High Contrast Imaging at the Photon Noise Limit with WFS-based PSF Calibration

Speckle Noise is the dominant source of error in high contrast imaging with adaptive optics system. We discuss the potential for wavefront sensing telemetry to calibrate speckle noise with sufficient precision and accuracy so that it can be removed in post-processing of science images acquired by high contrast imaging instruments. In such a self-calibrating system, exoplanet detection would be limited by photon noise and be significantly more robust and efficient than in current systems. We show initial laboratory and on-sky tests, demonstrating over short timescale that residual speckle noise is indeed calibrated to an accuracy exceeding readout and photon noise in the high contrast region. We discuss immplications for the design of space and ground high-contrast imaging systems.Comment: 12 pages, 7 figures, To appear in SPIE Proceedings of Astronomical Telescopes + Instrumentation, 2022. arXiv admin note: text overlap with arXiv:2109.1395

First on-sky demonstration of an integrated-photonic nulling interferometer: the GLINT instrument

The characterization of exoplanets is critical to understanding planet diversity and formation, their atmospheric composition, and the potential for life. This endeavour is greatly enhanced when light from the planet can be spatially separated from that of the host star. One potential method is nulling interferometry, where the contaminating starlight is removed via destructive interference. The GLINT instrument is a photonic nulling interferometer with novel capabilities that has now been demonstrated in on-sky testing. The instrument fragments the telescope pupil into sub-apertures that are injected into waveguides within a single-mode photonic chip. Here, all requisite beam splitting, routing, and recombination are performed using integrated photonic components. We describe the design, construction, and laboratory testing of our GLINT pathfinder instrument. We then demonstrate the efficacy of this method on sky at the Subaru Telescope, achieving a null-depth precision on sky of ∼10⁻⁴ and successfully determining the angular diameter of stars (via their null-depth measurements) to milliarcsecond accuracy. A statistical method for analysing such data is described, along with an outline of the next steps required to deploy this technique for cutting-edge science

Asgard/NOTT: L-band nulling interferometry at the VLTI I. Simulating the expected high-contrast performance

Context: NOTT (formerly Hi-5) is a new high-contrast L' band (3.5-4.0 \textmu m) beam combiner for the VLTI with the ambitious goal to be sensitive to young giant exoplanets down to 5 mas separation around nearby stars. The performance of nulling interferometers in these wavelengths is affected both by fundamental noise from the background and by the contributions of instrumental noises. This motivates the development of end-to-end simulations to optimize these instruments. Aims: To enable the performance evaluation and inform the design of such instruments on the current and future infrastructures, taking into account the different sources of noise, and their correlation. Methods: SCIFYsim is an end-to-end simulator for single mode filtered beam combiners, with an emphasis on nulling interferometers. It is used to compute a covariance matrix of the errors. Statistical detection tests based on likelihood ratios are then used to compute compound detection limits for the instrument. Results: With the current assumptions on the performance of the wavefront correction systems, the errors are dominated by correlated instrumental errors down to stars of magnitude 6-7 in the L band, beyond which thermal background from the telescopes and relay system becomes dominant. Conclusions: SCIFYsim is suited to anticipate some of the challenges of design, tuning, operation and signal processing for integrated optics beam combiners. The detection limits found for this early version of NOTT simulation with the unit telescopes are compatible with detections at contrasts up to $10^5$ in the L band at separations of 5 to 80 mas around bright stars

Asgard/NOTT: L-band nulling interferometry at the VLTI. II. Warm optical design and injection system

Asgard/NOTT (previously Hi-5) is a European Research Council (ERC)-funded project hosted at KU Leuven and a new visitor instrument for the Very Large Telescope Interferometer (VLTI). Its primary goal is to image the snow line region around young stars using nulling interferometry in the L-band (3.5 to 4.0)$\mu$m, where the contrast between exoplanets and their host stars is advantageous. The breakthrough is the use of a photonic beam combiner, which only recently allowed the required theoretical raw contrast of $10^{-3}$ in this spectral range. Nulling interferometry observations of exoplanets also require a high degree of balancing between the four pupils of the VLTI in terms of intensity, phase, and polarization. The injection into the beam combiner and the requirements of nulling interferometry are driving the design of the warm optics and the injection system. The optical design up to the beam combiner is presented. It offers a technical solution to efficiently couple the light from the VLTI into the beam combiner. During the coupling, the objective is to limit throughput losses to 5% of the best expected efficiency for the injection. To achieve this, a list of different loss sources is considered with their respective impact on the injection efficiency. Solutions are also proposed to meet the requirements on beam balancing for intensity, phase, and polarization. The different properties of the design are listed, including the optics used, their alignment and tolerances, and their impact on the instrumental performances in terms of throughput and null depth. The performance evaluation gives an expected throughput loss of less than <6.4% of the best efficiency for the injection and a null depth of $\sim2.10^{-3}$, mainly from optical path delay errors outside the scope of this work.Comment: Accepted for publication in JATIS. 23 pages, 11 figures, 8 table

Technical requirements and optical design of the Hi-5 spectrometer

Hi-5 is a proposed L’ band high-contrast nulling interferometric instrument for the visitor focus of the Very Large Telescope Interferometer (VLTI). As a part of the ERC consolidator project called SCIFY (Self-Calibrated Interferometry For exoplanet spectroscopY), the instrument aims to achieve sufficient dynamic range and angular resolution to directly image and characterize the snow line of young extra-solar planetary systems. The spectrometer is based on a dispersive grism and is located downstream of an integrated optics beam- combiner. To reach the contrast and sensitivity specifications, the outputs of the I/O chip must be sufficiently separated and properly sampled on the Hawaii-2RG detector. This has many implications for the photonic chip and spectrometer design. We present these technical requirements, trade-off studies, and phase-A of the optical design of the Hi-5 spectrometer in this paper. For both science and contract-driven reasons, the instrument design currently features three different spectroscopic modes (R=20, 400, and 2000). Designs and efficiency estimates for the grisms are also presented as well as the strategy to separate the two polarization states.SCIF

L-band nulling interferometry at the VLTI with Asgard/NOTT: status and plans

editorial reviewedNOTT (formerly Hi-5) is the L’-band (3.5-4.0 μm) nulling interferometer of Asgard, an instrument suite in preparation for the VLTI visitor focus. The primary scientific objectives of NOTT include characterizing (i) young planetary systems near the snow line, a critical region for giant planet formation, and (ii) nearby mainsequence stars close to the habitable zone, with a focus on detecting exozodiacal dust that could obscure Earthlike planets. In 2023-2024, the final warm optics have been procured and assembled in a new laboratory at KU Leuven. First fringes and null measurements were obtained using a Gallium Lanthanum Sulfide (GLS) photonic chip that was also tested at cryogenic temperatures. In this paper, we present an overall update of the NOTT project with a particular focus on the cold mechanical design, the first results in the laboratory with the final NOTT warm optics, and the ongoing Asgard integration activities. We also report on other ongoing activities such as the characterization of the photonic chip (GLS, LiNbO3, SiO), the development of the exoplanet science case, the design of the dispersion control module, and the progress with the self-calibration data reduction software.SCIF

Développement et exploitation scientifique d’un nouvel instrument interférométrique visible en optique guidée

Long baseline visible interferometry in astronomy is an observing technique which allows to get insights of an object with an outstanding angular resolution, unreachable with single-dish telescope. Interferometric measurements with ground-based instrumentation are currently limited in sensitivity and precision due to atmospheric turbulence. However, the new astrophysical needs, particularly the determination of fundamental parameters or the study of the closed environment and the surface of the stars, require to observe fainter objects with a better precision than now in visible interferometry. Ought to overcome the atmospheric turbulence, multispeckle interferometry has been developed by adapting speckle imaging technics used on single-dish telescope. Today, in order to improve the performance of the future combiners, instrumentation progresses to the use of a new generation detector called EMCCD, and the use of optical fibers which are coupled with adaptive optics. This path is chosen thank to the success of the use of the adaptive optics with the fringe tracking in the infrared interferometry in 2017 (Gravity Collaboration et al. 2017), in order to compensate turbulence. FRIEND prototype (Fibered and spectrally Resolved Interferometer - New Design) has been designed to characterize and estimate the performance of such a combination of technologies, in the visible spectral band. The improvement of the precision of the measurements from interferometric instruments is due to optical fibers and the dynamical range of the EMCCD. The counterpart of using the optical fibers is a loss in sensitivity due to a low injection rate of flux into the fibers because of the atmospheric turbulence. On the other hand, sensitivity is improved thanks to adaptive optics and EMCCDs. Indeed, adaptive optics increases the injection rate and EMCCDs can measure low fluxes. Lastly, FRIEND is a pathfinder for the future instrument SPICA which should recombine up to 6 telescopes (Mourard et al. 2017, 2018). Fringe-tracking aspects will have to be studied for SPICA; this topic is not dealt with in this thesis. In this work, I present the FRIEND prototype, which can recombine up to three telescopes and operates in the R band with dispersed fringes. It has Gaussian polarization-maintaining single mode optical fibers and an EMCCD. It is set up at the Center for High Angular Resolution Astronomy (CHARA), at Mount Wilson, in California. CHARA is currently being equipped with adaptive optics. I develop estimators of visibility modulus and closure phase, the data reduction software and an observing strategy. Thanks to that, I am able to show that adaptive optics improves the injection rate. I also demonstrate how important the stabilization of injection is to maximize the signal-to-noise ratio (SNR) per frame. Birefringence of the fibers decreases the performance of the instrument but we manage to compensate it. I show how such an instrument can measure low visibility with a better precision than now by developing and using a SNR model of FRIEND. Finally, FRIEND has entirely been tested on the known binary system ζ Ori A. These observations demonstrate how reliable and accurate the measurements of FRIEND are.L'interférométrie visible longue base est une technique d'observation en astronomie permettant de sonder les objets avec une résolution spatiale qu'il est impossible d'atteindre avec un télescope seul. La mise en œuvre au sol de cette méthode est limitée en sensibilité et précision de mesure à cause de la turbulence atmosphérique. Or les nouveaux besoins scientifiques, tels que la détermination des paramètres fondamentaux, l'étude de l'environnement proche ou de la surface des étoiles, requièrent la capacité d'observer des objets de moins en moins brillants et de faire des mesures de plus en plus précises, en interférométrie visible. Pour s'affranchir de la turbulence, l'interférométrie multimode a été développée en reprenant le concept de l'interférométrie des tavelures utilisée sur un seul télescope. Aujourd'hui, pour améliorer davantage les performances des futurs instruments, cette instrumentation évolue vers l'utilisation de la nouvelle génération de détecteur, l'Electron Multiplying Charge-Coupled Device (EMCCD), et de l'emploi des fibres optiques interfacées avec des optiques adaptatives. Cette avancée est motivée par le succès de l'utilisation conjointe de l'optique adaptative et du suivi de franges pour s'affranchir partiellement de la turbulence en interférométrie infrarouge, en 2017 avec l'instrument GRAVITY (Gravity Collaboration et al. 2017). Le prototype FRIEND (Fibered and spectrally Resolved Interferometer - New Design) a été conçu pour caractériser et évaluer les performances de la combinaison de ces éléments, dans le domaine visible. L'amélioration de la précision des instruments interférométriques est apportée par les fibres optiques et par la dynamique du signal délivré par une EMCCD. L'inconvénient de l'emploi des fibres dans le visible est une perte de la sensibilité du fait que le taux d'injection du flux dans celles-ci est très faible à cause de la turbulence atmosphérique. Mais il se trouve que l'optique adaptative et l'EMCCD permettent d'améliorer la sensibilité. En effet, l'optique adaptative maximise l'injection en réduisant l'influence de la turbulence atmosphérique, et l'EMCCD est capable de détecteur de faibles flux. FRIEND prépare ainsi le développement du futur instrument SPICA, recombinant jusqu'à six télescopes (Mourard et al. 2017, 2018). Celui-ci devra explorer la stabilisation des interférences grâce au suivi de franges. Cet aspect n'est pas abordé dans cette thèse. Je présente dans cette thèse le prototype FRIEND, capable de recombiner jusqu'à trois télescopes, opérant dans la bande R en franges dispersées. Il est doté de fibres optiques gaussiennes monomodes à maintien de polarisation et d'une EMCCD. Il est installé sur l'interféromètre visible Center for High Angular Resolution Astronomy (CHARA), au Mount Wilson, en Californie, qui est en train de s'équiper d'optiques adaptatives. J'ai développé des estimateurs de visibilité et de clôture de phase, la méthode de réduction des données de ce prototype et une stratégie d'observation. Grâce à ces outils, j'ai montré que les optiques adaptatives améliorent le taux d'injection dans les fibres. Il est alors apparu que la stabilisation de l'injection est importante pour maximiser le rapport signal-à-bruit dans chaque image. La biréfringence des fibres dégrade les performances de l'instrument mais elle a pu être compensée. J'ai montré qu'un instrument, basé sur la conception de FRIEND, permet d'accéder à des mesures de visibilité faibles avec une précision, inatteignable avec la génération actuelle, grâce au développement et l'utilisation d'un modèle de rapport signal-à-bruit. L'instrument a enfin été testé dans son intégralité sur le système binaire connu ζ Ori A. Cette observation montre la fiabilité et la précision des mesures interférométriques obtenues avec ce prototype, montrant l'intérêt de cette association de technologies pour les futurs interféromètres visibles

Development and scientific exploitation of a new guided optics visible in interferometric instrument

L'interférométrie visible longue base est une technique d'observation en astronomie permettant de sonder les objets avec une résolution spatiale qu'il est impossible d'atteindre avec un télescope seul. La mise en œuvre au sol de cette méthode est limitée en sensibilité et précision de mesure à cause de la turbulence atmosphérique. Or les nouveaux besoins scientifiques, tels que la détermination des paramètres fondamentaux, l'étude de l'environnement proche ou de la surface des étoiles, requièrent la capacité d'observer des objets de moins en moins brillants et de faire des mesures de plus en plus précises, en interférométrie visible. Pour s'affranchir de la turbulence, l'interférométrie multimode a été développée en reprenant le concept de l'interférométrie des tavelures utilisée sur un seul télescope. Aujourd'hui, pour améliorer davantage les performances des futurs instruments, cette instrumentation évolue vers l'utilisation de la nouvelle génération de détecteur, l'Electron Multiplying Charge-Coupled Device (EMCCD), et de l'emploi des fibres optiques interfacées avec des optiques adaptatives. Cette avancée est motivée par le succès de l'utilisation conjointe de l'optique adaptative et du suivi de franges pour s'affranchir partiellement de la turbulence en interférométrie infrarouge, en 2017 avec l'instrument GRAVITY (Gravity Collaboration et al. 2017). Le prototype FRIEND (Fibered and spectrally Resolved Interferometer - New Design) a été conçu pour caractériser et évaluer les performances de la combinaison de ces éléments, dans le domaine visible. L'amélioration de la précision des instruments interférométriques est apportée par les fibres optiques et par la dynamique du signal délivré par une EMCCD. L'inconvénient de l'emploi des fibres dans le visible est une perte de la sensibilité du fait que le taux d'injection du flux dans celles-ci est très faible à cause de la turbulence atmosphérique. Mais il se trouve que l'optique adaptative et l'EMCCD permettent d'améliorer la sensibilité. En effet, l'optique adaptative maximise l'injection en réduisant l'influence de la turbulence atmosphérique, et l'EMCCD est capable de détecteur de faibles flux. FRIEND prépare ainsi le développement du futur instrument SPICA, recombinant jusqu'à six télescopes (Mourard et al. 2017, 2018). Celui-ci devra explorer la stabilisation des interférences grâce au suivi de franges. Cet aspect n'est pas abordé dans cette thèse. Je présente dans cette thèse le prototype FRIEND, capable de recombiner jusqu'à trois télescopes, opérant dans la bande R en franges dispersées. Il est doté de fibres optiques gaussiennes monomodes à maintien de polarisation et d'une EMCCD. Il est installé sur l'interféromètre visible Center for High Angular Resolution Astronomy (CHARA), au Mount Wilson, en Californie, qui est en train de s'équiper d'optiques adaptatives. J'ai développé des estimateurs de visibilité et de clôture de phase, la méthode de réduction des données de ce prototype et une stratégie d'observation. Grâce à ces outils, j'ai montré que les optiques adaptatives améliorent le taux d'injection dans les fibres. Il est alors apparu que la stabilisation de l'injection est importante pour maximiser le rapport signal-à-bruit dans chaque image. La biréfringence des fibres dégrade les performances de l'instrument mais elle a pu être compensée. J'ai montré qu'un instrument, basé sur la conception de FRIEND, permet d'accéder à des mesures de visibilité faibles avec une précision, inatteignable avec la génération actuelle, grâce au développement et l'utilisation d'un modèle de rapport signal-à-bruit. L'instrument a enfin été testé dans son intégralité sur le système binaire connu ζ Ori A. Cette observation montre la fiabilité et la précision des mesures interférométriques obtenues avec ce prototype, montrant l'intérêt de cette association de technologies pour les futurs interféromètres visibles.Long baseline visible interferometry in astronomy is an observing technique which allows to get insights of an object with an outstanding angular resolution, unreachable with single-dish telescope. Interferometric measurements with ground-based instrumentation are currently limited in sensitivity and precision due to atmospheric turbulence. However, the new astrophysical needs, particularly the determination of fundamental parameters or the study of the closed environment and the surface of the stars, require to observe fainter objects with a better precision than now in visible interferometry. Ought to overcome the atmospheric turbulence, multispeckle interferometry has been developed by adapting speckle imaging technics used on single-dish telescope. Today, in order to improve the performance of the future combiners, instrumentation progresses to the use of a new generation detector called EMCCD, and the use of optical fibers which are coupled with adaptive optics. This path is chosen thank to the success of the use of the adaptive optics with the fringe tracking in the infrared interferometry in 2017 (Gravity Collaboration et al. 2017), in order to compensate turbulence. FRIEND prototype (Fibered and spectrally Resolved Interferometer - New Design) has been designed to characterize and estimate the performance of such a combination of technologies, in the visible spectral band. The improvement of the precision of the measurements from interferometric instruments is due to optical fibers and the dynamical range of the EMCCD. The counterpart of using the optical fibers is a loss in sensitivity due to a low injection rate of flux into the fibers because of the atmospheric turbulence. On the other hand, sensitivity is improved thanks to adaptive optics and EMCCDs. Indeed, adaptive optics increases the injection rate and EMCCDs can measure low fluxes. Lastly, FRIEND is a pathfinder for the future instrument SPICA which should recombine up to 6 telescopes (Mourard et al. 2017, 2018). Fringe-tracking aspects will have to be studied for SPICA; this topic is not dealt with in this thesis. In this work, I present the FRIEND prototype, which can recombine up to three telescopes and operates in the R band with dispersed fringes. It has Gaussian polarization-maintaining single mode optical fibers and an EMCCD. It is set up at the Center for High Angular Resolution Astronomy (CHARA), at Mount Wilson, in California. CHARA is currently being equipped with adaptive optics. I develop estimators of visibility modulus and closure phase, the data reduction software and an observing strategy. Thanks to that, I am able to show that adaptive optics improves the injection rate. I also demonstrate how important the stabilization of injection is to maximize the signal-to-noise ratio (SNR) per frame. Birefringence of the fibers decreases the performance of the instrument but we manage to compensate it. I show how such an instrument can measure low visibility with a better precision than now by developing and using a SNR model of FRIEND. Finally, FRIEND has entirely been tested on the known binary system ζ Ori A. These observations demonstrate how reliable and accurate the measurements of FRIEND are