12,507 research outputs found
A sparse approach to astronomical point source detection
In this work we introduce a method for the detection of point sources in images based on al l1-norm sparse approximation. The method is inspired on astronomical image analysis but is directly applicable to any kind of images. We introduce a "top-to-bottom" detection algorithm that can greatly reduce the computational burden of detection if the images are sufficiently well-behaved, in the sense that sources are truly sparse and the chances of source overlapping are small. We test our ideas with simulated faint sources embedded in white
noise, comparing the results with the matched filter detector for a number of detection thresholds. We show that the sparse detection approach leads to better results in the ROC curve than the matched filter detector. Moreover, with the sparse approach it is possible to provide an objective stopping criterion for the detection algorithm.The authors acknowledge partial financial support from the Spanish Ministry of Education (MEC) under project ESP2004-07067-C03-01 and from the joint CNR-CSIC research project 2008IT0059. MLC acknowledges an EGEE-III postdoctoral contract at IFCA
ZAP -- Enhanced PCA Sky Subtraction for Integral Field Spectroscopy
We introduce Zurich Atmosphere Purge (ZAP), an approach to sky subtraction
based on principal component analysis (PCA) that we have developed for the
Multi Unit Spectrographic Explorer (MUSE) integral field spectrograph. ZAP
employs filtering and data segmentation to enhance the inherent capabilities of
PCA for sky subtraction. Extensive testing shows that ZAP reduces sky emission
residuals while robustly preserving the flux and line shapes of astronomical
sources. The method works in a variety of observational situations from sparse
fields with a low density of sources to filled fields in which the target
source fills the field of view. With the inclusion of both of these situations
the method is generally applicable to many different science cases and should
also be useful for other instrumentation. ZAP is available for download at
http://muse-vlt.eu/science/tools.Comment: 12 pages, 7 figures, 1 table. Accepted to MNRA
The Characterised Noise Hi source finder: Detecting Hi galaxies using a novel implementation of matched filtering
The spectral line datacubes obtained from the Square Kilometre Array (SKA)
and its precursors, such as the Australian SKA Pathfinder (ASKAP), will be
sufficiently large to necessitate automated detection and parametrisation of
sources. Matched filtering is widely acknowledged as the best possible method
for the automated detection of sources. This paper presents the Characterised
Noise Hi (CNHI) source finder, which employs a novel implementation of matched
filtering. This implementation is optimised for the 3-D nature of the planned
Wide-field ASKAP Legacy L-band All- sky Blind surveY's (WALLABY) Hi spectral
line observations. The CNHI source finder also employs a novel sparse
representation of 3-D objects, with a high compression rate, to implement Lutz
one-pass algorithm on datacubes that are too large to process in a single pass.
WALLABY will use ASKAP's phenomenal 30 square degree field of view to image
\sim 70% of the sky. It is expected that WALLABY will find 500 000 Hi galaxies
out to z \sim 0.2.Comment: Part of the 2012 PASA Source Finding Special Issue, 10 figure
LOFAR Sparse Image Reconstruction
Context. The LOw Frequency ARray (LOFAR) radio telescope is a giant digital
phased array interferometer with multiple antennas distributed in Europe. It
provides discrete sets of Fourier components of the sky brightness. Recovering
the original brightness distribution with aperture synthesis forms an inverse
problem that can be solved by various deconvolution and minimization methods
Aims. Recent papers have established a clear link between the discrete nature
of radio interferometry measurement and the "compressed sensing" (CS) theory,
which supports sparse reconstruction methods to form an image from the measured
visibilities. Empowered by proximal theory, CS offers a sound framework for
efficient global minimization and sparse data representation using fast
algorithms. Combined with instrumental direction-dependent effects (DDE) in the
scope of a real instrument, we developed and validated a new method based on
this framework Methods. We implemented a sparse reconstruction method in the
standard LOFAR imaging tool and compared the photometric and resolution
performance of this new imager with that of CLEAN-based methods (CLEAN and
MS-CLEAN) with simulated and real LOFAR data Results. We show that i) sparse
reconstruction performs as well as CLEAN in recovering the flux of point
sources; ii) performs much better on extended objects (the root mean square
error is reduced by a factor of up to 10); and iii) provides a solution with an
effective angular resolution 2-3 times better than the CLEAN images.
Conclusions. Sparse recovery gives a correct photometry on high dynamic and
wide-field images and improved realistic structures of extended sources (of
simulated and real LOFAR datasets). This sparse reconstruction method is
compatible with modern interferometric imagers that handle DDE corrections (A-
and W-projections) required for current and future instruments such as LOFAR
and SKAComment: Published in A&A, 19 pages, 9 figure
Compact source detection in multi-channel microwave surveys: from SZ clusters to polarized sources
In this paper we describe the state-of-the art status of multi-frequency
detection techniques for compact sources in microwave astronomy. From the
simplest cases where the spectral behaviour is well-known (i.e. thermal SZ
clusters) to the more complex cases where there is little a priori information
(i.e. polarized radio sources) we will review the main advances and the most
recent results in the detection problem.Comment: 13 pages, 4 figures. Accepted for publication in the Special Issue
"Astrophysical Foregrounds in Microwave Surveys" of the journal Advances in
Astronom
Asteroid Models from Multiple Data Sources
In the past decade, hundreds of asteroid shape models have been derived using
the lightcurve inversion method. At the same time, a new framework of 3-D shape
modeling based on the combined analysis of widely different data sources such
as optical lightcurves, disk-resolved images, stellar occultation timings,
mid-infrared thermal radiometry, optical interferometry, and radar
delay-Doppler data, has been developed. This multi-data approach allows the
determination of most of the physical and surface properties of asteroids in a
single, coherent inversion, with spectacular results. We review the main
results of asteroid lightcurve inversion and also recent advances in multi-data
modeling. We show that models based on remote sensing data were confirmed by
spacecraft encounters with asteroids, and we discuss how the multiplication of
highly detailed 3-D models will help to refine our general knowledge of the
asteroid population. The physical and surface properties of asteroids, i.e.,
their spin, 3-D shape, density, thermal inertia, surface roughness, are among
the least known of all asteroid properties. Apart for the albedo and diameter,
we have access to the whole picture for only a few hundreds of asteroids. These
quantities are nevertheless very important to understand as they affect the
non-gravitational Yarkovsky effect responsible for meteorite delivery to Earth,
or the bulk composition and internal structure of asteroids.Comment: chapter that will appear in a Space Science Series book Asteroids I
X-ray Astronomical Point Sources Recognition Using Granular Binary-tree SVM
The study on point sources in astronomical images is of special importance,
since most energetic celestial objects in the Universe exhibit a point-like
appearance. An approach to recognize the point sources (PS) in the X-ray
astronomical images using our newly designed granular binary-tree support
vector machine (GBT-SVM) classifier is proposed. First, all potential point
sources are located by peak detection on the image. The image and spectral
features of these potential point sources are then extracted. Finally, a
classifier to recognize the true point sources is build through the extracted
features. Experiments and applications of our approach on real X-ray
astronomical images are demonstrated. comparisons between our approach and
other SVM-based classifiers are also carried out by evaluating the precision
and recall rates, which prove that our approach is better and achieves a higher
accuracy of around 89%.Comment: Accepted by ICSP201
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