423 research outputs found
Lyot-based Ultra-Fine Pointing Control System for Phase Mask Coronagraphs
High performance coronagraphic imaging at small inner working angle requires
efficient control of low order aberrations. The absence of accurate pointing
control at small separation not only degrades coronagraph starlight rejection
but also increases the risk of confusing planet's photons with starlight
leaking next to the coronagraph focal plane mask center. Addressing this issue
is essential for preventing coronagraphic leaks, and we have thus developed a
new concept, the Lyot-based pointing control system (LPCS), to control pointing
errors and other low order aberrations within a coronagraph. The LPCS uses
residual starlight reflected by the Lyot stop at the pupil plane. Our
simulation has demonstrated pointing errors measurement accuracy between 2-12
nm for tip-tilt at 1.6 micron with a four quadrant phase mask coronagraph.Comment: 7 pages, 5 figures, Proceedings of AO4ELTs3 conference, Paper 12667,
Florence, Italy, May 201
Expected Performance of a Self-Coherent Camera
Residual wavefront errors in optical elements limit the performance of
coronagraphs. To improve their efficiency, different types of devices have been
proposed to correct or calibrate these errors. In this paper, we study one of
these techniques proposed by Baudoz et al. 2006 and called Self-Coherent Camera
(SCC). The principle of this instrument is based on the lack of coherence
between the stellar light and the planet that is searched for. After recalling
the principle of the SCC, we simulate its performance under realistic
conditions and compare it with the performance of differential imaging.Comment: 6 pages, 4 figure
Lyot-based Low Order Wavefront Sensor for Phase-mask Coronagraphs: Principle, Simulations and Laboratory Experiments
High performance coronagraphic imaging of faint structures around bright
stars at small angular separations requires fine control of tip, tilt and other
low order aberrations. When such errors occur upstream of a coronagraph, they
results in starlight leakage which reduces the dynamic range of the instrument.
This issue has been previously addressed for occulting coronagraphs by sensing
the starlight before or at the coronagraphic focal plane. One such solution,
the coronagraphic low order wave-front sensor (CLOWFS) uses a partially
reflective focal plane mask to measure pointing errors for Lyot-type
coronagraphs.
To deal with pointing errors in low inner working angle phase mask
coronagraphs which do not have a reflective focal plane mask, we have adapted
the CLOWFS technique. This new concept relies on starlight diffracted by the
focal plane phase mask being reflected by the Lyot stop towards a sensor which
reliably measures low order aberrations such as tip and tilt. This reflective
Lyot-based wavefront sensor is a linear reconstructor which provides high
sensitivity tip-tilt error measurements with phase mask coronagraphs.
Simulations show that the measurement accuracy of pointing errors with
realistic post adaptive optics residuals are approx. 10^-2 lambda/D per mode at
lambda = 1.6 micron for a four quadrant phase mask. In addition, we demonstrate
the open loop measurement pointing accuracy of 10^-2 lambda/D at 638 nm for a
four quadrant phase mask in the laboratory.Comment: 9 Pages, 11 Figures, to be published in PASP June 2014 issu
Condensation of helium in aerogels and athermal dynamics of the Random Field Ising Model
High resolution measurements reveal that condensation isotherms of He in
a silica aerogel become discontinuous below a critical temperature. We show
that this behaviour does not correspond to an equilibrium phase transition
modified by the disorder induced by the aerogel structure, but to the
disorder-driven critical point predicted for the athermal out-of-equilibrium
dynamics of the Random Field Ising Model. Our results evidence the key role of
non-equilibrium effects in the phase transitions of disordered systems.Comment: 5 p + suppl. materia
Branch and Bound hybride pour un problème de job-shop soumis à des contraintes de ressources humaines
National audienceNous considérons un problème couplant ordonnancement de production et planification d'agents. On se place ainsi dans un atelier où la production à réaliser requiert divers types de machines dans des séquences variées de type job-shop. Chaque machine nécessite pour son utilisation la présence d'un employé qualifié à son pilotage. Les ressources humaines sont assujetties à des contraintes légales restreignant leur disponibilité. La production doit être entièrement ordonnancée et le critère d'optimisation retenu est la minimisation des coûts salariaux. Pour résoudre ce problème, nous avons développé une méthode exacte hybridant approche arborescente de type Procédure de Séparation et Evaluation Séquentielle et technique de génération de coupes de réalisabilité. Cette méthode exploite la décomposition naturelle du problème global en deux sous-problèmes : un problème de planification d'agents et un problème de job-shop à contraintes de disponibilité. Des méthodes de génération d'inégalités valides en pré-process (notamment du probing) ont en outre été étudiées. Notre approche s'avère particulièrement adaptée à la problématique ; ses résultats dominent en effet ceux obtenus avec l'un des meilleurs solveurs commerciaux actuels (Ilog Cplex 12.1) et ceux obtenus avec les méthodes décrites dans la littérature
Solving an integrated job-shop problem with human resource constraints
International audienceWe propose two exact methods to solve an integrated employee-timetable and job-shop-scheduling problem. The problem is to find a minimum cost employee-timetable, where employees have different competences and work during shifts, so that the production, that corresponds to a job-shop with resource availability constraints, can be achieved. We introduce two new exact procedures: (1) a decomposition and cut generation approach and (2) a hybridization of a cut generation process with a branch and bound strategy. We also propose initial cuts that strongly improve these methods as well as a standard MIP approach. The computational performances of those methods on benchmark instances are compared to that of other methods from the literature
Solving an integrated Job-Shop problem with human resource constraints
This paper investigates the integration of the employee timetabling and production scheduling problems. At the first level, we have to manage a classical employee timetabling problem. At the second level, we aim at supplying a feasible production schedule for a job-shop scheduling problem (NP-hard problem). Instead of using a hierarchical approach as in the current practice, we here integrate the two decision stages and propose two exact methods for solving the resulting problem. The former is similar to the cut generation algorithm proposed in Guyon et. al. 2010) for a problem integrating a classical employee timetabling problem and a polynomially solvable production scheduling problem. The latter is based on a Branch-And-Cut process that exploits the same feasibility cuts than the first approach. Preliminar experimental results on instances proposed in (Artigues et al. 2009) reveal a real interest for the approaches described here
Planification d'agents et ordonnancement de production : règles d'élimination et heuristique
Nous nous intéressons à l'optimisation des systèmes de production avec d'un côté l'ordonnancement de production, dont le but est d'affecter dans le temps des ressources à des tâches à réaliser, et d'un autre côté la gestion du personnel visant généralement à minimiser les coûts de main d'œuvre. Bien qu'il soit admis que, pour obtenir l'optimum global, il faille prendre en compte simultanément ces deux problématiques, en pratique le problème global est souvent résolu dans un processus de décision à deux niveaux. Dans cette étude, nous intégrons ces deux phases de décision et proposons différentes méthodes pour résoudre le problème résultant
Cut generation for an integrated employee timetabling and production scheduling problem
International audienceThis paper investigates the integration of the employee timetabling and production scheduling problems. At the first level, we manage a classical employee timetabling problem. At the second level, we aim at supplying a feasible production schedule for a set of interruptible tasks with qualification requirements and time-windows. Instead of hierarchically solving these two problems as in the current practice, we try here to integrate them and propose two exact methods to solve the resulting problem. The former is based on a Benders decomposition while the latter relies on a specific decomposition and a cut generation process. The relevance of these different approaches is discussed here through experimental results
- …