6,089 research outputs found
海事産業の国際化における国家脆弱性評価に関する研究
東京海洋大学博士学位論文 平成26年度(2014) 応用環境システム学専攻 課程博士 甲第361号指導教員 : 渡邉豊全文公表年月日: 2015-06-26東京海洋大学201
Direct imaging with highly diluted apertures. II. Properties of the point spread function of a hypertelescope
In the future, optical stellar interferometers will provide true images
thanks to larger number of telescopes and to advanced cophasing subsystems.
These conditions are required to have sufficient resolution elements (resel) in
the image and to provide direct images in the hypertelescope mode. It has
already been shown that hypertelescopes provide snapshot images with a
significant gain in sensitivity without inducing any loss of the useful field
of view for direct imaging applications. This paper aims at studying the
properties of the point spread functions of future large arrays using the
hypertelescope mode. Numerical simulations have been performed and criteria
have been defined to study the image properties. It is shown that the choice of
the configuration of the array is a trade-off between the resolution, the halo
level and the field of view. A regular pattern of the array of telescopes
optimizes the image quality (low halo level and maximum encircled energy in the
central peak), but decreases the useful field of view. Moreover, a
non-redundant array is less sensitive to the space aliasing effect than a
redundant array.Comment: 10 pages paper with referee in A&
Apodized-pupil Lyot coronagraphs: multistage designs for extremely large telescopes
Earlier apodized-pupil Lyot coronagraphs (APLC) have been studied and
developed to enable high-contrast imaging for exoplanet detection and
characterization with present-day ground-based telescopes. With the current
interest in the development of the next generation of telescopes, the future
extremely large telescopes (ELTs), alternative APLC designs involving
multistage configuration appear attractive. The interest of these designs for
application to ELTs is studied. Performance and sensitivity of multistage APLC
to ELT specificities are analyzed and discussed, taking into account several
ineluctable coronagraphic telescope error sources by means of numerical
simulations. Additionally, a first laboratory experiment with a two-stages-APLC
in the near-infrared (H-band) is presented to further support the numerical
treatment. Multistage configurations are found to be inappropriate to ELTs. The
theoretical gain offered by a multistage design over the classical single-stage
APLC is largely compromised by the presence of inherent error sources occurring
in a coronagraphic telescope, and in particular in ELTs. The APLC remains an
attractive solution for ELTs, but rather in its conventional single-stage
configuration.Comment: A&A accepte
Optimization of starshades: focal plane versus pupil plane
We search for the best possible transmission for an external occulter
coronagraph that is dedicated to the direct observation of terrestrial
exoplanets. We show that better observation conditions are obtained when the
flux in the focal plane is minimized in the zone in which the exoplanet is
observed, instead of the total flux received by the telescope. We describe the
transmission of the occulter as a sum of basis functions. For each element of
the basis, we numerically computed the Fresnel diffraction at the aperture of
the telescope and the complex amplitude at its focus. The basis functions are
circular disks that are linearly apodized over a few centimeters (truncated
cones). We complemented the numerical calculation of the Fresnel diffraction
for these functions by a comparison with pure circular discs (cylinder) for
which an analytical expression, based on a decomposition in Lommel series, is
available. The technique of deriving the optimal transmission for a given
spectral bandwidth is a classical regularized quadratic minimization of
intensities, but linear optimizations can be used as well. Minimizing the
integrated intensity on the aperture of the telescope or for selected regions
of the focal plane leads to slightly different transmissions for the occulter.
For the focal plane optimization, the resulting residual intensity is
concentrated behind the geometrical image of the occulter, in a blind region
for the observation of an exoplanet, and the level of background residual
starlight becomes very low outside this image. Finally, we provide a tolerance
analysis for the alignment of the occulter to the telescope which also favors
the focal plane optimization.This means that telescope offsets of a few
decimeters do not strongly reduce the efficiency of the occulter
Phase-Induced Amplitude Apodization of Telescope Pupils for Extrasolar Terrestrial Planet Imaging
In this paper, an alternative to the classical pupil apodization techniques
(use of an amplitude pupil mask) is proposed. It is shown that an apodized
pupil suitable for imaging of Extrasolar planets can be obtained by reflection
of an unapodized flat wavefront on 2 mirrors. By carefully choosing the shape
of these 2 mirrors, it is possible to obtain a contrast better than 10^{9} at a
distance smaller than 2 \lambda/d from the optical axis. Because this technique
preserves both the angular resolution and light gathering capabilities of the
unapodized pupil, it allows efficient detection of terrestrial extrasolar
planets with a 1.5m telescope in the visible.Comment: 9 pages, 9 figures, Accepted for publication in A&A. Postscript file
with full-resolution figures can be found at
http://www.naoj.org/staff/guyon/publications/PIAA.p
Dependability in wireless networks: can we rely on WiFi?
WiFi - short for "wireless fidelity" - is the commercial name for the 802.11 products that have flooded the corporate wireless local area network (WLAN) market and are becoming rapidly ingrained in our daily lives via public hotspots and digital home networks. Authentication and confidentiality are crucial issues for corporate WiFi use, but privacy and availability tend to dominate pervasive usage. However, because a technology's dependability requirements are proportional to its pervasiveness, newer applications mandate a deeper understanding of how much we can rely on WiFi and its security promises. In this article, we present an overview of WiFi vulnerabilities and investigate their proximate and ultimate origins. The intended goal is to provide a foundation to discuss WiFi dependability and its impact on current and future usage scenarios. Although a wireless network's overall security depends on the network stack to the application layer, this article focuses on specific vulnerabilities at the physical (PHY) and data (MAC) layers of 802.11 network
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