333 research outputs found
Transformations de programmes pertinentes pour la sécurité du logiciel
Avec le développement des outils d'analyse tels que les décompilateurs ou les désassembleurs, la nécessité de protéger les codes contre le risque d'attaque de rétro-ingénierie est apparue. Un moyen efficace de prévention est d'obfusquer le code source, assembleur ou binaire. Un obfuscateur de code est une application qui convertit un programme afin de le rendre moins compréhensible et donc plus difficile à rétro-concevoir. Pour l'utilisateur final, l'application d'origine et l'application obfusquée doivent sembler identiques, le comportement général ne doit pas être altéré. Le principe d'obfuscation repose sur des transformations de code, quelques-unes d'entre elles sont étudiées, leur efficacité et leur coût d'exécution sont évalués dans ce rapport
Snapshot coronagraphy with an interferometer in space
Diluted arrays of many optical apertures will be able to provide h
igh-resolution snapshot images if the beams are combined according to the
densified-pupil scheme. We show that the same principle can also provide
coronagraphic images, for detecting faint sources near a bright unresolved one.
Recent refinements of coronagraphic techniques, i.e. the use of a phase mask,
active apodization and dark-speckle analysis, are also applicable for enhanced
contrast. Implemented in the form of a proposed 50-500m Exo-Earth Discoverer
array in space, the principle can serve to detect Earth-like exo-planets in the
infra-red. It can also provide images of faint nebulosity near stars, active
galactic nuclei and quasars. Calculations indicate that exo-planets are
detectable amidst the zodiacal and exo-zodiacal emission faster than with a
Bracewell array of equivalent area, a consequence of the spatial selectivity in
the image.Comment: 23 pages, 10 figures, to appear in Icaru
Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes
Less than 3% of the known exoplanets were directly imaged for two main
reasons. They are angularly very close to their parent star, which is several
magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated
instrumentation with large telescopes and accurate wavefront control devices
for high-angular resolution and coronagraphs for attenuating the stellar light.
Coronagraphs are usually chromatic and they cannot perform high-contrast
imaging over a wide spectral bandwidth. That chromaticity will be critical for
future instruments. Enlarging the coronagraph spectral range is a challenge for
future exoplanet imaging instruments on both space-based and ground-based
telescopes. We propose the multi-stage four-quadrant phase mask that associates
several monochromatic four-quadrant phase mask coronagraphs in series.
Monochromatic device performance has already been demonstrated and the
manufacturing procedures are well-under control since their development for
previous instruments on VLT and JWST. The multi-stage implementation simplicity
is thus appealing. We present the instrument principle and we describe the
laboratory performance for large spectral bandwidths and for both pupil shapes
for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The
multi-stage four-quadrant phase mask reduces the stellar flux over a wide
spectral range (30%) and it is a very good candidate to be associated with a
spectrometer for future exoplanet imaging instruments in ground- and
space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&
Evidence for two spatially separated UV continuum emitting regions in the Cloverleaf broad absorption line quasar
Testing the standard Shakura-Sunyaev model of accretion is a challenging task
because the central region of quasars where accretion takes place is unresolved
with telescopes. The analysis of microlensing in gravitationally lensed quasars
is one of the few techniques that can test this model, yielding to the
measurement of the size and of temperature profile of the accretion disc. We
present spectroscopic observations of the gravitationally lensed broad
absorption line quasar H1413+117, which reveal partial microlensing of the
continuum emission that appears to originate from two separated regions: a
microlensed region, corresponding to the compact accretion disc; and a
non-microlensed region, more extended and contributing to at least 30\% of the
total UV-continuum flux. Because this extended continuum is occulted by the
broad absorption line clouds, it is not associated with the host galaxy, but
rather with light scattered in the neighbourhood of the central engine. We
measure the amplitude of microlensing of the compact continuum over the
rest-frame wavelength range 1000-7000 \AA. Following a Bayesian scheme, we
confront our measurements to microlensing simulations of an accretion disc with
a temperature varying as . We find a most likely source
half-light radius of cm (i.e., 0.002\,pc) at
0.18\,m, and a most-likely index of . The standard disc
() model is not ruled out by our data, and is found within the 95\%
confidence interval associated with our measurements. We demonstrate that, for
H1413+117, the existence of an extended continuum in addition to the disc
emission only has a small impact on the inferred disc parameters, and is
unlikely to solve the tension between the microlensing source size and standard
disc sizes, as previously reported in the literature.Comment: Accepted for publication in Astronomy and Astrophysics. 12 pages.
Minor changes w.r.t. v1 (language editing, Fig. 5-6
Taming the degeneration of Bessel beams at anisotropic-isotropic interface: toward 3D control of confined vortical waves
Despite their self-reconstruction properties in heterogeneous media, Bessel
beams are known to degenerate when they are refracted from an isotropic to an
anisotropic medium. In this paper, we study the converse situation wherein an
anisotropic Bessel beam is refracted into an isotropic medium. It is shown that
these anisotropic Bessel beams also degenerate, leading to confined vortical
waves that may serve as localized particle trap for acoustical tweezers. The
linear nature of this degeneration allows the 3D control of this trap position
by wavefront correction. Theory is confronted to experiments performed in the
field of acoustics. A swirling surface acoustic wave is synthesized at the
surface of a piezoelectric crystal by a MEMS integrated system and radiated
inside a miniature liquid vessel. The wavefront correction is operated with
inverse filter technique. This work opens perspectives for contactless on-chip
manipulation devices
Subwavelength gratings for phase mask coronagraphy: the 4QZOG and AGPM coronagraphs
We present two new phase mask coronagraphs implemented with subwavelength diffractive optical elements. The first one is an evolution of the four quadrant phase mask coronagraph (FQPM), which resolves the π phase shift chromaticity issue: the four quadrant zeroth order grating (4QZOG). The second one is a totally new design consisting of an optical
vortex induced by a space-variant grating: the annular groove phase mask (AGPM) coronagraph is fully symmetric and free from any "shaded zones". The potential performances of the 4QZOG and AGPM coronagraph are very good, ensuring, for instance, a theoretical contrast of 1.4 x 10^(-7) at 3λ/D over the whole K band. These coronagraphs could be used alone on single-pupil telescopes either in space or on the ground (with an adaptive optics system) to detect exoplanets
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