The planar optics phase sensor: a study for the VLTI 2nd generation fringe tracker

In a few years, the second generation instruments of the Very Large Telescope Interferometer (VLTI) will routinely provide observations with 4 to 6 telescopes simultaneously. To reach their ultimate performance, they will need a fringe sensor capable to measure in real time the randomly varying optical paths differences. A collaboration between LAOG (PI institute), IAGL, OCA and GIPSA-Lab has proposed the Planar Optics Phase Sensor concept to ESO for the 2[SUP]nd[/SUP] Generation Fringe Tracker. This concept is based on the integrated optics technologies, enabling the conception of extremely compact interferometric instruments naturally providing single-mode spatial filtering. It allows operations with 4 and 6 telescopes by measuring the fringes position thanks to a spectrally dispersed ABCD method. We present here the main analysis which led to the current concept as well as the expected on-sky performance and the proposed design

Mise en phase des grands interféromètres: Méthode de La Diversité de Phase Chromatique - Développement et Implémentation sur le démonstrateur hypertélescope fibré SIRIUS

In order to increase the imaging capabilities and the attainable resolution of the observing instruments in astronomy, the High Angular Resolution prospective proposes to increase the number of sub-apertures of the optical interferometers. Thanks to densification techniques, future interferometers will be capable of imaging faint and small (apparent size) astrophysical targets. The resulting instrumental constraints define the technical and technological challenge in which my Thesis work is performed. The propagation conditions and the recombining quality of the collected beams govern the performances of the image, in terms of stability and instrument sensitivity. To ensure a coherent interferometric mix and the possibility of observing during long integration times, it is necessary to maintain the optical path difference to a value lesser than a fraction of wavelength thanks to a cophasing device. I propose a method dedicated to cophase large interferometers: the Chromatic Phase Diversity. It is based on the spectral analysis of multi-wavelength images allowing to determine in real time the optical path difference to be compensated by the instrument delay lines. After a theoretical and numerical study of the method, I describe its practical implementation on the fibered hypertelescope testbench SIRIUS developed at the Observatoire de la Côte d'Azur (Nice, France).Afin d'augmenter les capacités d'imagerie et de résolution des instruments d'observation en astronomie, la prospective Haute Résolution Angulaire propose d'augmenter le nombre de sous-pupilles des interféromètres optiques. Associés à des techniques de densification de pupille, les futurs interféromètres seront capables d'imager des cibles astrophysiques faibles et/ou de taille apparente réduite. Il en résulte des contraintes instrumentales constituant le défi technique et technologique dans lequel s'inscrit mon travail de Thèse. Les conditions de propagation et la qualité de recombinaison des faisceaux collectés par chaque sous-pupille régissent les performances en termes de stabilité de l'image et de sensibilité des réseaux optiques. Pour garantir un mélange interférométrique cohérent et la possibilité d'observer sur des temps d'intégrations supérieurs à quelques millisecondes, il est nécessaire de maintenir la différence de marche optique à une valeur inférieure à la fraction de longueur d'onde grâce à un dispositif de cophasage. Je propose une méthode dédiée à la mise en phase des grands interféromètres : la Diversité de Phase Chromatique. Celle-ci est fondée sur une analyse spectrale des images à plusieurs longueurs d'onde permettant de déterminer en temps réel les différences de marche optique à compenser par les lignes à retard de l'instrument. Après une étude théorique et numérique de la méthode à travers l'analyse de cas réalistes, je présente sa mise en œuvre pratique sur le banc hypertélescope fibré SIRIUS développé à l'Observatoire de la Côte d'Azur

Spatial-frequency tuning of visual contour integration

We examine the mechanism that subserves visual contour detection and particularly its tuning for the spatial frequency of contour components. The measured the detection of contours composed of Gabor micropatterns within a held of randomly oriented distracter elements. Distracters were randomly assigned one of two spatial frequencies, and elements lying along the contour alternated between these values. We report that the degree of tolerable spatial-frequency difference between successive contour elements is inversely proportional to the orientation difference between them. Spatial-frequency tuning (half-width at half-height) for straight contours is similar to 1.3 octaves but, for contours with a 30 degrees difference between successive elements, drops to similar to 0.7 octaves. Integration of curved contours operates at a narrower bandwidth. Much orientation information in natural images arises from edges, and we propose that this narrowing of tuning is related to the reduction in interscale support that accompanies increasing edge curvature. (C) 1998 Optical Society of America

Multi-axial integrated optics solution for POPS, a 2nd-generation VLTI fringe tracker

POPS (Planar Optical Phase Sensor) is a second-generation fringe tracker for the Very Large Telescope Interferometer (VLTI), intended to simultaneously measure the cophasing and coherencing errors of up to six Unit Telescopes (UT) or Auxiliary Telescopes (AT) in real time. The most promising concepts are probably based on the utilization of Integrated Optics (IO) components, and were the scope of a Phase A study led by Observatoire de Grenoble (LAOG). Herein is described a tentative design built around a multi-axial IO chip whose fringes are dispersed downstream on a detector array, and a Chromatic Phase Diversity algorithm presented in another paper of this conference . We depict the foreseen opto-mechanical, detection and software implementations, and provide numerical results from a realistic simulation model in terms of group and phase delay measurement accuracy and limiting magnitudes in the K band. The ultimate performance of the method is discussed and compared with the original 2[SUP]nd[/SUP] generation VLTI fringe tracker requirements

Implementation of the chromatic phase diversity method on the SIRIUS test bench

International audienc

Group and phase delay sensing for cophasing large optical arrays

International audienceThe next generation of optical interferometers will provide high-resolution imaging of celestial objects by using either the aperture synthesis technique or the direct imaging principle. To determine the technical requirements, we have developed an interferometric test bench, called SIRIUS. To preserve the quality of the image, fast corrections of the optical path differences within a fraction of a wavelength have to be applied: this is the cophasing of the array, whereas making it coherent aims at stabilizing the optical path differences within a fraction of the coherence length. In the SIRIUS test bench, coherence and cophasing are achieved by fibred delay lines. Air delay lines are also used for the raw delay equalization. We present an original implementation of a piston sensor, called chromatic phase diversity, which is adaptable to any interferometer, whatever the configuration of the entrance pupil and the number of sub-pupils and whatever the interferometric combiner. Our method is based on the dispersed fringes principle and uses a derived version of the dispersed speckles method. The numerical simulation shows the performance of the method in terms of cophasing, accuracy and limiting magnitude. Experimental tests have been carried out both with optical turbulence and without. They show good results in both cases, despite some instrument-related limitations that can be eliminated. We show that our method is able to handle an amplitude of correction of ±11(λ/2) with an accuracy of ∼λ/30 over many minutes