Phase-shifting technique for improving the imaging capacity of sparse-aperture optical interferometers

In this paper, we describe the principle of a multi-aperture interferometer that uses a phase-shifting technique and is suitable for quick, snapshot imaging of astrophysical objects at extreme angular resolution through Fourier inversion. A few advantages of the proposed design are highlighted, among which are radiometric efficiency, Field of View equivalent to those of Fizeau interferometers, and a preliminary calibration procedure allowing characterization of instrumental errors. For large telescope numbers, the proposed design also results in considerable simplification of the optical and mechanical design. Numerical simulations suggest that it should be possible to couple hundreds of telescopes on a single 4K x 4K detector array, using only conventional optical components or emerging technologies.Comment: 40 pages and 9 figure

Optimisation et cophasage d'un dispositif d'imagerie directe à haute résolution et haut contraste : l'hypertélescope temporel

Thanks to high angular resolution abilities, the hypertelescope concept forms a promising way to develop new astronomical instruments. However, its feasibility have to be experimentally demonstrated and especially a solution to the cophasing problem has to be found. This is the aim of this PHD work. In the XLIM laboratory, we have developed a fibered hypertelescope prototype called Temporal HyperTelescope (THT). The concept of this height-telescope-instrument is detailed in the first chapter of this document. A study of the instrument defaults has been carried out to evaluate its theoretical imaging abilities. In a second step, a cophasing system has been implemented. It is based on a joint use of a genetic algorithm and the phase diversity technique. Its efficiency has been experimentally validated in laboratory thanks to the acquisition of a binary star image showing a 9.1 magnitude unbalance and requiring a 3-nanometer-optical-path-lengh-resolution. Then, we have tested this method in the photon counting regime and the experimental results demonstrate that the instrument is able to acquire images even in these extreme running conditions. Finally, the last chapter of this document gives few points to be improved and suggests a draft of space based instrument which could be realized in the next decade.Les hypertélescopes, grâce à leur capacité d'imagerie directe à haute résolution, constituent une voie prometteuse pour le développement de nouveaux instruments dédiés à l'astrophysique. Il reste cependant à démontrer expérimentalement leur faisabilité et notamment à trouver une solution au difficile problème du cophasage. C'est dans cette optique qu'ont été réalisés les travaux présentés dans cette thèse. Le banc de test THT, développé au laboratoire XLIM de Limoges, est le prototype entièrement fibré d'une version particulière d'hypertélescope, appelée hypertélescope temporel. L'historique ainsi que le concept de cet instrument constitué d'un réseau de huit télescopes sont présentés dans le premier chapitre de ce manuscrit. Une étude des défauts expérimentaux intrinsèques à l'instrument a été réalisée afin d'évaluer ses capacités théoriques d'imagerie. Dans un second temps, un dispositif de cophasage a été mis en place. Il est basé sur l'utilisation couplée d'un algorithme génétique et de la technique de diversité de phase. Son efficacité a été validé expérimentalement grâce à l'acquisition en laboratoire d'une image d'un système binaire d'étoiles présentant un écart en magnitude de 9,1 nécessitant un contrôle de tous les chemins optiques avec une résolution d'environ 3 nm. Nous avons ensuite testé cette méthode dans le cas d'un fonctionnement en régime de comptage de photons. Les résultats expérimentaux obtenus démontrent que même dans ces conditions difficiles, les qualités d'imagerie du dispositif sont conservées. Finalement, le dernier chapitre de ce document donne différentes pistes de développement et propose une ébauche d'un projet spatial réalisable à moyen terme

Co-phasing of a diluted aperture synthesis instrument for direct imaging: Experimental demonstration on a temporal hypertelescope

International audienceContext. The diluted aperture synthesis is one of the most promising ways of obtaining direct images with an angular resolution in the milliarcsecond range. By applying apodization techniques to a hypertelescope, it is possible to discriminate between objects with a high contrast in intensity with a reasonable number of telescopes (<10). Aims. To reach such performances, we attempt to develop a co-phasing system capable of stabilizing the optical path differences with an accuracy better than λ/100 RMS. Methods. We propose a method based on a joint use of a sub-aperture piston phase-diversity technique and a genetic algorithm to co-phase a laboratory prototype called a temporal hypertelescope (THT). First, we simulated the behavior of this instrument and inferred the related statistical properties of our co-phasing method. In a second step, we implemented this co-phasing system on our THT prototype. Results. We obtain an experimental stabilization of the optical path differences of about λ/300 RMS over 1000 s. Thanks to this result, we are able to acquire an image of a high-contrast binary system. We also validate that the instrument accurately estimates the object characteristics, i.e. 25 μrad for the angular separation and ΔH = 9.1 magnitude difference between the main star and its companion

First experimental demonstration of temporal hypertelescope operation

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Workshop hypertelescope Nice

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Co-phasing of a diluted aperture synthesis instrument for direct imagingII. Experimental demonstration in the photon-counting regime with a temporal hypertelescope

International audienceContext. Amongst the new techniques currently developed for high-resolution and high-dynamics imaging, the hypertelescope architecture is very promising for direct imaging of objects such as exoplanets. The performance of this instrument strongly depends on the co-phasing process accuracy. In a previous high-flux experimental study with an eight-telescope array, we successfully implemented a co-phasing system based on the joint use of a genetic algorithm and a sub-aperture piston phase diversity using the object itself as a source for metrology.Aims. To fit the astronomical context, we investigate the impact of photon noise on the co-phasing performance operating our laboratory prototype at low flux. This study provides experimental results on the sensitivity and the dynamics that could be reached for real astrophysical observations.Methods. Simulations were carried out to optimize the critical parameters to be applied in the co-phasing system running in the photon-counting regime. We used these parameters experimentally to acquire images with our temporal hypertelescope test bench for different photon flux levels. A data reduction method allows highly contrasted images to be extracted.Results. The optical path differences have been servo-controlled over one hour with an accuracy of 22.0 nm and 15.7 nm for 200 and 500 photons/frame, respectively. The data reduction greatly improves the signal-to-noise ratio and allows us to experimentally obtain highly contrasted images. The related normalized point spread function is characterized by a 1.1 × 10-4 and 5.4 × 10-5 intensity standard deviation over the dark field (for 15 000 snapshots with 200 and 500 photons/frame, respectively).Conclusions. This laboratory experiment demonstrates the potential of our hypertelescope concept, which could be directly transposed to a space-based telescope array. Assuming eight telescopes with a 30 cm diameter, the I-band limiting magnitude of the main star would be 7.3, allowing imaging of a companion with a 17.3 mag

Two new proposals for high resolution imaging in astronomy: Parametric interferometers and temporal hypertelescopes

National audienceHigh resolution stellar interferometers are very powerful and efficient instruments to get a better knowledge of our Universe through direct imaging or spatial coherence. Many important results are currently obtained with interferometers such as Keck telescopes or VLTI. For 30 years, we have developed at Limoges lab a set of new techniques to simplify the architecture of a telescope array dedicated to high resolution imaging. In a first step, I will present you a tutorial on the current status of high resolution imaging techniques mainly based on the coherence analysis. Second, I will explain how we intend to use non-linear optics in this field. Our team proposes to explore the possibility of using non-linear optical techniques to avoid lots of technical difficulties related to infrared optics (components transmission, thermal noises, thermal cooling,...). This is a promising alternative technique for the detection of infrared optical signals. Indeed, we have experimentally demonstrated that sum frequency generation in a non-linear crystal (PPLN) does not result in an additional bias on the interferometric data provided by a stellar interferometer up to the photon counting regime. Last, I will describe a new kind of instrument allowing to provide direct images. The temporal hypertelescope is a new version of spatial hypertelescope proposed by A. Labeyrie. Our temporal version simplifies the implementation through a transposition from the spatial to the temporal domain. The experimental demonstration has been achieved with an 8-telescope laboratory prototype. A servo-control system has been implemented using a genetic algorithm and a phase diversity technique allowing to reach a $10^3-10^4$ dynamics. At the present time, we are studying noise limits up to the photon counting regime

Asservissement et cophasage des antennes synthétiques dédiées à l'imagerie haute résolution pour l'astronomie

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Laboratory Demonstration of Spatial-Coherence Analysis of a Blackbody through an Up-Conversion Interferometer

International audienceIn the field of high resolution imaging in astronomy, we experimentally demonstrate the spatial-coherence analysis of a blackbody using an up-conversion interferometer in the photon counting regime. The infrared radiation of the blackbody is converted to a visible one in both arms of the interferometer thanks to the sum-frequency generation processes achieved in Ti-diffused periodically poled lithium niobate waveguides. The coherence analysis is performed through a dedicated imaging stage which mimics a classical telescope array analyzing an astrophysical source. The validity of these measurements is confirmed by the comparison with spatial-coherence analysis through a reference interferometer working at infrared wavelengths

Application of frequency conversion of starlight to high-resolution imaging interferometry. On-sky sensitivity test of a single arm of the interferometer

International audienceWe investigate the sensitivity of frequency conversion of starlight using a non-linear optical sum frequency process. This study is being carried out in the context of future applications of optical interferometry dedicated to high-resolution imaging. We have implemented a complete experimental chain from telescope to detector. The starlight frequency is shifted from the infrared to the visible using an optically non-linear crystal. To fulfil the requirements of interferometry, our experimental setup uses spatially single-mode and polarization maintaining components. Due to the small size of the collecting aperture (8 inches Celestron C8) with a 3 nm spectral bandwidth, on-sky tests were performed on bright stars in the H band. The detection was achieved in a true photon counting operation, using synchronous detection. Betelgeuse (HMag =−3.9), Antares (HMag =−3.6) and Pollux (HMag =−1) were successfully converted and detected in visible light. Despite the low transmission of our experiment, our results prove that the efficiency of frequency conversion offers sufficient sensitivity for future interferometric applications