2,247 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
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
The application of compressive sampling to radio astronomy I: Deconvolution
Compressive sampling is a new paradigm for sampling, based on sparseness of
signals or signal representations. It is much less restrictive than
Nyquist-Shannon sampling theory and thus explains and systematises the
widespread experience that methods such as the H\"ogbom CLEAN can violate the
Nyquist-Shannon sampling requirements. In this paper, a CS-based deconvolution
method for extended sources is introduced. This method can reconstruct both
point sources and extended sources (using the isotropic undecimated wavelet
transform as a basis function for the reconstruction step). We compare this
CS-based deconvolution method with two CLEAN-based deconvolution methods: the
H\"ogbom CLEAN and the multiscale CLEAN. This new method shows the best
performance in deconvolving extended sources for both uniform and natural
weighting of the sampled visibilities. Both visual and numerical results of the
comparison are provided.Comment: Published by A&A, Matlab code can be found:
http://code.google.com/p/csra/download
Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor
Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales
Mosaicking with cosmic microwave background interferometers
Measurements of cosmic microwave background (CMB) anisotropies by
interferometers offer several advantages over single-dish observations. The
formalism for analyzing interferometer CMB data is well developed in the
flat-sky approximation, valid for small fields of view. As the area of sky is
increased to obtain finer spectral resolution, this approximation needs to be
relaxed. We extend the formalism for CMB interferometry, including both
temperature and polarization, to mosaics of observations covering arbitrarily
large areas of the sky, with each individual pointing lying within the flat-sky
approximation. We present a method for computing the correlation between
visibilities with arbitrary pointing centers and baselines and illustrate the
effects of sky curvature on the l-space resolution that can be obtained from a
mosaic.Comment: 9 pages; submitted to Ap
Advances in Calibration and Imaging Techniques in Radio Interferometry
This paper summarizes some of the major calibration and image reconstruction
techniques used in radio interferometry and describes them in a common
mathematical framework. The use of this framework has a number of benefits,
ranging from clarification of the fundamentals, use of standard numerical
optimization techniques, and generalization or specialization to new
algorithms
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