620 research outputs found
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
Hypertelescopes: The Challenge of Direct Imaging at High Resolution
This book is a collection of 19 articles which reflect the courses given at the Collège de France/Summer school “Reconstruction d'images − Applications astrophysiques“ held in Nice and Fréjus, France, from June 18 to 22, 2012. The articles presented in this volume address emerging concepts and methods that are useful in the complex process of improving our knowledge of the celestial objects, including Earth
High-contrast Imaging from Space: Speckle Nulling in a Low Aberration Regime
High-contrast imaging from space must overcome two major noise sources to
successfully detect a terrestrial planet angularly close to its parent star:
photon noise from diffracted star light, and speckle noise from star light
scattered by instrumentally-generated wavefront perturbation. Coronagraphs
tackle only the photon noise contribution by reducing diffracted star light at
the location of a planet. Speckle noise should be addressed with
adaptative-optics systems. Following the tracks of Malbet, Yu and Shao (1995),
we develop in this paper two analytical methods for wavefront sensing and
control that aims at creating dark holes, i.e. areas of the image plane cleared
out of speckles, assuming an ideal coronagraph and small aberrations. The first
method, speckle field nulling, is a fast FFT-based algorithm that requires the
deformable-mirror influence functions to have identical shapes. The second
method, speckle energy minimization, is more general and provides the optimal
deformable mirror shape via matrix inversion. With a NxN deformable mirror, the
size of matrix to be inverted is either N^2xN^2 in the general case, or only
NxN if influence functions can be written as the tensor product of two
one-dimensional functions. Moreover, speckle energy minimization makes it
possible to trade off some of the dark hole area against an improved contrast.
For both methods, complex wavefront aberrations (amplitude and phase) are
measured using just three images taken with the science camera (no dedicated
wavefront sensing channel is used), therefore there are no non-common path
errors. We assess the theoretical performance of both methods with numerical
simulations, and find that these speckle nulling techniques should be able to
improve the contrast by several orders of magnitude.Comment: 31 pages, 8 figures, 1 table. Accepted for publication in ApJ (should
appear in February 2006
Coherent backscattering of light by cold atoms: theory meets experiment
Coherent backscattering (CBS) of quasi-resonant light by cold atoms presents
some specific features due to the internal structure of the atomic scatterers.
We present the first quantitative comparison between the experimentally
observed CBS cones and Monte-Carlo calculations which take into account the
shape of the atomic cloud as well as the internal atomic structure.Comment: 5 pages, 3 figures, to appear in Eur. Phys. Let
First images on the sky from a hyper telescope
We show star images obtained with a miniature ``densified pupil imaging
interferometer'' also called a hyper-telescope. The formation of such images
violates a ``golden rule of imaging interferometers'' which appeared to forbid
the use of interferometric arrangements differing from a Fizeau interferometer.
These produce useless images when the sub-apertures spacing is much wider than
their size, owing to diffraction through the sub-apertures. The hyper-telescope
arrangement solves these problems opening the way towards multi-kilometer
imaging arrays in space. We experimentally obtain an intensity gain of 24 +- 3X
when a densified-pupil interferometer is compared to an equivalent Fizeau-type
interferometer and show images of the double star alpha Gem. The initial
results presented confirm the possibility of directly obtaining high resolution
and high dynamic range images in the recombined focal plane of a large
interferometer if enough elements are used.Comment: 6 pages, LaTeX, standard A&A macros + BibTeX macros. Accepted for
publication in Astronomy and Astrophysics Supplement
Light transport in cold atoms and thermal decoherence
By using the coherent backscattering interference effect, we investigate
experimentally and theoretically how coherent transport of light inside a cold
atomic vapour is affected by the residual motion of atomic scatterers. As the
temperature of the atomic cloud increases, the interference contrast
dramatically decreases emphazising the role of motion-induced decoherence for
resonant scatterers even in the sub-Doppler regime of temperature. We derive
analytical expressions for the corresponding coherence time.Comment: 4 pages - submitted to Physical Review Letter
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
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