412,883 research outputs found
The Local Structure of Space-Variant Images
Local image structure is widely used in theories of both machine and biological vision. The form of the differential operators describing this structure for space-invariant images has been well documented (e.g. Koenderink, 1984). Although space-variant coordinates are universally used in mammalian visual systems, the form of the operators in the space-variant domain has received little attention. In this report we derive the form of the most common differential operators and surface characteristics in the space-variant domain and show examples of their use. The operators include the Laplacian, the gradient and the divergence, as well as the fundamental forms of the image treated as a surface. We illustrate the use of these results by deriving the space-variant form of corner detection and image enhancement algorithms. The latter is shown to have interesting properties in the complex log domain, implicitly encoding a variable grid-size integration of the underlying PDE, allowing rapid enhancement of large scale peripheral features while preserving high spatial frequencies in the fovea.Office of Naval Research (N00014-95-I-0409
High resolution reddening map in the direction of the stellar system Terzan 5
We have used optical images acquired with the Hubble Space Telescope to
construct the first high-resolution extinction map in the direction of Terzan
5, a peculiar stellar system in the inner bulge of our Galaxy. The map has a
spatial resolution of 8" X 8", over a total FoV of 200" X 200". The absorption
clouds show a patchy structure on a typical scale of 20" and extinction
variations as large as delta E(B-V) = 0.67 mag, especially in the direction of
the center of the system. These correspond to an absolute color excess ranging
from E(B-V)=2.15 mag, up to 2.82 mag. After the correction for differential
reddening, two distinct red giant branches become clearly visible in the color
magnitude diagram of Terzan 5 and they well correspond to the two
sub-populations with different iron abundances recently discovered in this
system.Comment: Published by the Astrophysical Journal Letters, Volume 755, Issue 2,
article id. L32 (2012). 15 pages, 5 figure
Multiple spiral patterns in the transitional disk of HD 100546
Protoplanetary disks around young stars harbor many structures related to
planetary formation. Of particular interest, spiral patterns were discovered
among several of these disks and are expected to be the sign of gravitational
instabilities leading to giant planets formation or gravitational perturbations
caused by already existing planets. In this context, the star HD100546 presents
some specific characteristics with a complex gas and dusty disk including
spirals as well as a possible planet in formation. The objective of this study
is to analyze high contrast and high angular resolution images of this
emblematic system to shed light on critical steps of the planet formation. We
retrieved archival images obtained at Gemini in the near IR (Ks band) with the
instrument NICI and processed the data using advanced high contrast imaging
technique taking advantage of the angular differential imaging. These new
images reveal the spiral pattern previously identified with HST with an
unprecedented resolution, while the large-scale structure of the disk is mostly
erased by the data processing. The single pattern at the southeast in HST
images is now resolved into a multi-armed spiral pattern. Using two models of a
gravitational perturber orbiting in a gaseous disk we attempted to bring
constraints on the characteristics of this perturber assuming each spiral being
independent and we derived qualitative conclusions. The non-detection of the
northeast spiral pattern observed in HST allows to put a lower limit on the
intensity ratio between the two sides of the disk, which if interpreted as
forward scattering yields a larger anisotropic scattering than derived in the
visible. Also, we found that the spirals are likely spatially resolved with a
thickness of about 5-10AU. Finally, we did not detect the candidate forming
planet recently discovered in the Lp band, with a mass upper limit of 16-18 MJ.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 8
figure
The Third Image of the Large-Separation Lensed Quasar SDSS J1029+2623
We identify a third image in the unique quasar lens SDSS J1029+2623, the
second known quasar lens produced by a massive cluster of galaxies. The
spectrum of the third image shows similar emission and absorption features, but
has a redder continuum than the other two images which can be explained by
differential extinction or microlensing. We also identify several lensed arcs.
Our observations suggest a complicated structure of the lens cluster at z~0.6.
We argue that the three lensed images are produced by a naked cusp on the basis
of successful mass models, the distribution of cluster member galaxies, and the
shapes and locations of the lensed arcs. Lensing by a naked cusp is quite rare
among galaxy-scale lenses but is predicted to be common among large-separation
lensed quasars. Thus the discovery can be viewed as support for an important
theoretical prediction of the standard cold dark matter model.Comment: 5 pages, 3 figures, accepted for publication in ApJ Letter
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