203,241 research outputs found
Automated detection of extended sources in radio maps: progress from the SCORPIO survey
Automated source extraction and parameterization represents a crucial
challenge for the next-generation radio interferometer surveys, such as those
performed with the Square Kilometre Array (SKA) and its precursors. In this
paper we present a new algorithm, dubbed CAESAR (Compact And Extended Source
Automated Recognition), to detect and parametrize extended sources in radio
interferometric maps. It is based on a pre-filtering stage, allowing image
denoising, compact source suppression and enhancement of diffuse emission,
followed by an adaptive superpixel clustering stage for final source
segmentation. A parameterization stage provides source flux information and a
wide range of morphology estimators for post-processing analysis. We developed
CAESAR in a modular software library, including also different methods for
local background estimation and image filtering, along with alternative
algorithms for both compact and diffuse source extraction. The method was
applied to real radio continuum data collected at the Australian Telescope
Compact Array (ATCA) within the SCORPIO project, a pathfinder of the ASKAP-EMU
survey. The source reconstruction capabilities were studied over different test
fields in the presence of compact sources, imaging artefacts and diffuse
emission from the Galactic plane and compared with existing algorithms. When
compared to a human-driven analysis, the designed algorithm was found capable
of detecting known target sources and regions of diffuse emission,
outperforming alternative approaches over the considered fields.Comment: 15 pages, 9 figure
Extremely fast focal-plane wavefront sensing for extreme adaptive optics
We present a promising approach to the extremely fast sensing and correction
of small wavefront errors in adaptive optics systems. As our algorithm's
computational complexity is roughly proportional to the number of actuators, it
is particularly suitable to systems with 10,000 to 100,000 actuators. Our
approach is based on sequential phase diversity and simple relations between
the point-spread function and the wavefront error in the case of small
aberrations. The particular choice of phase diversity, introduced by the
deformable mirror itself, minimizes the wavefront error as well as the
computational complexity. The method is well suited for high-contrast
astronomical imaging of point sources such as the direct detection and
characterization of exoplanets around stars, and it works even in the presence
of a coronagraph that suppresses the diffraction pattern. The accompanying
paper in these proceedings by Korkiakoski et al. describes the performance of
the algorithm using numerical simulations and laboratory tests.Comment: SPIE Paper 8447-7
Statistical and systematic uncertainties in pixel-based source reconstruction algorithms for gravitational lensing
Gravitational lens modeling of spatially resolved sources is a challenging
inverse problem with many observational constraints and model parameters. We
examine established pixel-based source reconstruction algorithms for de-lensing
the source and constraining lens model parameters. Using test data for four
canonical lens configurations, we explore statistical and systematic
uncertainties associated with gridding, source regularisation, interpolation
errors, noise, and telescope pointing. Specifically, we compare two gridding
schemes in the source plane: a fully adaptive grid that follows the lens
mapping but is irregular, and an adaptive Cartesian grid. We also consider
regularisation schemes that minimise derivatives of the source (using two
finite difference methods) and introduce a scheme that minimises deviations
from an analytic source profile. Careful choice of gridding and regularisation
can reduce "discreteness noise" in the surface that is inherent in the
pixel-based methodology. With a gridded source, some degree of interpolation is
unavoidable, and errors due to interpolation need to be taken into account
(especially for high signal-to-noise data). Different realisations of the noise
and telescope pointing lead to slightly different values for lens model
parameters, and the scatter between different "observations" can be comparable
to or larger than the model uncertainties themselves. The same effects create
scatter in the lensing magnification at the level of a few percent for a peak
signal-to-noise ratio of 10, which decreases as the data quality improves.Comment: 20 pages, 18 figures, accepted to MNRAS, see
http://physics.rutgers.edu/~tagoreas/papers/ for high resolution image
Adaptive content mapping for internet navigation
The Internet as the biggest human library ever assembled keeps on growing. Although all kinds of information carriers (e.g. audio/video/hybrid file formats) are available, text based documents dominate. It is estimated that about 80% of all information worldwide stored electronically exists in (or can be converted into) text form. More and more, all kinds of documents are generated by means of a text processing system and are therefore available electronically. Nowadays, many printed journals are also published online and may even discontinue to appear in print form tomorrow. This development has many convincing advantages: the documents are both available faster (cf. prepress services) and cheaper, they can be searched more easily, the physical storage only needs a fraction of the space previously necessary and the medium will not age. For most people, fast and easy access is the most interesting feature of the new age; computer-aided search for specific documents or Web pages becomes the basic tool for information-oriented work. But this tool has problems. The current keyword based search machines available on the Internet are not really appropriate for such a task; either there are (way) too many documents matching the specified keywords are presented or none at all. The problem lies in the fact that it is often very difficult to choose appropriate terms describing the desired topic in the first place. This contribution discusses the current state-of-the-art techniques in content-based searching (along with common visualization/browsing approaches) and proposes a particular adaptive solution for intuitive Internet document navigation, which not only enables the user to provide full texts instead of manually selected keywords (if available), but also allows him/her to explore the whole database
Adaptive Binning of X-ray data with Weighted Voronoi Tesselations
We present a technique to adaptively bin sparse X-ray data using weighted
Voronoi tesselations (WVTs). WVT binning is a generalisation of Cappellari &
Copin's (2001) Voronoi binning algorithm, developed for integral field
spectroscopy. WVT binning is applicable to many types of data and creates
unbiased binning structures with compact bins that do not lead the eye. We
apply the algorithm to simulated data, as well as several X-ray data sets, to
create adaptively binned intensity images, hardness ratio maps and temperature
maps with constant signal-to-noise ratio per bin. We also illustrate the
separation of diffuse gas emission from contributions of unresolved point
sources in elliptical galaxies. We compare the performance of WVT binning with
other adaptive binning and adaptive smoothing techniques. We find that the CIAO
tool csmooth creates serious artefacts and advise against its use to interpret
diffuse X-ray emission.Comment: 14 pages; submitted to MNRAS; code freely available at
http://www.phy.ohiou.edu/~diehl/WVT/index.html with user manual, examples and
high-resolution version of this pape
Finding faint HI structure in and around galaxies: scraping the barrel
Soon to be operational HI survey instruments such as APERTIF and ASKAP will
produce large datasets. These surveys will provide information about the HI in
and around hundreds of galaxies with a typical signal-to-noise ratio of
10 in the inner regions and 1 in the outer regions. In addition, such
surveys will make it possible to probe faint HI structures, typically located
in the vicinity of galaxies, such as extra-planar-gas, tails and filaments.
These structures are crucial for understanding galaxy evolution, particularly
when they are studied in relation to the local environment. Our aim is to find
optimized kernels for the discovery of faint and morphologically complex HI
structures. Therefore, using HI data from a variety of galaxies, we explore
state-of-the-art filtering algorithms. We show that the intensity-driven
gradient filter, due to its adaptive characteristics, is the optimal choice. In
fact, this filter requires only minimal tuning of the input parameters to
enhance the signal-to-noise ratio of faint components. In addition, it does not
degrade the resolution of the high signal-to-noise component of a source. The
filtering process must be fast and be embedded in an interactive visualization
tool in order to support fast inspection of a large number of sources. To
achieve such interactive exploration, we implemented a multi-core CPU (OpenMP)
and a GPU (OpenGL) version of this filter in a 3D visualization environment
().Comment: 17 pages, 9 figures, 4 tables. Astronomy and Computing, accepte
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