Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Polychromatic guide star: feasibility study

International audienceAdaptive optics at astronomical telescopes aims at correcting in real time the phase corrugations of incoming wavefronts caused by the turbulent atmosphere, as early proposed by Babcock. Measuring the phase errors requires a bright source located within the isoplanatic patch of the program source. The probability that such a reference source exists is a function of the wavelength, of the required image quality (Strehl ratio), of the turbulence optical properties, and of the direction of the observation. It turns out that the sky coverage is disastrously low in particular in the visible wavelength range where, unfortunately, the gain in spatial resolution brought by adaptive optics is the largest. Foy and Labeyrie have proposed to overcome this difficulty by creating an artificial point source in the sky in the direction of the observation relying on the backscattered light due to a laser beam. This laser guide star (hereinafter referred to as LGS) can be bright enough to allow us to accurately measure the wavefront phase errors, except for two modes which are the piston (not relevant in this case) and the tilt. Pilkington has emphasized that the round trip time of the laser beam to the mesosphere, where the LGS is most often formed, is significantly shorter than the typical tilt coherence time; then the inverse-return-of-light principle causes deflections of the outgoing and the ingoing beams to cancel. The apparent direction of the LGS is independent of the tilt. Therefore the tilt cannot be measured only from the LGS. Until now, the way to overcome this difficulty has been to use a natural guide star to sense the tilt. Although the tilt is sensed through the entire telescope pupil, one cannot use a faint source because $APEX 90% of the variance of the phase error is in the tilt. Therefore, correcting the tilt requires a higher accuracy of the measurements than for higher orders of the wavefront. Hence current adaptive optics devices coupled with a LGS face low sky coverage. Several methods have been proposed to get a partial sky coverage for the tilt. The only one providing us with a full sky coverage is the polychromatic LGS (hereafter referred to as PLGS). We present here a progress report of the R&D; program Etoile Laser Polychromatique et Optique Adaptative (ELP-OA) carried out in France to develop the PLGS concept. After a short recall of the principles of the PLGS, we will review the goal of ELP-OA and the steps to get over to bring it into play. We finally shortly described the effort in Europe to develop the LGS

Aujourd'hui le Moyen Age, archéologie et vie quotidienne en France Méridionale

[catalogue d'exposition], 1981-1983, Sénanque, Marseille, Arles, Toulon, Perpignan, Montpellier, Nice, GapInternational audienceCe catalogue d’exposition itinérante, fait la synthèse des principaux travaux du Laboratoire d’Archéologie Médiévale Méditerranéenne d’Aix-en-Provence (URA 6 du CRA CNRS) en 1981 réunis par l’équipe de Gabrielle Démians d’Archimbaud. L’habitat, les gestes de la vie quotidienne, le commerce et l’artisanat sont illustrés par les fouilles effectuées dans le Midi de la France, par de nombreux objets, en céramique, verre, pierre, os et métal ainsi que par les sources écrites, et les analyses de laboratoire, innovantes à cette époque

NATO Advanced Study Institute on Optics in Astrophysics

Astrophysics is facing challenging aims such as deep cosmology at redshift higher than 10 to constrain cosmology models, or the detection of exoplanets, and possibly terrestrial exoplanets, and several others. It requires unprecedented ambitious R&D programs, which have definitely to rely on a tight cooperation between astrophysics and optics communities. The book addresses most of the most critical interdisciplinary domains where they interact, or where they will do. A first need is to collect more light, i.e. telescopes still larger than the current 8-10 meter class ones. Decametric, and even hectometric, optical (from UV to IR wavelengths) telescopes are being studied. Whereas up to now the light collecting surface of new telescopes was approximately 4 times that of the previous generation, now this factor is growing to 10 to 100. This quantum leap urges to implement new methods or technologies developed in the optics community, both in academic labs and in the industry. Given the astrophysical goals and technological constraints, new generation adaptive optics with a huge number of actuators and laser guide stars devices have to be developed, from theoretical bases to experimental works. Two other newcomers in observational astrophysics are interferometric arrays of optical telescopes and gravitational wave detectors. Up-to-date reviews of detectors and of spectrographs are given, as well as forefront R&D in the field of optical coatings and of guided optics. Possible new ways to handle photons are also addressed, based on quantum physics. More and more signal processing algorithms are a part and parcel of any modern instrumentation. Thus finally the book gives two reviews about wavefront processing and about image restoration and deconvolution algorithms for ill conditioned cases

Peyremoutou : une verrerie du XVII e siècle dans la Montagne Noire (Tarn)

The glass-kiln of Peyremoutou found thanks to the excavation is one vestige of the numerous workshops set up in La Montagne Noire in the modem age. This melting-kiln whose base-plate still bears the stamp of the crucibles to a more complex set that we'll try to find in our next excavation. According to the archaeological material (ceramics, glasses and tools) and according to the texts, this workshop dated from the 17 th century, belonged to the famous glassblowers named de Robert. Some medieval fragments of window-glasses and ceramics enable to suppose there was a former occupation. But thèse éléments unhappily out of stratigraphy are not sufficient to define this occupation.Le four à verre de Peyremoutou découvert par la fouille est un vestige des nombreux ateliers établis dans la Montagne Noire à l'époque Moderne. Ce four de fusion dont la sole porte encore les marques des creusets, faisait partie d'un ensemble plus complexe que la suite des travaux s'attacheront à révéler. D'après le matériel archéologique (céramiques, verres et outil) et les textes, cet atelier daté du XVIIe siècle appartenait à la célèbre famille verrière des de Robert. Quelques indices -mali.eureusement hors stratigraphie- laissent supposer une occupation antérieure à l'installation des verriers modernes : des fragments de verre et de céramique médiévaux ne suffisent pas à définir cette occupation.Bourrel Bernard, Averous Jean-Claude, Foy Danièle. Peyremoutou : une verrerie du XVII e siècle dans la Montagne Noire (Tarn). In: Archéologie du Midi médiéval. Tome 1, 1983. pp. 93-102

Le mobilier de l’Antiquité tardive

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Guide pédagogique : le handicap visuel : postsecondaire /

Bibliogr.: f. 85-8