3,526 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
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
Mott transition in Cr-doped V2O3 studied by ultrafast reflectivity: electron correlation effects on the transient response
The ultrafast response of the prototype Mott-Hubbard system (V1-xCrx)2O3 was
systematically studied with fs pump-probe reflectivity, allowing us to clearly
identify the effects of the metal-insulator transition on the transient
response. The isostructural nature of the phase transition in this material
made it possible to follow across the phase diagram the behaviour of the
detected coherent acoustic wave, whose average value and lifetime depend on the
thermodynamic phase and on the correlated electron density of states. It is
also shown how coherent lattice oscillations can play an important role in some
changes affecting the ultrafast electronic peak relaxation at the phase
transition, changes which should not be mistakenly attributed to genuine
electronic effects. These results clearly show that a thorough understanding of
the ultrafast response of the material over several tenths of ps is necessary
to correctly interpret its sub-ps excitation and relaxation regime, and appear
to be of general interest also for other strongly correlated materials.Comment: 6 pages, 3 figures. Europhysics Letters (in press
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 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
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
CEA Bolometer Arrays: the First Year in Space
The CEA/LETI and CEA/SAp started the development of far-infrared filled bolometer arrays for space applications
over a decade ago. The unique design of these detectors makes possible the assembling of large focal planes
comprising thousands of bolometers running at 300 mK with very low power dissipation. Ten arrays of 16x16
pixels were thoroughly tested on the ground, and integrated in the Herschel/PACS instrument before launch in
May 2009. These detectors have been successfully commissioned and are now operating in their nominal environment
at the second Lagrangian point of the Earth-Sun system. In this paper we briefly explain the functioning
of CEA bolometer arrays, and we present the properties of the detectors focusing on their noise characteristics,
the effect of cosmic rays on the signal, the repeatability of the measurements, and the stability of the system
Delayed Photoionization Feedback in a Super Star Cluster in SBS0335-052E
SBS0335-052 is a well studied Blue Compact Dwarf galaxy with one of the
lowest metallicities of any known galaxy. It also contains 6 previously
identified Super Star Clusters. We combine archival HST NICMOS images in the Pa
alpha line and the 1.6 micron continuum of the eastern component, SBS0335-052E,
with other space and ground based data to perform a multi-wavelength analysis
of the super star clusters. We concentrate on the southern most clusters,
designated S1 and S2, which appear to be the youngest clusters and are the
strongest emitters of Pa alpha, radio, and x-ray flux. Our analysis leads to a
possible model for S1 and perhaps S2 as a cluster of very young, massive stars
with strong stellar winds. The wind density can be high enough to absorb the
majority of ionizing photons within less than 1000 AU of the stars, creating
very compact HII regions that emit optically thick radiation at radio
wavelengths. These winds would then effectively quench the photoionizing flux
very close to the stars. This can delay the onset of negative feedback by
photoionization and photodissociation on star formation in the clusters. This
is significant since SBS0335-052E resembles the conditions that were probably
common for high redshift star formation in galaxies near the epoch of
reionization.Comment: Accepted for publication in the Astrophysical Journa
The detection of post-monsoon tropospheric ozone variability over south Asia using IASI data
The ozone (O<sub>3</sub>) variability over south Asia during the 2008 post-monsoon season has been assessed using measurements from the MetOP-A/IASI instrument and O<sub>3</sub> profiles retrieved with the SOftware for a Fast Retrieval of IASI Data (SOFRID). The information content study and error analyses carried out in this paper show that IASI Level 1 data can be used to retrieve tropospheric O<sub>3</sub> columns (TOC, surface-225 hPa) and UTLS columns (225–70 hPa) with errors smaller than 20%. Validation with global radiosonde O<sub>3</sub> profiles obtained during a period of 6 months show the excellent agreement between IASI and radiosonde for the UTLS with correlation coefficient <i>R</i> > 0.91 and good agreement in the troposphere with correlation coefficient <i>R</i> > 0.74. For both the UTLS and the troposphere Relative Standard Deviations (RSD) are lower than 23%. Comparison with in-situ measurements from the MOZAIC program around Hyderabad demonstrates that IASI is able to capture the TOC inter and intra-seasonal variability in central India. Nevertheless, the agreement is mitigated by the fact that the smoothing of the true O<sub>3</sub> profiles by the retrieval results in a reduction of the TOC variability detected by IASI relative to the variability observed by in situ instruments. The post-monsoon temporal variability of the vertical profile of O<sub>3</sub> around Hyderabad has been investigated with MOZAIC observations. These observations from airborne instruments show that tropospheric O<sub>3</sub> is steadily elevated during most of the studied period with the exception of two sharp drops following the crossing of tropical storms over India. Lagrangian simulations with the FLEXPART model indicate that elevated O<sub>3</sub> concentrations in the middle troposphere near Hyderabad are associated with the transport of UTLS air-masses that have followed the Subtropical Westerly Jet (SWJ) and subsided over northern India together with boundary layer polluted air-masses transported from the Indo-gangetic plain by the north-easterly trades. Low O<sub>3</sub> concentrations result from the uplift and westward transport of pristine air-masses from the marine boundary layer of the Bay of Bengal by tropical storms. In order to extend the analysis of tropospheric O<sub>3</sub> variability to the whole of south Asia, we have used IASI-SOFRID O<sub>3</sub> data. We show that IASI O<sub>3</sub> data around Hyderabad were able to capture the fast variability revealed by MOZAIC. Furthermore, their spatio-temporal coverage demonstrates that the behaviour of tropospheric O<sub>3</sub> observed near Hyderabad extended over most of central and south India and part of the Bay of Bengal. This result highlights the ability of the IASI sensor to capture fast changes in chemical composition related to dramatic tropical weather conditions
- …