1,799 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
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
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
A multi-scale multi-frequency deconvolution algorithm for synthesis imaging in radio interferometry
Aims : We describe MS-MFS, a multi-scale multi-frequency deconvolution
algorithm for wide-band synthesis-imaging, and present imaging results that
illustrate the capabilities of the algorithm and the conditions under which it
is feasible and gives accurate results.
Methods : The MS-MFS algorithm models the wide-band sky-brightness
distribution as a linear combination of spatial and spectral basis functions,
and performs image-reconstruction by combining a linear-least-squares approach
with iterative minimization. This method extends and combines the
ideas used in the MS-CLEAN and MF-CLEAN algorithms for multi-scale and
multi-frequency deconvolution respectively, and can be used in conjunction with
existing wide-field imaging algorithms. We also discuss a simpler hybrid of
spectral-line and continuum imaging methods and point out situations where it
may suffice.
Results : We show via simulations and application to multi-frequency VLA data
and wideband EVLA data, that it is possible to reconstruct both spatial and
spectral structure of compact and extended emission at the continuum
sensitivity level and at the angular resolution allowed by the highest sampled
frequency.Comment: 17 pages, 11 figure
Station-Keeping Requirements for Constellations of Free-Flying Collectors Used for Astronomical Imaging in Space
The accuracy requirements on station-keeping for constellations of
free-flying collectors coupled as (future) imaging arrays in space for
astrophysics applications are examined. The basic imaging element of these
arrays is the two-element interferometer. Accurate knowledge of two quantities
is required: the \textit{projected baseline length}, which is the distance
between the two interferometer elements projected on the plane tranverse to the
line of sight to the target; and the \textit{optical path difference}, which is
the difference in the distances from that transverse plane to the beam
combiner. ``Rules-of-thumb'' are determined for the typical accuracy required
on these parameters. The requirement on the projected baseline length is a
\textit{knowledge} requirement and depends on the angular size of the targets
of interest; it is generally at a level of half a meter for typical stellar
targets, decreasing to perhaps a few centimeters only for the widest attainable
fields of view. The requirement on the optical path difference is a
\textit{control} requirement and is much tighter, depending on the bandwidth of
the signal; it is at a level of half a wavelength for narrow (few %) signal
bands, decreasing to for the broadest bandwidths expected
to be useful. Translation of these requirements into engineering requirements
on station-keeping accuracy depends on the specific details of the collector
constellation geometry. Several examples are provided to guide future
application of the criteria presented here. Some implications for the design of
such collector constellations and for the methods used to transform the
information acquired into images are discussed.Comment: 13 pages, 6 figures, accepted 6/29/07 for the August 2007 issue of
PAS
Microarcsecond Radio Imaging using Earth Orbit Synthesis
The observed interstellar scintillation pattern of an intra-day variable
radio source is influenced by its source structure. If the velocity of the
interstellar medium responsible for the scattering is comparable to the
earth's, the vector sum of these allows an observer to probe the scintillation
pattern of a source in two dimensions and, in turn, to probe two-dimensional
source structure on scales comparable to the angular scale of the scintillation
pattern, typically as for weak scattering. We review the theory on
the extraction of an ``image'' from the scintillation properties of a source,
and show how earth's orbital motion changes a source's observed scintillation
properties during the course of a year. The imaging process, which we call
Earth Orbit Synthesis, requires measurements of the statistical properties of
the scintillations at epochs spread throughout the course of a year.Comment: ApJ in press. 25 pages, 7 fig
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