38,833 research outputs found
Combining Undersampled Dithered Images
Undersampled images, such as those produced by the HST WFPC-2, misrepresent
fine-scale structure intrinsic to the astronomical sources being imaged.
Analyzing such images is difficult on scales close to their resolution limits
and may produce erroneous results. A set of ``dithered'' images of an
astronomical source generally contains more information about its structure
than any single undersampled image, however, and may permit reconstruction of a
``superimage'' with Nyquist sampling. I present a tutorial on a method of image
reconstruction that builds a superimage from a complex linear combination of
the Fourier transforms of a set of undersampled dithered images. This method
works by algebraically eliminating the high order satellites in the periodic
transforms of the aliased images. The reconstructed image is an exact
representation of the data-set with no loss of resolution at the Nyquist scale.
The algorithm is directly derived from the theoretical properties of aliased
images and involves no arbitrary parameters, requiring only that the dithers
are purely translational and constant in pixel-space over the domain of the
object of interest. I show examples of its application to WFC and PC images. I
argue for its use when the best recovery of point sources or morphological
information at the HST diffraction limit is of interest.Comment: 22 pages, 9 EPS figures, submitted to PAS
Atmospheric Chemistry for Astrophysicists: A Self-consistent Formalism and Analytical Solutions for Arbitrary C/O
We present a self-consistent formalism for computing and understanding the
atmospheric chemistry of exoplanets from the viewpoint of an astrophysicist.
Starting from the first law of thermodynamics, we demonstrate that the van't
Hoff equation (which describes the equilibrium constant), Arrhenius equation
(which describes the rate coefficients) and procedures associated with the
Gibbs free energy (minimisation, rescaling) have a common physical and
mathematical origin. We address an ambiguity associated with the equilibrium
constant, which is used to relate the forward and reverse rate coefficients,
and restate its two definitions. By necessity, one of the equilibrium constants
must be dimensionless and equate to an exponential function involving the Gibbs
free energy, while the other is a ratio of rate coefficients and must therefore
possess physical units. We demonstrate that the Arrhenius equation takes on a
functional form that is more general than previously stated without recourse to
tagging on ad hoc functional forms. Finally, we derive analytical models of
chemical systems, in equilibrium, with carbon, hydrogen and oxygen. We include
acetylene and are able to reproduce several key trends, versus temperature and
carbon-to-oxygen ratio, published in the literature. The rich variety of
behavior that mixing ratios exhibit as a function of the carbon-to-oxygen ratio
is merely the outcome of stoichiometric book-keeping and not the direct
consequence of temperature or pressure variations.Comment: Accepted by ApJ. 9 pages, 4 figure
The Te-d1 mode reflection coefficient of a ground-plane mounted parallel-plate waveguide illuminating a reflecting sheet
Symmetric parallel plate waveguide reflection coefficient analyze
The influence of conducting flaps on the reflection coefficient of a parallel-plate waveguide illuminating a conducting sheet
Conducting flap effects on reflection coefficient of parallel-plate waveguide illuminating conducting shee
Aperture reflection coefficient of a parallel- plate waveguide by wedge diffraction analysis
Aperture reflection coefficient of parallel plate waveguide by wedge diffraction analysi
Renormalization-group approach to superconductivity: from weak to strong electron-phonon coupling
We present the numerical solution of the renormalization group (RG) equations
derived in Ref. [1], for the problem of superconductivity in the presence of
both electron-electron and electron-phonon coupling at zero temperature. We
study the instability of a Fermi liquid to a superconductor and the RG flow of
the couplings in presence of retardation effects and the crossover from weak to
strong coupling. We show that our numerical results provide an ansatz for the
analytic solution of the problem in the asymptotic limits of weak and strong
coupling.Comment: 8 pages, 3 figures, conference proceedings for the Electron
Correlations and Materials Properties, in Kos, Greece, July 5-9, 200
Reply to Comment by D. Spemann et al [EPL 98 (2012) 57006, arXiv:1204.2992]
This article is a reply to the Comment by D. Spemann et al (arXiv:1204.2992)
in response to our paper 'Revealing common artifacts due to ferromagnetic
inclusions in highly oriented pyrolytic graphite' (EPL, 97 (2012) 47001).Comment: Reply to arXiv:1204.2992 Comment by D. Spemann et al re
arXiv:1201.6374 by Sepioni et a
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