13,619 research outputs found
The calibration of interferometric visibilities obtained with single-mode optical interferometers. Computation of error bars and correlations
I present in this paper a method to calibrate data obtained from optical and
infrared interferometers. I show that correlated noises and errors need to be
taken into account for a very good estimate of individual error bars but also
when model fitting the data to derive meaningful model parameters whose
accuracies are not overestimated. It is also shown that under conditions of
high correlated noise, faint structures of the source can be detected. This
point is important to define strategies of calibration for difficult programs
such as exoplanet detection. The limits of validity of the assumptions on the
noise statistics are discussed
Subtracting the photon noise bias from single-mode optical interferometer visibilities
I present in this paper a method to subtract the bias due to source photon
noise from visibilities measured with a single-mode optical interferometer.
Properties of the processed noise are demonstrated and examples of subtraction
on real data are presented.Comment: To appear in Astronomy & Astrophysic
Pupil remapping for high contrast astronomy: results from an optical testbed
The direct imaging and characterization of Earth-like planets is among the
most sought-after prizes in contemporary astrophysics, however current optical
instrumentation delivers insufficient dynamic range to overcome the vast
contrast differential between the planet and its host star. New opportunities
are offered by coherent single mode fibers, whose technological development has
been motivated by the needs of the telecom industry in the near infrared. This
paper presents a new vision for an instrument using coherent waveguides to
remap the pupil geometry of the telescope. It would (i) inject the full pupil
of the telescope into an array of single mode fibers, (ii) rearrange the pupil
so fringes can be accurately measured, and (iii) permit image reconstruction so
that atmospheric blurring can be totally removed. Here we present a laboratory
experiment whose goal was to validate the theoretical concepts underpinning our
proposed method. We successfully confirmed that we can retrieve the image of a
simulated astrophysical object (in this case a binary star) though a pupil
remapping instrument using single mode fibers.Comment: Accepted in Optics Expres
High dynamic range imaging with a single-mode pupil remapping system : a self-calibration algorithm for redundant interferometric arrays
The correction of the influence of phase corrugation in the pupil plane is a
fundamental issue in achieving high dynamic range imaging. In this paper, we
investigate an instrumental setup which consists in applying interferometric
techniques on a single telescope, by filtering and dividing the pupil with an
array of single-mode fibers. We developed a new algorithm, which makes use of
the fact that we have a redundant interferometric array, to completely
disentangle the astronomical object from the atmospheric perturbations (phase
and scintillation). This self-calibrating algorithm can also be applied to any
- diluted or not - redundant interferometric setup. On an 8 meter telescope
observing at a wavelength of 630 nm, our simulations show that a single mode
pupil remapping system could achieve, at a few resolution elements from the
central star, a raw dynamic range up to 10^6; depending on the brightness of
the source. The self calibration algorithm proved to be very efficient,
allowing image reconstruction of faint sources (mag = 15) even though the
signal-to-noise ratio of individual spatial frequencies are of the order of
0.1. We finally note that the instrument could be more sensitive by combining
this setup with an adaptive optics system. The dynamic range would however be
limited by the noise of the small, high frequency, displacements of the
deformable mirror.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Functional Methods and Effective Potentials for Nonlinear Composites
A formulation of variational principles in terms of functional integrals is
proposed for any type of local plastic potentials. The minimization problem is
reduced to the computation of a path integral. This integral can be used as a
starting point for different approximations. As a first application, it is
shown how to compute to second-order the weak-disorder perturbative expansion
of the effective potentials in random composite. The three-dimensional results
of Suquet and Ponte-Casta\~neda (1993) for the plastic dissipation potential
with uniform applied tractions are retrieved and extended to any space
dimension, taking correlations into account. In addition, the viscoplastic
potential is also computed for uniform strain rates.Comment: 20 pages, accepted for publication in JMP
Updated results on prototype chalcogenide fibers for 10-um wavefront spatial filtering
The detection of terrestrial planets by Darwin/TPF missions will require
extremely high quality wavefronts. Single-mode fibers have proven to be
powerful beam cleaning components in the near-infrared, but are currently not
available in the mid-infrared where they would be critically needed for
Darwin/TPF. In this paper, we present updated measurements on the prototype
chalcogenide fibers we are developing for the purpose of mid-infrared spatial
filtering. We demonstrate the guiding property of our 3rd generation component
and we characterize its filtering performances on a 4 mm length: the far-field
radiation pattern matches a Gaussian profile at the level of 3% rms and 13%
pk-pk.Comment: 4 pages, 5 figures, to appear in the proceedings of the conference
"Toward Other Earths, Darwin/TPF and the search for extrasolar terrestrial
planets", held in Heidelberg, Germany, 22-25 April 2003, ESA SP-53
Sound velocity and absorption measurements under high pressure using picosecond ultrasonics in diamond anvil cell. Application to the stability study of AlPdMn
We report an innovative high pressure method combining the diamond anvil cell
device with the technique of picosecond ultrasonics. Such an approach allows to
accurately measure sound velocity and attenuation of solids and liquids under
pressure of tens of GPa, overcoming all the drawbacks of traditional
techniques. The power of this new experimental technique is demonstrated in
studies of lattice dynamics, stability domain and relaxation process in a
metallic sample, a perfect single-grain AlPdMn quasicrystal, and rare gas, neon
and argon. Application to the study of defect-induced lattice stability in
AlPdMn up to 30 GPa is proposed. The present work has potential for application
in areas ranging from fundamental problems in physics of solid and liquid
state, which in turn could be beneficial for various other scientific fields as
Earth and planetary science or material research
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