396 research outputs found
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&
Multi-spectral piston sensor for co-phasing giant segmented mirrors and multi-aperture interferometric arrays
This paper presents the optical design of a multi-spectral piston sensor
suitable to co-phasing giant segmented mirrors equipping the Future Extremely
Large Telescopes (ELTs). The general theory of the sensor is described in
detail and numerical simulations have been carried out, demonstrating that
direct piston and tip-tilt measurements are feasible within accuracies
respectively close to 20 nm and 10 nano-radians. Those values are compatible
with the co-phasing requirements, although the method seems to be perturbed by
uncorrected atmospheric seein
Large Faraday rotation of resonant light in a cold atomic cloud
We experimentally studied the Faraday rotation of resonant light in an
optically-thick cloud of laser-cooled rubidium atoms. Measurements yield a
large Verdet constant in the range of 200 000 degrees/T/mm and a maximal
polarization rotation of 150 degrees. A complete analysis of the polarization
state of the transmitted light was necessary to account for the role of the
probe laser's spectrum
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
Multiple imaging by gravitational waves
Gravitational waves act like lenses for the light propagating through them.
This phenomenon is described using the vector formalism employed for ordinary
gravitational lenses, which was proved to be applicable also to a
non-stationary spacetime, with the appropriate modifications. In order to have
multiple imaging, an approximate condition analogous to that for ordinary
gravitational lenses must be satisfied. Certain astrophysical sources of
gravitational waves satisfy this condition, while the gravitational wave
background, on average, does not. Multiple imaging by gravitational waves is,
in principle, possible, but the probability of observing such a phenomenon is
extremely low.Comment: 23 pages, LaTeX, no figures, to appear in Int. J. Mod. Phys.
A New Channel for the Detection of Planetary Systems Through Microlensing: I. Isolated Events Due to Planet Lenses
We propose and evaluate the feasibility of a new strategy to search for
planets via microlensing. This new strategy is designed to detect planets in
"wide" orbits, i.e., with orbital separation, greater than .
Planets in wide orbits may provide the dominant channel for the microlensing
discovery of planets, particularly low-mass (e.g., Earth-mass) planets. This
paper concentrates on events in which a single planet serves as a lens, leading
to an isolated event of short duration. We point out that a distribution of
events due to lensing by stars with wide-orbit planets is necessarily
accompanied by a distribution of shorter- duration events. The fraction of
events in the latter distribution is proportional to the average value of
, where is the ratio between \pl and stellar masses. The position
of the peak or peaks also provides a measure of the mass ratios typical of
planetary systems. We study detection strategies that can optimize our ability
to discover isolated short-duration events due to lensing by planets, and find
that monitoring employing sensitive photometry is particularly useful. If
planetary systems similar to our own are common, even modest changes in
detection strategy should lead to the discovery of a few isolated events of
short duration every year. We therefore also address the issue of the
contamination due to stellar populations of any microlensing signal due to
low-mass MACHOs. We describe how, even for isolated events of short duration,
it will be possible to test the hypothesis that the lens was a planet instead
of a low-mass MACHO, if the central star of the planetary system contributes a
measurable fraction of the baseline flux.Comment: 37 pages, 6 figure. To be published in the Astrophysical Journal.
This is part one of a series of papers on microlensing by planetary systems
containing wide-orbit planets; the series represents a reorganization and
extension of astro-ph/971101
Experimental perspectives for systems based on long-range interactions
The possibility of observing phenomena peculiar to long-range interactions,
and more specifically in the so-called Quasi-Stationary State (QSS) regime is
investigated within the framework of two devices, namely the Free-Electron
Laser (FEL) and the Collective Atomic Recoil Laser (CARL). The QSS dynamics has
been mostly studied using the Hamiltonian Mean-Field (HMF) toy model,
demonstrating in particular the presence of first versus second order phase
transitions from magnetized to unmagnetized regimes in the case of HMF. Here,
we give evidence of the strong connections between the HMF model and the
dynamics of the two mentioned devices, and we discuss the perspectives to
observe some specific QSS features experimentally. In particular, a dynamical
analog of the phase transition is present in the FEL and in the CARL in its
conservative regime. Regarding the dissipative CARL, a formal link is
established with the HMF model. For both FEL and CARL, calculations are
performed with reference to existing experimental devices, namely the
FERMI@Elettra FEL under construction at Sincrotrone Trieste (Italy) and the
CARL system at LENS in Florence (Italy)
Un modèle écophysiologique de croissance des foraminifères pour améliorer les reconstructions climatiques
- …