1,565 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&
A Nulling Wide Field Imager for Exoplanets Detection and General Astrophysics
We present a solution to obtain a high-resolution image of a wide field with
the central source removed by destructive interference. The wide-field image is
created by aperture synthesis with a rotating sparse array of telescopes in
space. Nulling of the central source is achieved using a phase-mask
coronagraph. The full (u,v) plane coverage delivered by the 60m, six 3-meter
telescope array is particularly well-suited for the detection and
characterization of exoplanets in the infrared (DARWIN and Terrestrial Planet
Finder (TPF) missions) as well as for other generic science observations.
Detection (S/N=10) of an Earth-like planet is achieved in less than 10 hours
with a 1 micron bandwidth at 10 micron.Comment: 18 pages, 16 figures. Accepted for publication in A&
Adaptive optics imaging of P Cygni in Halpha
We obtained Halpha diffraction limited data of the LBV star P Cyg using the
ONERA Adaptive Optics (AO) facility BOA at the OHP 1.52m telescope on October
1997. Taking P Cyg and the reference star 59 Cyg AO long exposures we find that
P Cyg clearly exhibits a large and diffuse intensity distribution compared to
the 59 Cyg's point-like source. A deconvolution of P Cyg using 59 Cyg as the
Point Spread Function was performed by means of the Richardson-Lucy algorithm.
P Cyg clearly appears as an unresolved star surrounded by a clumped envelope.
The reconstructed image of P Cyg is compared to similar spatial resolution maps
obtained from radio aperture synthesis imaging. We put independent constraints
on the physics of P Cyg which agree well with radio results. We discuss future
possibilities to constrain the wind structure of P Cyg by using
multi-resolution imaging, coronagraphy and long baseline interferometry to
trace back its evolutionary status.Comment: 10 pages, 19 Encapsulated Postscript figure
Coordination chemistry of amide-functionalised tetraazamacrocycles: structural, relaxometric and cytotoxicity studies
Three different tetraazamacrocyclic ligands containing four amide substituents that feature groups (namely allyl, styryl and propargyl groups) suitable for polymerisation have been synthesised. Gadolinium(III) complexes of these three ligands have been prepared as potential monomers for the synthesis of polymeric MRI contrast agents. To assess the potential of these monomers as MRI contrast agents, their relaxation enhancement properties and cytotoxicity have been determined. A europium(III) complex of one of these ligands (with propargyl substituents) is also presented together with its PARACEST properties. In addition, to gain further insight into the coordination chemistry of the tetra-propargyl substituted ligand, the corresponding zinc(II) and cadmium(II) complexes have been prepared. The X-ray crystal structures of the tetra-propargyl ligand and its corresponding gadolinium(III), zinc(II) and cadmium(II) complexes are also presented
On-sky observations with an achromatic hybrid phase knife coronagraph in the visible
CONTEXT: The four-quadrant phase mask stellar coronagraph, introduced by D.
Rouan et al., is capable of achieving very high dynamical range imaging and was
studied in the context of the direct detection of extra-solar planets.
Achromatic four-quadrant phase mask is currently being developed for broadband
IR applications. AIMS: We report on laboratory and on-sky tests of a prototype
coronagraph in the visible. This prototype, the achromatic hybrid phase knife
coronagraph, was derived from the four-quadrant phase mask principle. METHODS:
The instrumental setup implementing the coronagraph itself was designed to
record the pre- and post-coronagraphic images simultaneously so that an
efficient real-time image selection procedure can be performed. We describe the
coronagraph and the associated tools that enable robust and repeatable
observations. We present an algorithm of image selection that has been tested
against the real on-sky data of the binary star HD80081 (* 38 Lyn). RESULTS
Although the observing conditions were poor, the efficiency of the proposed
method is proven. From this experiment, we derive procedures that can apply to
future focal instruments associating adaptive optics and coronagraphy,
targeting high dynamic range imaging in astronomy, such as detecting
extra-solar planets
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
Speckle noise and dynamic range in coronagraphic images
This paper is concerned with the theoretical properties of high contrast
coronagraphic images in the context of exoplanet searches. We derive and
analyze the statistical properties of the residual starlight in coronagraphic
images, and describe the effect of a coronagraph on the speckle and photon
noise. Current observations with coronagraphic instruments have shown that the
main limitations to high contrast imaging are due to residual quasi-static
speckles. We tackle this problem in this paper, and propose a generalization of
our statistical model to include the description of static, quasi-static and
fast residual atmospheric speckles. The results provide insight into the
effects on the dynamic range of wavefront control, coronagraphy, active speckle
reduction, and differential speckle calibration. The study is focused on
ground-based imaging with extreme adaptive optics, but the approach is general
enough to be applicable to space, with different parameters.Comment: 31 pages, 18 figure
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