we predict the wrapped vortex can attenuate the peak of the star image by a factor of 1e4 over a 29% bandpass and 1e5 over a 18% bandpass with transmission of more than 50% of the planet flux at ~4 lambda/D. We confirm these predictions in laboratory in visible light between 550nm and 870nm. We also obtain laboratory dark hole images in which exoplanets with fluxes that are 3e-8 times the host star flux could be detected at 3sigma. Taking advantage of a new technology for etching continuous 2D-functions, new type of masks can be easily manufactured opening new possibilities for broadband coronagraphy.

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arXiv2019

The Fast Atmospheric Self-Coherent Camera Technique: Laboratory Results and Future Directions

Direct detection and detailed characterization of exoplanets using extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes (ELTs). However, wavefront errors will limit the sensitivity of this endeavor. Limitations for ground-based telescopes arise from both quasi-static and residual AO-corrected atmospheric wavefront errors, the latter of which generates short-lived aberrations that will average into a halo over a long exposure. We have developed and tested the framework for a solution to both of these problems using the self-coherent camera (SCC), to be applied to ground-based telescopes, called the Fast Atmospheric SCC Technique (FAST). In this paper we present updates of new and ongoing work for FAST, both in numerical simulation and in the laboratory. We first present numerical simulations that illustrate the scientific potential of FAST, including, with current 10-m telescopes, the direct detection of exoplanets reflected light and exo-Jupiters in thermal emission and, with future ELTs, the detection of habitable exoplanets. In the laboratory, we present the first characterizations of our proposed, and now fabricated, coronagraphic masks.

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arXiv2019

Active minimization of non-common path aberrations in long-exposure imaging of exoplanetary systems

Context. Spectroscopy of exoplanets is very challenging because of the high star-planet contrast. A technical difficulty in the design of imaging instruments is the noncommon path aberrations (NCPAs) between the adaptive optics (AO) sensing and the science camera, which induce planet-resembling stellar speckles in the coronagraphic science images. In an observing sequence of several long exposures, quickly evolving NCPAs average out and leave behind an AO halo that adds photon noise to the planet detection. Static NCPA can be calibrated a posteriori using differential imaging techniques. However, NCPAs that evolve during the observing sequence do not average out and cannot be calibrated a posteriori. These quasi-static NCPAs are one of the main limitations of the current direct imaging instruments such as SPHERE, GPI, and SCExAO. Aims. Our aim is to actively minimize the quasi-static speckles induced in long-exposure images. To do so, we need to measure the quasi-static speckle field above the AO halo. Methods. The self-coherent camera (SCC) is a proven technique which measures the speckle complex field in the coronagraphic science images. It is routinely used on the THD2 bench to reach contrast levels of <10^{-8} in the range 5-12 λ/D in space-related conditions. To test the SCC in ground conditions on THD2, we optically simulated the residual aberrations measured behind the SPHERE/VLT AO system under good observing conditions. Results. We demonstrate in the laboratory that the SCC can minimize the quasi-static speckle intensity in the science images down to a limitation set by the AO halo residuals. The SCC reaches 1σ raw contrast levels below 10^{-6} in the region 5-12 λ/D at 783.25 nm in our experiments.

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arXiv2018

Review of high-contrast imaging systems for current and future ground-based and space-based telescopes III. Technology opportunities and pathways

The Optimal Optical CoronagraphWorkshop at the Lorentz Center in September 2017 in Leiden, the Netherlands gathered a diverse group of 25 researchers working on exoplanet instrumentation to stimulate the emergence and sharing of new ideas. This contribution is the final part of a series of three papers summarizing the outcomes of the workshop, and presents an overview of novel optical technologies and systems that are implemented or considered for high-contrast imaging instruments on both ground-based and space telescopes. The overall objective of high contrast instruments is to provide direct observations and characterizations of exoplanets at contrast levels as extreme as 10^-10. We list shortcomings of current technologies, and identify opportunities and development paths for new technologies that enable quantum leaps in performance. Specifically, we discuss the design and manufacturing of key components like advanced deformable mirrors and coronagraphic optics, and their amalgamation in "adaptive coronagraph" systems. Moreover, we discuss highly integrated system designs that combine contrast-enhancing techniques and characterization techniques (like high-resolution spectroscopy) while minimizing the overall complexity. Finally, we explore extreme implementations using all-photonics solutions for ground-based telescopes and dedicated huge apertures for space telescopes.

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arXiv2018

Experimental validation of joint phase and amplitude wave-front sensing with coronagraphic phase diversity for high-contrast imaging

Context. The next generation of space-borne instruments dedicated to the direct detection of exoplanets requires unprecedented levels of wavefront control precision. Coronagraphic wavefront sensing techniques for these instruments must measure both the phase and amplitude of the optical aberrations using the scientific camera as a wavefront sensor. Aims. In this paper, we develop an extension of coronagraphic phase diversity to the estimation of the complex electric field, that is, the joint estimation of phase and amplitude. Methods. We introduced the formalism for complex coronagraphic phase diversity. We have demonstrated experimentally on the Très Haute Dynamique testbed at the Observatoire de Paris that it is possible to reconstruct phase and amplitude aberrations with a subnanometric precision using coronagraphic phase diversity. Finally, we have performed the first comparison between the complex wavefront estimated using coronagraphic phase diversity (which relies on time-modulation of the speckle pattern) and the one reconstructed by the self-coherent camera (which relies on the spatial modulation of the speckle pattern). Results. We demonstrate that coronagraphic phase diversity retrieves complex wavefront with subnanometric precision with a good agreement with the reconstruction performed using the self-coherent camera. Conclusions. This result paves the way to coronagraphic phase diversity as a coronagraphic wave-front sensor candidate for very high contrast space missions.

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arXiv2018

Fast focal plane wavefront sensing on ground-based telescopes

Exoplanet detection and characterization through extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes. This achievement, however, will be limited in sensitivity by both quasi-static wavefront errors and residual AO-corrected atmospheric wavefront errors. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to remove these aberrations as soon as they are detected. We have developed the framework for one such method, using the self-coherent camera (SCC), to be applied to ground-based telescopes, called Fast Atmospheric SCC Technique (FAST; Gerard et al., submitted). Our FAST solution requires an optimally designed coronagraph (the SCC FPM) and post-processing algorithm and is in principle able to reach a "raw" contrast of a few times the photon noise limit, continually improving with integration time. In this paper, we present new ongoing work in exploring the manufacturing limitations of the SCC FPM as well as a general framework to implement and optimize a FAST deformable mirror control loop.

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arXiv2018

Fast Coherent Differential Imaging on Ground-Based Telescopes using the Self-Coherent Camera

Direct imaging and spectral characterization of exoplanets using extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes and space observatories. However, quasi-static wavefront errors will limit the sensitivity of this endeavor. Additional limitations for ground-based telescopes arise from residual AO-corrected atmospheric wavefront errors, generating millisecond-lifetime speckles that average into a halo over a long exposure. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to minimize these aberrations as soon as they are detected. We develop the framework for one such method based on the self-coherent camera (SCC) to be applied to ground-based telescopes, called Fast Atmospheric SCC Technique (FAST). We show that with the use of a specially designed coronagraph and coherent differential imaging algorithm, recording images every few milliseconds allows for a subtraction of atmospheric and static speckles while maintaining a close to unity algorithmic exoplanet throughput. Detailed simulations reach a contrast close to the photon noise limit after 30 seconds for a 1 % bandpass in H band on both 0$^\text{th}$ and 5$^\text{th}$ magnitude stars. For the 5th magnitude case, this is about 110 times better in raw contrast than what is currently achieved from ExAO instruments if we extrapolate for an hour of observing time, illustrating that sensitivity improvement from this method could play an essential role in the future detection and characterization of lower mass exoplanets.

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arXiv2017

A self-consistent cloud model for brown dwarfs and young giant exoplanets: comparison with photometric and spectroscopic observations

We developed a simple, physical and self-consistent cloud model for brown dwarfs and young giant exoplanets. We compared different parametrisations for the cloud particle size, by either fixing particle radii, or fixing the mixing efficiency (parameter fsed) or estimating particle radii from simple microphysics. The cloud scheme with simple microphysics appears as the best parametrisation by successfully reproducing the observed photometry and spectra of brown dwarfs and young giant exoplanets. In particular, it reproduces the L-T transition, due to the condensation of silicate and iron clouds below the visible/near-IR photosphere. It also reproduces the reddening observed for low-gravity objects, due to an increase of cloud optical depth for low gravity. In addition, we found that the cloud greenhouse effect shifts chemical equilibriums, increasing the abundances of species stable at high temperature. This effect should significantly contribute to the strong variation of methane abundance at the L-T transition and to the methane depletion observed on young exoplanets. Finally, we predict the existence of a continuum of brown dwarfs and exoplanets for absolute J magnitude=15-18 and J-K color=0-3, due to the evolution of the L-T transition with gravity. This self-consistent model therefore provides a general framework to understand the effects of clouds and appears well-suited for atmospheric retrievals.

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arXiv2017

Speckle correction in polychromatic light with the self-coherent camera for the direct detection of exoplanets

Direct detection is a very promising field in exoplanet science. It allows the detection of companions with large separation and allows their spectral analysis. A few planets have already been detected and are under spectral analysis. But the full spectral characterization of smaller and colder planets requires higher contrast levels over large spectral bandwidths. Coronagraphs can be used to reach these contrasts, but their efficiency is limited by wavefront aberrations. These deformations induce speckles, star lights leaks, in the focal plane after the coronagraph. The wavefront aberrations should be estimated directly in the science image to avoid usual limitations by differential aberrations in classical adaptive optics. In this context, we introduce the Self- Coherent Camera (SCC). The SCC uses the coherence of the star light to produce a spatial modulation of the speckles in the focal plane and estimate the associated electric complex field. Controlling the wavefront with a deformable mirror, high contrasts have already been reached in monochromatic light with this technique. The performance of the current version of the SCC is limited when widening the spectral bandwidth. We will present a theoretical analysis of these issues and their possible solution. Finally, we will present test bench performance in polychromatic light.

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arXiv2017

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 (0.1"). 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).

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arXiv2017

Experimental parametric study of the self-coherent camera

Direct imaging of exoplanets requires the detection of very faint objects orbiting close to very bright stars. In this context, the SPICES mission was proposed to the European Space Agency for planet characterization at visible wavelength. SPICES is a 1.5m space telescope which uses a coronagraph to strongly attenuate the central source. However, small optical aberrations, which appear even in space telescopes, dramatically decrease coronagraph performance. To reduce these aberrations, we want to estimate, directly on the coronagraphic image, the electric field, and, with the help of a deformable mirror, correct the wavefront upstream of the coronagraph. We propose an instrument, the Self-Coherent Camera (SCC) for this purpose. By adding a small "reference hole" into the Lyot stop, located after the coronagraph, we can produce interferences in the focal plane, using the coherence of the stellar light. We developed algorithms to decode the information contained in these Fizeau fringes and retrieve an estimation of the field in the focal plane. After briefly recalling the SCC principle, we will present the results of a study, based on both experiment and numerical simulation, analyzing the impact of the size of the reference hole.

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arXiv2014

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 λ/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.

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arXiv2013

Independent confirmation of β Pictoris b imaging with NICI

Context. β 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, β 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 β Pictoris b in Dec. 2010 images at a separation of 404 \pm 10 mas and P A = 212.1 \pm 0.7° . It is the first time this planet is observed with a telescope different than the VLT.

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