3,337 research outputs found
Multi-Scale CLEAN deconvolution of radio synthesis images
Radio synthesis imaging is dependent upon deconvolution algorithms to
counteract the sparse sampling of the Fourier plane. These deconvolution
algorithms find an estimate of the true sky brightness from the necessarily
incomplete sampled visibility data. The most widely used radio synthesis
deconvolution method is the CLEAN algorithm of Hogbom. This algorithm works
extremely well for collections of point sources and surprisingly well for
extended objects. However, the performance for extended objects can be improved
by adopting a multi-scale approach. We describe and demonstrate a conceptually
simple and algorithmically straightforward extension to CLEAN that models the
sky brightness by the summation of components of emission having different size
scales. While previous multiscale algorithms work sequentially on decreasing
scale sizes, our algorithm works simultaneously on a range of specified scales.
Applications to both real and simulated data sets are given.Comment: Submitted to IEEE Special Issue on Signal Processin
Abstract carrier space formalism for the irreducible tensor operators of compact quantum group algebras
Defining conditions for irreducible tensor operators associated with the
unitary irreducible corepresentations of compact quantum group algebras are
deduced within the framework of the abstract carrier space formalism. It is
shown that there are {\em{two}} types of irreducible tensor operator, which may
be called `ordinary' and `twisted'. The consistency of the definitions is
demonstrated, and various consequences are deduced, including generalizations
of the Wigner-Eckart theorem for both the ordinary and twisted operators.
Examples of irreducible tensor operators for the standard deformation of the
function algebra of the compact Lie group are described to demonstrate
the applicability of the new definitions.Comment: To be published in J.Math.Phys., 32 pages, RevTe
Radio and optical interferometric imaging
Since diffraction-limited imaging with a single aperture yields angular resolution approx. lambda/D, the attainment of high angular resolution with single apertures requires the construction of correspondingly large monolithic apertures, the whole surface of which must be figured to much less than a wavelength. At the longer wavelengths, it is impossible to build a sufficiently large single aperture: for example, at lambda 21 cm, arcsec resolution requires an aperture of diameter approx. 50 km. At the shorter wavelengths, the atmosphere imposes a natural limit in resolution of about one arcsec. However, another route is possible; that is, using synthetic apertures to image the sky. Synthetic apertures are now in use in many fields, e.g., radio interferometry, radar imaging, and magnetic-resonance imaging. Radio-interferometric techniques developed in radio astronomy over the past 40 years are now being applied to optical and IR astronomical imaging by a number of groups. Furthermore, the problem of figuring synthetic apertures is considerably simpler, and can be implemented in a computer: new 'self-calibration' techniques allow imaging even in the presence of phase errors due to the atmosphere
Higgs transitions of spin ice
Frustrated magnets such as spin ice exhibit Coulomb phases, where
correlations have power-law forms at long distances. Applied perturbations can
cause ordering transitions which cannot be described by the usual Landau
paradigm, and are instead naturally viewed as Higgs transitions of an emergent
gauge theory. Starting from a classical statistical model of spin ice, it is
shown that a variety of possible phases and transitions can be described by
this approach. Certain cases are identified where continuous transitions are
argued to be likely; the predicted critical behavior may be tested in
experiments or numerical simulations.Comment: 23 pages, 10 figures; v2: published version with minor changes;
ancillary file "Figures3D.nb" is a Mathematica (v7) notebook containing
figures as rotatable 3D graphics (see http://www.wolfram.com/cdf-player/ for
a free viewer
Optimal Image Reconstruction in Radio Interferometry
We introduce a method for analyzing radio interferometry data which produces
maps which are optimal in the Bayesian sense of maximum posterior probability
density, given certain prior assumptions. It is similar to maximum entropy
techniques, but with an exact accounting of the multiplicity instead of the
usual approximation involving Stirling's formula. It also incorporates an Occam
factor, automatically limiting the effective amount of detail in the map to
that justified by the data. We use Gibbs sampling to determine, to any desired
degree of accuracy, the multi-dimensional posterior density distribution. From
this we can construct a mean posterior map and other measures of the posterior
density, including confidence limits on any well-defined function of the
posterior map.Comment: 41 pages, 11 figures. High resolution figures 8 and 9 available at
http://www.astro.uiuc.edu/~bwandelt/SuttonWandelt200
The non-coplanar baselines effect in radio interferometry: The W-Projection algorithm
We consider a troublesome form of non-isoplanatism in synthesis radio
telescopes: non-coplanar baselines. We present a novel interpretation of the
non-coplanar baselines effect as being due to differential Fresnel diffraction
in the neighborhood of the array antennas.
We have developed a new algorithm to deal with this effect. Our new
algorithm, which we call "W-projection", has markedly superior performance
compared to existing algorithms. At roughly equivalent levels of accuracy,
W-projection can be up to an order of magnitude faster than the corresponding
facet-based algorithms. Furthermore, the precision of result is not tightly
coupled to computing time.
W-projection has important consequences for the design and operation of the
new generation of radio telescopes operating at centimeter and longer
wavelengths.Comment: Accepted for publication in "IEEE Journal of Selected Topics in
Signal Processing
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