4,438 research outputs found
Compressed sensing for radio interferometric imaging: review and future direction
Radio interferometry is a powerful technique for astronomical imaging. The
theory of Compressed Sensing (CS) has been applied recently to the ill-posed
inverse problem of recovering images from the measurements taken by radio
interferometric telescopes. We review novel CS radio interferometric imaging
techniques, both at the level of acquisition and reconstruction, and discuss
their superior performance relative to traditional approaches. In order to
remain as close to the theory of CS as possible, these techniques necessarily
consider idealised interferometric configurations. To realise the enhancement
in quality provided by these novel techniques on real radio interferometric
observations, their extension to realistic interferometric configurations is
now of considerable importance. We also chart the future direction of research
required to achieve this goal.Comment: 4 pages, 4 figures, Proceedings of IEEE International Conference on
Image Processing (ICIP) 201
Complex data processing: fast wavelet analysis on the sphere
In the general context of complex data processing, this paper reviews a
recent practical approach to the continuous wavelet formalism on the sphere.
This formalism notably yields a correspondence principle which relates wavelets
on the plane and on the sphere. Two fast algorithms are also presented for the
analysis of signals on the sphere with steerable wavelets.Comment: 20 pages, 5 figures, JFAA style, paper invited to J. Fourier Anal.
and Appli
On the computation of directional scale-discretized wavelet transforms on the sphere
We review scale-discretized wavelets on the sphere, which are directional and
allow one to probe oriented structure in data defined on the sphere.
Furthermore, scale-discretized wavelets allow in practice the exact synthesis
of a signal from its wavelet coefficients. We present exact and efficient
algorithms to compute the scale-discretized wavelet transform of band-limited
signals on the sphere. These algorithms are implemented in the publicly
available S2DW code. We release a new version of S2DW that is parallelized and
contains additional code optimizations. Note that scale-discretized wavelets
can be viewed as a directional generalization of needlets. Finally, we outline
future improvements to the algorithms presented, which can be achieved by
exploiting a new sampling theorem on the sphere developed recently by some of
the authors.Comment: 13 pages, 3 figures, Proceedings of Wavelets and Sparsity XV, SPIE
Optics and Photonics 2013, Code is publicly available at http://www.s2dw.org
Fast directional continuous spherical wavelet transform algorithms
We describe the construction of a spherical wavelet analysis through the
inverse stereographic projection of the Euclidean planar wavelet framework,
introduced originally by Antoine and Vandergheynst and developed further by
Wiaux et al. Fast algorithms for performing the directional continuous wavelet
analysis on the unit sphere are presented. The fast directional algorithm,
based on the fast spherical convolution algorithm developed by Wandelt and
Gorski, provides a saving of O(sqrt(Npix)) over a direct quadrature
implementation for Npix pixels on the sphere, and allows one to perform a
directional spherical wavelet analysis of a 10^6 pixel map on a personal
computer.Comment: 10 pages, 3 figures, replaced to match version accepted by IEEE
Trans. Sig. Pro
S2LET: A code to perform fast wavelet analysis on the sphere
We describe S2LET, a fast and robust implementation of the scale-discretised
wavelet transform on the sphere. Wavelets are constructed through a tiling of
the harmonic line and can be used to probe spatially localised, scale-depended
features of signals on the sphere. The scale-discretised wavelet transform was
developed previously and reduces to the needlet transform in the axisymmetric
case. The reconstruction of a signal from its wavelets coefficients is made
exact here through the use of a sampling theorem on the sphere. Moreover, a
multiresolution algorithm is presented to capture all information of each
wavelet scale in the minimal number of samples on the sphere. In addition S2LET
supports the HEALPix pixelisation scheme, in which case the transform is not
exact but nevertheless achieves good numerical accuracy. The core routines of
S2LET are written in C and have interfaces in Matlab, IDL and Java. Real
signals can be written to and read from FITS files and plotted as Mollweide
projections. The S2LET code is made publicly available, is extensively
documented, and ships with several examples in the four languages supported. At
present the code is restricted to axisymmetric wavelets but will be extended to
directional, steerable wavelets in a future release.Comment: 8 pages, 6 figures, version accepted for publication in A&A. Code is
publicly available from http://www.s2let.or
Higher-Order Spectra of Weak Lensing Convergence Maps in Parameterized Theories of Modified Gravity
We compute the low- limit of the family of higher-order spectra for
projected (2D) weak lensing convergence maps. In this limit, these spectra are
computed to an arbitrary order using {\em tree-level} perturbative
calculations. We use the flat-sky approximation and Eulerian perturbative
results based on a generating function approach. We test these results for the
lower-order members of this family, i.e. the skew- and kurt-spectra against
state-of-the-art simulated all-sky weak lensing convergence maps and find our
results to be in very good agreement. We also show how these spectra can be
computed in the presence of a realistic sky-mask and Gaussian noise. We
generalize these results to three-dimensions (3D) and compute the {\em
equal-time} higher-order spectra. These results will be valuable in analyzing
higher-order statistics from future all-sky weak lensing surveys such as the
{\em Euclid} survey at low- modes. As illustrative examples, we compute
these statistics in the context of the {\em Horndeski} and {\em Beyond
Horndeski} theories of modified gravity. They will be especially useful in
constraining theories such as the Gleyzes-Langlois-Piazza-Vernizzi (GLPV)
theories and Degenerate Higher-Order Scalar-Tensor (DHOST) theories as well as
the commonly used normal-branch of Dvali-Gabadadze-Porrati (nDGP) model,
clustering quintessence models, and scenarios with massive neutrinos.Comment: 22 pages, 5 figure
Markov chain Monte Carlo analysis of Bianchi VII_h models
We have extended the analysis of Jaffe et al. to a complete Markov chain
Monte Carlo (MCMC) study of the Bianchi type models including a
dark energy density, using 1-year and 3-year Wilkinson Microwave Anisotropy
Probe (WMAP) cosmic microwave background (CMB) data. Since we perform the
analysis in a Bayesian framework our entire inference is contained in the
multidimensional posterior distribution from which we can extract marginalised
parameter constraints and the comparative Bayesian evidence. Treating the
left-handed Bianchi CMB anisotropy as a template centred upon the `cold-spot'
in the southern hemisphere, the parameter estimates derived for the total
energy density, `tightness' and vorticity from 3-year data are found to be:
, , with orientation ). This template is preferred by a factor of roughly
unity in log-evidence over a concordance cosmology alone. A Bianchi type
template is supported by the data only if its position on the sky is heavily
restricted. The low total energy density of the preferred template, implies a
geometry that is incompatible with cosmologies inferred from recent CMB
observations. Jaffe et al. found that extending the Bianchi model to include a
term in creates a degeneracy in the plane. We explore this region fully by MCMC and find that the
degenerate likelihood contours do not intersect areas of parameter space that 1
or 3 year WMAP data would prefer at any significance above . Thus we
can confirm that a physical Bianchi model is not responsible for
this signature.Comment: 8 pages, 10 figures, significant update to include more accurate
results and conclusions to match version accepted by MNRA
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