390 research outputs found
Coronagraphic phase diversity: performance study and laboratory demonstration
The final performance of current and future instruments dedicated to
exoplanet detection and characterization (such as SPHERE on the European Very
Large Telescope, GPI on Gemini North, or future instruments on Extremely Large
Telescopes) is limited by uncorrected quasi-static aberrations. These
aberrations create long-lived speckles in the scientific image plane, which can
easily be mistaken for planets. Common adaptive optics systems require
dedicated components to perform wave-front analysis. The ultimate wave-front
measurement performance is thus limited by the unavoidable differential
aberrations between the wavefront sensor and the scientific camera. To reach
the level of detectivity required by high-contrast imaging, these differential
aberrations must be estimated and compensated for. In this paper, we
characterize and experimentally validate a wave-front sensing method that
relies on focal-plane data. Our method, called COFFEE (for COronagraphic
Focal-plane wave-Front Estimation for Exoplanet detection), is based on a
Bayesian approach, and it consists in an extension of phase diversity to
high-contrast imaging. It estimates the differential aberrations using only two
focal-plane coronagraphic images recorded from the scientific camera itself. In
this paper, we first present a thorough characterization of COFFEE's
performance by means of numerical simulations. This characterization is then
compared with an experimental validation of COFFEE using an in-house adaptive
optics bench and an apodized Roddier & Roddier phase mask coronagraph. An
excellent match between experimental results and the theoretical study is
found. Lastly, we present a preliminary validation of COFFEE's ability to
compensate for the aberrations upstream of a coronagraph.Comment: A&A accepte
High-order myopic coronagraphic phase diversity (COFFEE) for wave-front control in high-contrast imaging systems
The estimation and compensation of quasi-static aberrations is mandatory to
reach the ultimate performance of high-contrast imaging systems. COFFEE is a
focal plane wave-front sensing method that consists in the extension of phase
diversity to high-contrast imaging systems. Based on a Bayesian approach, it
estimates the quasi-static aberrations from two focal plane images recorded
from the scientific camera itself. In this paper, we present COFFEE's extension
which allows an estimation of low and high order aberrations with nanometric
precision for any coronagraphic device. The performance is evaluated by
realistic simulations, performed in the SPHERE instrument framework. We develop
a myopic estimation that allows us to take into account an imperfect knowledge
on the used diversity phase. Lastly, we evaluate COFFEE's performance in a
compensation process, to optimize the contrast on the detector, and show it
allows one to reach the 10^-6 contrast required by SPHERE at a few resolution
elements from the star. Notably, we present a non-linear energy minimization
method which can be used to reach very high contrast levels (better than 10^-7
in a SPHERE-like context)Comment: Accepted in Optics Expres
The New Control and Interlock System for the SPS Main Power Converters
The Control and Interlock System (CIS) of the SPS main power converters was designed in the mid-70s and became increasingly difficult to maintain. A new system based on Programmable Logic Controllers has been developed by an external contractor in close collaboration with CERN. The system is now operational and fully integrated in the SPS/LEP control infrastructure. The CIS is the first major contracted industrial solution used to control accelerator equipment directly involved in the production of particle beams at CERN. This paper gives an overview of the SPS main power converter installation and describes both the contractual and technical solution adopted for the CIS. It first explains how the system was specified and how the contractual relationship was defined to respect CERNs purchasing rules and the operational requirements of the SPS accelerator. The architectural design of the new system is presented with special emphasis on how the conflict between safety and availability has been addressed
Control and interlock system for the SPS main power converters
The control and interlock system of the main power converters was 20 years old and needed to be replaced. In order to face the shrinking resources of CERN, it was decided to adopt, as far as possible, standard industrial solutions and to contract out the development of the new system to industry. A tender was sent to European firms and the contract was awarded to GTD, a Spanish Engineering Firm, in May 1997. The SPS accelerator restarted in March 98 with the new Control and Interlock System
Correction of distortion for optimal image stacking in Wide Field Adaptive Optics: Application to GeMS data
The advent of Wide Field Adaptive Optics (WFAO) systems marks the beginning
of a new era in high spatial resolution imaging. The newly commissioned Gemini
South Multi-Conjugate Adaptive Optics System (GeMS) combined with the infrared
camera Gemini South Adaptive Optics Imager (GSAOI), delivers quasi
diffraction-limited images over a field of 2 arc-minutes across. However,
despite this excellent performance, some variable residues still limit the
quality of the analyses. In particular, distortions severely affect GSAOI and
become a critical issue for high-precision astrometry and photometry. In this
paper, we investigate an optimal way to correct for the distortion following an
inverse problem approach. Formalism as well as applications on GeMS data are
presented.Comment: 10 pages, 6 figure
Risk factors for mortality-morbidity after emergency-urgent colorectal surgery
Background: The aim of this study was to assess the risk factors associated with mortality and morbidity following emergency or urgent colorectal surgery. Materials and methods: All data regarding the 462 patients who underwent emergency colonic resection in our institution between November 2002 and December 2007 were prospectively entered into a computerized database. Results: The median age of patients was 73 (range 17-98)years. The most common indications for surgery were: 171 adenocarcinomas (37%), 129 complicated diverticulitis (28%), and 35 colonic ischemia (7.5%). Overall mortality and morbidity rates were 14% and 36%, respectively. In multivariate analysis, the only parameter significantly associated with postoperative mortality was blood loss >500cm3 (odds ratio (OR) = 3.33, 95% confidence interval (CI) 1.63-6.82, p = 0.001). There were three parameters which correlated with postoperative morbidity: ASA score ≥3 (OR = 2.9, 95% CI 1.9-4.5, p < 0.001), colonic ischemia (OR = 3.4, 95% CI 1.4-7.7, p = 0.006), and stoma creation (OR = 2.2, 95% CI 1.4-3.4, p = 0.0003). Conclusions: The main risk factors for postoperative morbidity and mortality following emergency colorectal surgery are related to: (1) patients' ASA score, (2) colonic ischemia, and (3) perioperative bleeding. These variables should be considered in the elaboration of future scoring systems to predict outcome of emergency colorectal surger
Basic conceptual structures theory
Although the theory of Conceptual Structures is over 10 years old, basic notions (like canonical graphs) are far from settled and are subject to constant extensions and reformulations. However, most of these are done in an informal way, which doesn't help in clarifying the issues involved. It is our hope that this paper will provide a first step towards the complete and rigorous account of Conceptual Structures (CS) Theory, which is needed for ongoing standardization and implementation efforts.
Towards that goal, we present formal definitions of some of the central notions of CS theory (type, referent, concept, relation, conceptual graph, canonical formation rules, canon, and canonical graph) in its simplest form, i.e. no contexts nor coreference links are allowed and referents must be individuals. We thereby introduce higher-order types in order to enable the use of conceptual graphs at the metalevel, the restriction operation of the canonical formation rules is extended to make use of the relation hierarchy, we show the relationship between denotation and conformity relation, and we give a rigorous meaning to the canonical basis, among other things
Elaboration and characterization of Fe1–xO thin films sputter deposited from magnetite target
Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with Ar–O2 gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe1–xOthin films, directly sputtered froma magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature.We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted bymodifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported
- …