2,539 research outputs found
Apodized phase mask coronagraphs for arbitrary apertures. II. Comprehensive review of solutions for the vortex coronagraph
With a clear circular aperture, the vortex coronagraph perfectly cancels an
on-axis point source and offers a 0.9 or 1.75 lambda/D inner working angle for
topological charge 2 or 4, respectively. Current and near-future large
telescopes are on-axis, however, and the diffraction effects of the central
obscuration, and the secondary supports are strong enough to prevent the
detection of companions 1e-3 - 1e-5 as bright as, or fainter than, their host
star. Recent advances show that a ring apodizer can restore the performance of
this coronagraph by compensating for the diffraction effects of a circular
central obscuration in a 1D modeling of the pupil. We extend this work and
optimize apodizers for arbitrary apertures in 2D in order to tackle the
diffraction effects of the spiders and other noncircular artefacts in the
pupil. We use a numerical optimization scheme to compute hybrid coronagraph
designs that combine the advantages of the vortex coronagraph (small in IWA)
and of shaped pupils coronagraphs (robustness to central obscuration and pupil
asymmetric structures). We maximize the apodizer transmission, while
constraints are set on the extremum values of the electric field that is
computed in chosen regions of the Lyot plane through closed form expressions.
Optimal apodizers are computed for topological charges 2 and 4 vortex
coronagraphs and for telescope apertures with 10-30% central obscurations and
0-1% thick spiders. We characterize the impacts of the obscuration ratio and
the thickness of the spiders on the throughput and the IWA for the two
topological charges.Comment: 23 pages, 12 figures, 2 table
Apodized vortex coronagraph designs for segmented aperture telescopes
Current state-of-the-art high contrast imaging instruments take advantage of
a number of elegant coronagraph designs to suppress starlight and image nearby
faint objects, such as exoplanets and circumstellar disks. The ideal
performance and complexity of the optical systems depends strongly on the shape
of the telescope aperture. Unfortunately, large primary mirrors tend to be
segmented and have various obstructions, which limit the performance of most
conventional coronagraph designs. We present a new family of vortex
coronagraphs with numerically-optimized gray-scale apodizers that provide the
sensitivity needed to directly image faint exoplanets with large, segmented
aperture telescopes, including the Thirty Meter Telescope (TMT) as well as
potential next-generation space telescopes.Comment: To appear in SPIE proceedings vol. 991
Ring-apodized vortex coronagraphs for obscured telescopes. I. Transmissive ring apodizers
The vortex coronagraph (VC) is a new generation small inner working angle
(IWA) coronagraph currently offered on various 8-meter class ground-based
telescopes. On these observing platforms, the current level of performance is
not limited by the intrinsic properties of actual vortex devices, but by
wavefront control residuals and incoherent background (e.g. thermal emission of
the sky) or the light diffracted by the imprint of the secondary mirror and
support structures on the telescope pupil. In the particular case of unfriendly
apertures (mainly large central obscuration) when very high contrast is needed
(e.g. direct imaging of older exoplanets with extremely large telescopes or
space- based coronagraphs), a simple VC, as most coronagraphs, can not deliver
its nominal performance because of the contamination due to the diffraction
from the obscured part of the pupil. Here we propose a novel yet simple concept
that circumvents this problem. We combine a vortex phase mask in the image
plane of a high-contrast instrument with a single pupil-based amplitude ring
apodizer, tailor designed to exploit the unique convolution properties of the
VC at the Lyot-stop plane. We show that such a ring-apodized vortex coronagraph
(RAVC) restores the perfect attenuation property of the VC regardless of the
size of the central obscuration, and for any (even) topological charge of the
vortex. More importantly the RAVC maintains the IWA and conserves a fairly high
throughput, which are signature properties of the VC.Comment: 10 pages, 6 figure
Kinetics of lime/bentonite pozzolanic reactions at 20 and 50 °C: Batch tests and modeling
International audienceThe effects of duration (1-100 days) and temperature (20 and 50 °C) were assessed from batch tests for Ca-bentonite mixed with 10 wt.% lime. The pozzolanic processes were monitored over time by 29Si NMR (Cement Concr. Res. 42, 2012), TGA-DTA, XRD and chemical analysis. Modeling considered kinetics and thermodynamics of mineralogical transformations and cation exchange. Kinetic laws were dependent on pH and temperature (Arrhenius energy). Lime hydration occurs within hours, modifying the bentonite exchangeable population and increasing the pH. These alkaline conditions initiate the pozzolanic reactions in a second stage. The rate-limiting step is the dissolution kinetics of the bentonite minerals, i.e. a relatively fast and total consumption of cristobalite in parallel to a long-term slower dissolution of montmorillonite. First C-S-H and then C-A-S-H are formed consequently. Temperature speeds up the pozzolanic reaction kinetics by a factor 5 from 20 to 50 °C, corresponding to an apparent activation energy of 40-50 kJ/mol
The European Commission’s Activation of Article 7: Better Late than Never?
On Wednesday, the European Commission reacted to the continuing deterioration of the rule of law situation in Poland. The remaining question, of course, is why this argument has been used in the context of 7(1) as opposed of 7(2) given that the situation on the ground in Poland is clearly – in the view of the Commission, the Venice Commission and countless other actors – one of clear and persistent breach of values, as opposed to a threat thereof. The explanation might lie beyond the simple difficulty of the procedural requirements related to the sanctioning stage
Caractérisation pédologique des berges sensibles: problématique de l'accès du bétail aux cours d'eau
VLT/SPHERE robust astrometry of the HR8799 planets at milliarcsecond-level accuracy Orbital architecture analysis with PyAstrOFit
HR8799 is orbited by at least four giant planets, making it a prime target
for the recently commissioned Spectro-Polarimetric High-contrast Exoplanet
REsearch (VLT/SPHERE). As such, it was observed on five consecutive nights
during the SPHERE science verification in December 2014. We aim to take full
advantage of the SPHERE capabilities to derive accurate astrometric
measurements based on H-band images acquired with the Infra-Red Dual-band
Imaging and Spectroscopy (IRDIS) subsystem, and to explore the ultimate
astrometric performance of SPHERE in this observing mode. We also aim to
present a detailed analysis of the orbital parameters for the four planets. We
report the astrometric positions for epoch 2014.93 with an accuracy down to 2.0
mas, mainly limited by the astrometric calibration of IRDIS. For each planet,
we derive the posterior probability density functions for the six Keplerian
elements and identify sets of highly probable orbits. For planet d, there is
clear evidence for nonzero eccentricity (), without completely
excluding solutions with smaller eccentricities. The three other planets are
consistent with circular orbits, although their probability distributions
spread beyond , and show a peak at for planet e. The
four planets have consistent inclinations of about with respect to the
sky plane, but the confidence intervals for the longitude of ascending node are
disjoint for planets b and c, and we find tentative evidence for
non-coplanarity between planets b and c at the level.Comment: 23 pages, 14 figure
Resistive switching induced by electronic avalanche breakdown in GaTaSeTe narrow gap Mott Insulators
Mott transitions induced by strong electric fields are receiving a growing
interest. Recent theoretical proposals have focused on the Zener dielectric
breakdown in Mott insulators, however experimental studies are still too scarce
to conclude about the mechanism. Here we report a study of the dielectric
breakdown in the narrow gap Mott insulators GaTaSeTe. We find
that the I-V characteristics and the magnitude of the threshold electric field
(E) do not correspond to a Zener breakdown, but rather to an avalanche
breakdown. E increases as a power law of the Mott Hubbard gap (E),
in surprising agreement with the universal law E E
reported for avalanche breakdown in semiconductors. However, the delay time for
the avalanche that we observe in Mott insulators is over three orders of
magnitude longer than in conventional semiconductors. Our results suggest that
the electric field induces local insulator-to-metal Mott transitions that
create conductive domains which grow to form filamentary paths across the
sample
Development of a new type of high pressure calorimetric cell, mechanically agitated and equipped with a dynamic pressure control system: Application to the characterization of gas hydrates
A novel prototype of calorimetric cell has been developed allowing experiments under pressure with an in situ agitation system and a dynamic control of the pressure inside the cell. The use of such a system opens a wide range of potential practical applications for determining properties of complex fluids in both pressurized and agitated conditions. The technical details of this prototype and its calibration procedure are described, and an application devoted to the determination of phase equilibrium and phase change enthalpy of gas hydrates is presented. Our results, obtained with a good precision and reproducibility, were found in fairly good agreement with those found in literature, illustrate the various interests to use this novel apparatu
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