381 research outputs found
The SPHERE data center: a reference for high contrast imaging processing
The objective of the SPHERE Data Center is to optimize the scientific return
of SPHERE at the VLT, by providing optimized reduction procedures, services to
users and publicly available reduced data. This paper describes our motivation,
the implementation of the service (partners, infrastructure and developments),
services, description of the on-line data, and future developments. The SPHERE
Data Center is operational and has already provided reduced data with a good
reactivity to many observers. The first public reduced data have been made
available in 2017. The SPHERE Data Center is gathering a strong expertise on
SPHERE data and is in a very good position to propose new reduced data in the
future, as well as improved reduction procedures.Comment: SF2A proceeding
Motion frozen 18F-FDG cardiac PET
BackgroundPET reconstruction incorporating spatially variant 3D Point Spread Function (PSF) improves contrast and image resolution. "Cardiac Motion Frozen" (CMF) processing eliminates the influence of cardiac motion in static summed images. We have evaluated the combined use of CMF- and PSF-based reconstruction for high-resolution cardiac PET.MethodsStatic and 16-bin ECG-gated images of 20 patients referred for (18)F-FDG myocardial viability scans were obtained on a Siemens Biograph-64. CMF was applied to the gated images reconstructed with PSF. Myocardium to blood contrast, maximum left ventricle (LV) counts to defect contrast, contrast-to-noise (CNR) and wall thickness with standard reconstruction (2D-AWOSEM), PSF, ED-gated PSF, and CMF-PSF were compared.ResultsThe measured wall thickness was 18.9 ± 5.2 mm for 2D-AWOSEM, 16.6 ± 4.5 mm for PSF, and 13.8 ± 3.9 mm for CMF-PSF reconstructed images (all P < .05). The CMF-PSF myocardium to blood and maximum LV counts to defect contrasts (5.7 ± 2.7, 10.0 ± 5.7) were higher than for 2D-AWOSEM (3.5 ± 1.4, 6.5 ± 3.1) and for PSF (3.9 ± 1.7, 7.7 ± 3.7) (CMF vs all other, P < .05). The CNR for CMF-PSF (26.3 ± 17.5) was comparable to PSF (29.1 ± 18.3), but higher than for ED-gated dataset (13.7 ± 8.8, P < .05).ConclusionCombined CMF-PSF reconstruction increased myocardium to blood contrast, maximum LV counts to defect contrast and maintained equivalent noise when compared to static summed 2D-AWOSEM and PSF reconstruction
Tilt-over mode in a precessing triaxial ellipsoid
The tilt-over mode in a precessing triaxial ellipsoid is studied
theoretically and numerically. Inviscid and viscous analytical models
previously developed for the spheroidal geometry by Poincar\'e [Bull. Astr. 27,
321 (1910)] and Busse [J. Fluid Mech., 33, 739 (1968)] are extended to this
more complex geometry, which corresponds to a tidally deformed spinning
astrophysical body. As confirmed by three-dimensional numerical simulations,
the proposed analytical model provides an accurate description of the
stationary flow in an arbitrary triaxial ellipsoid, until the appearance at
more vigorous forcing of time dependent flows driven by tidal and/or
precessional instabilities.Comment: http://link.aip.org/link/doi/10.1063/1.350435
Three-dimensional instability during vortex merging
4 p.The interaction of two parallel vortices of equal circulation is observed experimentally. For low Reynolds numbers (), the vortices remain two-dimensional and merge into a single one, when their time-dependent core size exceeds approximately 30\% of the vortex separation distance. At higher , a three-dimensional instability is discovered, showing the characteristics of an elliptic instability of the vortex cores. The instability rapidly generates small-scale turbulent motion, which initiates merging for smaller core sizes and produces a bigger final vortex than for laminar 2D flow
The Carlina-type diluted telescope: Stellar fringes on Deneb
Context. The performance of interferometers has largely been increased over
the last ten years. But the number of observable objects is still limited due
to the low sensitivity and imaging capability of the current facilities.
Studies have been done to propose a new generation of interferometers. Aims.
The Carlina concept studied at the Haute-Provence Observatory consists in an
optical interferometer configured as a diluted version of the Arecibo radio
telescope: above the diluted primary mirror made of fixed co-spherical
segments, a helium balloon or cables suspended between two mountains and/or
pylons, carries a gondola containing the focal optics. This concept does not
require delay lines. Methods. Since 2003, we have been building a technical
demonstrator of this diluted telescope. The main goals of this project were to
find the opto-mechanical solutions to stabilize the optics attached under
cables at several tens of meters above the ground, and to characterize this
diluted telescope under real conditions. In 2012, we have obtained metrology
fringes, and co-spherized the primary mirrors within one micron accuracy. In
2013, we have tested the whole optical train: servo loop, metrology, and the
focal gondola. Results. We obtained stellar fringes on Deneb in September 2013.
In this paper, we present the characteristics of these observations: quality of
the guiding, S /N reached, and possible improvements for a future system.
Conclusions. It is an important step that demonstrates the feasibility of
building a diluted telescope using cables strained between cliffs or pylons.
Carlina, like the MMT or LBT, could be one of the first members of a new class
of telescopes named Large Diluted Telescopes. Its optical architecture has many
advantages for future projects: Planet Formation Imager, Post-ELTs,
Interferometer in space.Comment: 8 pages, 7 figures, Astronomy & Astrophysic
Sanitary housing conditions modify the performance and behavioural response of weaned pigs to feed- and housing-related stressors
Interaction of vortices in viscous planar flows
We consider the inviscid limit for the two-dimensional incompressible
Navier-Stokes equation in the particular case where the initial flow is a
finite collection of point vortices. We suppose that the initial positions and
the circulations of the vortices do not depend on the viscosity parameter \nu,
and we choose a time T > 0 such that the Helmholtz-Kirchhoff point vortex
system is well-posed on the interval [0,T]. Under these assumptions, we prove
that the solution of the Navier-Stokes equation converges, as \nu -> 0, to a
superposition of Lamb-Oseen vortices whose centers evolve according to a
viscous regularization of the point vortex system. Convergence holds uniformly
in time, in a strong topology which allows to give an accurate description of
the asymptotic profile of each individual vortex. In particular, we compute to
leading order the deformations of the vortices due to mutual interactions. This
allows to estimate the self-interactions, which play an important role in the
convergence proof.Comment: 39 pages, 1 figur
Formation of interfacial molybdenum carbide for DLC lubricated by MoDTC: Origin of wear mechanism
A large amount of research has been devoted to the effect of molybdenum dithiocarbamate (MoDTC) additives on the lubricating performances of carbon-based coatings, showing that a high wear rate is produced when the MoDTC is blended with the base oil. However, the mechanisms leading to the coating removal are not fully understood yet.
In this work, the friction and wear performances of an amorphous hydrogenated DLC coating doped with silicon and oxygen have been analysed when lubricated by MoDTC-containing oils. Tribological experiments have been conducted with DLC/steel and DLC/DLC contacts under boundary lubrication conditions using a ball-on-flat tribometer. To understand the wear mechanism, the chemical composition of the tribofilm formed on the steel ball counterpart was investigated by X-ray Photoelectron Spectroscopy (XPS). Transmission Electron Microscopy (TEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDX). A new DLC wear model has been proposed and validated
Mesoscopic organization reveals the constraints governing C. elegans nervous system
One of the biggest challenges in biology is to understand how activity at the
cellular level of neurons, as a result of their mutual interactions, leads to
the observed behavior of an organism responding to a variety of environmental
stimuli. Investigating the intermediate or mesoscopic level of organization in
the nervous system is a vital step towards understanding how the integration of
micro-level dynamics results in macro-level functioning. In this paper, we have
considered the somatic nervous system of the nematode Caenorhabditis elegans,
for which the entire neuronal connectivity diagram is known. We focus on the
organization of the system into modules, i.e., neuronal groups having
relatively higher connection density compared to that of the overall network.
We show that this mesoscopic feature cannot be explained exclusively in terms
of considerations, such as optimizing for resource constraints (viz., total
wiring cost) and communication efficiency (i.e., network path length).
Comparison with other complex networks designed for efficient transport (of
signals or resources) implies that neuronal networks form a distinct class.
This suggests that the principal function of the network, viz., processing of
sensory information resulting in appropriate motor response, may be playing a
vital role in determining the connection topology. Using modular spectral
analysis, we make explicit the intimate relation between function and structure
in the nervous system. This is further brought out by identifying functionally
critical neurons purely on the basis of patterns of intra- and inter-modular
connections. Our study reveals how the design of the nervous system reflects
several constraints, including its key functional role as a processor of
information.Comment: Published version, Minor modifications, 16 pages, 9 figure
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