6,462 research outputs found
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
Revisiting the theory of interferometric wide-field synthesis
After several generations of interferometers in radioastronomy, wide-field
imaging at high angular resolution is today a major goal for trying to match
optical wide-field performances. All the radio-interferometric, wide-field
imaging methods currently belong to the mosaicking family. Based on a 30 years
old, original idea from Ekers & Rots, we aim at proposing an alternate
formalism. Starting from their ideal case, we successively evaluate the impact
of the standard ingredients of interferometric imaging. A comparison with
standard nonlinear mosaicking shows that both processing schemes are not
mathematically equivalent, though they both recover the sky brightness. In
particular, the weighting scheme is very different in both methods. Moreover,
the proposed scheme naturally processes the short spacings from both
single-dish antennas and heterogeneous arrays. Finally, the sky gridding of the
measured visibilities, required by the proposed scheme, may potentially save
large amounts of hard-disk space and cpu processing power over mosaicking when
handling data sets acquired with the on-the-fly observing mode. We propose to
call this promising family of imaging methods wide-field synthesis because it
explicitly synthesizes visibilities at a much finer spatial frequency
resolution than the one set by the diameter of the interferometer antennas.Comment: 22 pages, 6 PostScript figures. Accepted for publication in Astronomy
& Astrophysics. Uses aa LaTeX macros
Imaging and Nulling with the Space Interferometry Mission
We present numerical simulations for a possible synthesis imaging mode of the
Space Interferometer Mission (SIM). We summarize the general techniques that
SIM offers to perform imaging of high surface brightness sources, and discuss
their strengths and weaknesses. We describe an interactive software package
that is used to provide realistic, photometrically correct estimates of SIM
performance for various classes of astronomical objects. In particular, we
simulate the cases of gaseous disks around black holes in the nuclei of
galaxies, and zodiacal dust disks around young stellar objects. Regarding the
first, we show that a Keplerian velocity gradient of the line-emitting gaseous
disk -- and thus the mass of the putative black hole -- can be determined with
SIM to unprecedented accuracy in about 5 hours of integration time for objects
with H_alpha surface brigthness comparable to the prototype M 87. Detections
and observations of exo-zodiacal dust disks depend critically on the disk
properties and the nulling capabilities of SIM. Systems with similar disk size
and at least one tenth of the dust content of beta Pic can be detected by SIM
at distances between 100 pc and a few kpc, if a nulling efficiency of 1/10000
is achieved. Possible inner clear regions indicative of the presence of massive
planets can also be detected and imaged. On the other hand, exo-zodiacal disks
with properties more similar to the solar system will not be found in
reasonable integration times with SIM.Comment: 28 pages, incl. 8 postscript figures, excl. 10 gif-figures Submitted
to Ap
Image formation in synthetic aperture radio telescopes
Next generation radio telescopes will be much larger, more sensitive, have
much larger observation bandwidth and will be capable of pointing multiple
beams simultaneously. Obtaining the sensitivity, resolution and dynamic range
supported by the receivers requires the development of new signal processing
techniques for array and atmospheric calibration as well as new imaging
techniques that are both more accurate and computationally efficient since data
volumes will be much larger. This paper provides a tutorial overview of
existing image formation techniques and outlines some of the future directions
needed for information extraction from future radio telescopes. We describe the
imaging process from measurement equation until deconvolution, both as a
Fourier inversion problem and as an array processing estimation problem. The
latter formulation enables the development of more advanced techniques based on
state of the art array processing. We demonstrate the techniques on simulated
and measured radio telescope data.Comment: 12 page
Mammographic image restoration using maximum entropy deconvolution
An image restoration approach based on a Bayesian maximum entropy method
(MEM) has been applied to a radiological image deconvolution problem, that of
reduction of geometric blurring in magnification mammography. The aim of the
work is to demonstrate an improvement in image spatial resolution in realistic
noisy radiological images with no associated penalty in terms of reduction in
the signal-to-noise ratio perceived by the observer. Images of the TORMAM
mammographic image quality phantom were recorded using the standard
magnification settings of 1.8 magnification/fine focus and also at 1.8
magnification/broad focus and 3.0 magnification/fine focus; the latter two
arrangements would normally give rise to unacceptable geometric blurring.
Measured point-spread functions were used in conjunction with the MEM image
processing to de-blur these images. The results are presented as comparative
images of phantom test features and as observer scores for the raw and
processed images. Visualization of high resolution features and the total image
scores for the test phantom were improved by the application of the MEM
processing. It is argued that this successful demonstration of image
de-blurring in noisy radiological images offers the possibility of weakening
the link between focal spot size and geometric blurring in radiology, thus
opening up new approaches to system optimization.Comment: 18 pages, 10 figure
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
Smear fitting: a new deconvolution method for interferometric data
A new technique is presented for producing images from interferometric data.
The method, ``smear fitting'', makes the constraints necessary for
interferometric imaging double as a model, with uncertainties, of the sky
brightness distribution. It does this by modelling the sky with a set of
functions and then convolving each component with its own elliptical gaussian
to account for the uncertainty in its shape and location that arises from
noise. This yields much sharper resolution than CLEAN for significantly
detected features, without sacrificing any sensitivity. Using appropriate
functional forms for the components provides both a scientifically interesting
model and imaging constraints that tend to be better than those used by
traditional deconvolution methods. This allows it to avoid the most serious
problems that limit the imaging quality of those methods. Comparisons of smear
fitting to CLEAN and maximum entropy are given, using both real and simulated
observations. It is also shown that the famous Rayleigh criterion (resolution =
wavelength / baseline) is inappropriate for interferometers as it does not
consider the reliability of the measurements.Comment: 16 pages, 38 figures (some have been lossily compressed for
astro-ph). Uses the hyperref LaTeX package. Accepted for publication by the
Monthly Notices of the Royal Astronomical Societ
Jump-sparse and sparse recovery using Potts functionals
We recover jump-sparse and sparse signals from blurred incomplete data
corrupted by (possibly non-Gaussian) noise using inverse Potts energy
functionals. We obtain analytical results (existence of minimizers, complexity)
on inverse Potts functionals and provide relations to sparsity problems. We
then propose a new optimization method for these functionals which is based on
dynamic programming and the alternating direction method of multipliers (ADMM).
A series of experiments shows that the proposed method yields very satisfactory
jump-sparse and sparse reconstructions, respectively. We highlight the
capability of the method by comparing it with classical and recent approaches
such as TV minimization (jump-sparse signals), orthogonal matching pursuit,
iterative hard thresholding, and iteratively reweighted minimization
(sparse signals)
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