4,554 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
Normal Nearby Galaxies
Following on from IRAS, ISO has provided a huge advancement in our knowledge
of the phenomenology of the infrared (IR) emission of normal galaxies and the
underlying physical processes. Highlights include: the discovery of an extended
cold dust emission component, present in all types of gas-rich galaxies and
carrying the bulk of the dust luminosity; the definitive characterisation of
the spectral energy distribution in the IR, revealing the channels through
which stars power the IR light; the derivation of realistic geometries for
stars and dust from ISO imaging; the discovery of cold dust associated with HI
extending beyond the optical body of galaxies; the remarkable similarity of the
near-IR (NIR)/ mid-IR (MIR) SEDs for spiral galaxies, revealing the importance
of the photo-dissociation regions in the energy budget for that wavelength
range; the importance of the emission from the central regions in shaping up
the intensity and the colour of the global MIR luminosity; the discovery of the
``hot'' NIR continuum emission component of interstellar dust; the predominance
of the diffuse cold neutral medium as the origin for the main interstellar
cooling line, [CII] 158 micron, in normal galaxies.Comment: 47 pages, 15 figures, to be published in the ISO Special Issue of
Space Science Reviews: "ISO science legacy - a compact review of ISO major
achievements", Springer 2005. See http://www.iso.vilspa.esa.es/science/SSR/
for a higher resolution version and for all papers in the volum
Variation around a Pyramid theme: optical recombination and optimal use of photons
We propose a new type of Wave Front Sensor (WFS) derived from the Pyramid WFS
(PWFS). This new WFS, called the Flattened Pyramid-WFS (FPWFS), has a reduced
Pyramid angle in order to optically overlap the four pupil images into an
unique intensity. This map is then used to derive the phase information. In
this letter this new WFS is compared to three existing WFSs, namely the PWFS,
the Modulated PWFS (MPWFS) and the Zernike WFS (ZWFS) following tests about
sensitivity, linearity range and low photon flux behavior. The FPWFS turns out
to be more linear than a modulated pyramid for the high-spatial order
aberrations but it provides an improved sensitivity compared to the
non-modulated pyramid. The noise propagation may even be as low as the ZWFS for
some given radial orders. Furthermore, the pixel arrangement being more
efficient than for the PWFS, the FPWFS seems particularly well suited for
high-contrast applications.Comment: 4 pages, 4 figures, accepted for publication in Optics Letters -
Version corrected for affiliation
Post processing of differential images for direct extrasolar planet detection from the ground
The direct imaging from the ground of extrasolar planets has become today a
major astronomical and biological focus. This kind of imaging requires
simultaneously the use of a dedicated high performance Adaptive Optics [AO]
system and a differential imaging camera in order to cancel out the flux coming
from the star. In addition, the use of sophisticated post-processing techniques
is mandatory to achieve the ultimate detection performance required. In the
framework of the SPHERE project, we present here the development of a new
technique, based on Maximum A Posteriori [MAP] approach, able to estimate
parameters of a faint companion in the vicinity of a bright star, using the
multi-wavelength images, the AO closed-loop data as well as some knowledge on
non-common path and differential aberrations. Simulation results show a 10^-5
detectivity at 5sigma for angular separation around 15lambda/D with only two
images.Comment: 12 pages, 6 figures, This paper will be published in the proceedings
of the conference Advances in Adaptive Optics (SPIE 6272), part of SPIE's
Astronomical Telescopes & Instrumentation, 24-31 May 2006, Orlando, F
Shear-induced α → γ transformation in nanoscale Fe-C composite
High-resolution transmission electron microscopy and three-dimensional atom probe observations show clearly that a reverse transformation of body-centred cubic ferrite to face-centred cubic austenite occurs during severe plastic deformation of a pearlitic steel resulting in a nanocrystalline structure, something that never occurs in conventional deformation of coarse-grained iron and steels. The driving force and the mechanisms of this reverse transformation are discussed. It is shown that nanostructure and shear stresses are essential for this process, and the results confirm molecular dynamics predictions of such transformations in nanocrystalline iron
The central region of spiral galaxies as seen by Herschel: M 81, M 99, and M 100
With appropriate spatial resolution, images of spiral galaxies in thermal infrared (~10 μm and beyond) often reveal a bright central component, distinct from the stellar bulge, superimposed on a disk with prominent spiral arms. ISO and Spitzer studies have shown that much of the scatter in the mid-infrared colors of spiral galaxies is related to changes in the relative importance of these two components, rather than to other modifications, such as the morphological type or star formation rate, that affect the properties of the galaxy as a whole. With the Herschel imaging capability from 70 to 500 μm, we revisit this two-component approach at longer wavelengths, to see if it still provides a working description of the brightness
distribution of galaxies, and to determine its implications on the interpretation of global far-infrared properties of galaxies. We quantify the luminosity of the central component by both a decomposition of the radial surface brightness profile and a direct extraction in 2D.
We find the central component contribution is variable within the three galaxies in our sample, possibly connected more directly to the presence of a bar than to the morphological type. The central component’s relative contribution is at its maximum in the mid-infrared range and drops around 160 μm to reach a constant value beyond 200 μm. The central component contains a greater fraction of hot dust than the disk component, and while the colors of the central components are scattered, colors of the disk components are more homogenous from one galaxy to the next
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
Signs of dynamical effects for Cd, Sn, Te, Xe, Ba and Sm nuclear charge radii
The experimental charge radius values along Cd, Sn, Te, Xe, Ba and Sm isotopic series have been extracted from isotope shift measurements using different methods to calibrate the electronic factor and mass shift effects. Static and dynamic charge radii have been calculated in the framework of a microscopic configuration mixing approach on the ground of Hartree–Fock–Bogoliubov solutions obtained with the D1S Gogny effective nucleon–nucleon interaction. Low-energy spectroscopic observables have also been obtained. The theoretical and experimental results are compared and discussed. It is shown that dynamical effects must be taken into account especially for γ-soft and weakly deformed nuclei
A standardised method for measuring in situ denitrification in shallow aquifers: numerical validation and measurements in riparian wetlands
A tracer test to examine in situ denitrification in shallow groundwater by a piezometer with a packer system used bromide as a tracer of dilution and acetylene (10%) to block the denitrification process at the nitrous oxide stage. During the test, dissolved oxygen, nitrate (NO3-), bromide (Br-), nitrous oxide (N2O) and dissolved organic carbon (DOC) were measured. To calibrate the experimental method, comparison with numerical simulations of the groundwater transfer were carried out, taking into account the environmental characteristics. The method was tested by measurements undertaken in different environmental conditions (geology, land use and hydrology) in two riparian wetlands. Denitrification rates measured by this method ranged from 5.7 10-6 g N-NO3-L-1 h-1 to 1.97 10-3 g N-NO3-L-1 h-1 The method is applicable in shallow aquifers with a permeability from 10-2 to 10-4m s-1
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