6,564 research outputs found
A New Shear Estimator for Weak Lensing Observations
We present a new shear estimator for weak lensing observations which properly
accounts for the effects of a realistic point spread function (PSF). Images of
faint galaxies are subject to gravitational shearing followed by smearing with
the instrumental and/or atmospheric PSF. We construct a `finite resolution
shear operator' which when applied to an observed image has the same effect as
a gravitational shear applied prior to smearing. This operator allows one to
calibrate essentially any shear estimator. We then specialize to the case of
weighted second moment shear estimators. We compute the shear polarizability
which gives the response of an individual galaxy's polarization to a
gravitational shear. We then compute the response of the population of
galaxies, and thereby construct an optimal weighting scheme for combining shear
estimates from galaxies of various shapes, luminosities and sizes. We define a
figure of merit --- an inverse shear variance per unit solid angle --- which
characterizes the quality of image data for shear measurement. The new method
is tested with simulated image data. We discuss the correction for anisotropy
of the PSF and propose a new technique involving measuring shapes from images
which have been convolved with a re-circularizing PSF. We draw attention to a
hitherto ignored noise related bias and show how this can be analyzed and
corrected for. The analysis here draws heavily on the properties of real PSF's
and we include as an appendix a brief review, highlighting those aspects which
are relevant for weak lensing.Comment: 39 pages, 9 figure
Stability Analysis of Single-Phase Low-Voltage AC Microgrids With Constant Power Terminals
This express brief presents the stability analysis of single-phase microgrids (SP-MG) operating under master-slave connection with constant power terminals. The SP-MG is composed of linear elements, nonlinear loads and distributed generators modeled as P Q constant terminals interconnected through power electronic converters. A Lyapunov?s direct method through a Hamiltonian representation of the grid is used to demonstrate stability. The non-autonomous model of the SP-MG is transformed into an autonomous equivalent model based on the dynamics of the error. The proposed analysis shows that if there exists an admissible trajectory x * solution of the power flow equations then the SP-MG is stable in the sense of Lyapunov.Fil: Montoya, O. D.. Universidad Tecnologica de Bolivar; ColombiaFil: Garces, A.. Universidad Tecnológica de Pereira; ColombiaFil: Avila Becerril, S.. Universidad Nacional Autónoma de México; MéxicoFil: Espinosa Pérez, G.. Universidad Nacional Autónoma de México; MéxicoFil: Serra, Federico Martin. Universidad Nacional de San Luis. Facultad de Ingeniería y Ciencias Agropecuarias. Laboratorio de Control Automático; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Investigaciones en Tecnología Química. Universidad Nacional de San Luis. Facultad de Química, Bioquímica y Farmacia. Instituto de Investigaciones en Tecnología Química; Argentin
Filtration of atmospheric noise in narrow-field astrometry with very large telescopes
This paper presents a non-classic approach to narrow field astrometry that
offers a significant improvement over conventional techniques due to enhanced
reduction of atmospheric image motion. The method is based on two key elements:
apodization of the entrance pupil and the enhanced virtual symmetry of
reference stars. Symmetrization is implemented by setting special weights to
each reference star. Thus a reference field itself forms a virtual net filter
that effectively attenuates the image motion spectrum. Atmospheric positional
error was found to follow a power dependency ~ R^{K \mu /2} D^{-K/2+1/3} on
angular field size R and aperture D; here K is some optional even integer
2<=K<=sqrt{8N+1}-1 limited by a number N of reference stars, and \mu <= 1 is a
term dependent on K and the magnitude and sky star distribution in the field.
As compared to conventional techniques for which K=2, the improvement in
accuracy increases by some orders. Limitations to astrometric performance of
monopupil large ground-based telescopes are estimated. The total atmospheric
and photon noise for at a 10 m telescope at good 0.4" seeing was found to be,
depending on sky star density, 10 to 60 microarcsec per 10 min exposure in R
band. For a 100 m telescope and FWHM=0.1" (low-order adaptive optics
corrections) the potential accuracy is 0.2 to 2 microarcsec.Comment: 18 pages, 17 figure
An Optimal Protocol to Analyze the Rat Spinal Cord Proteome
Since the function of the spinal cord depends on the proteins found there, better defing the normal Spinal Cord Proteome is an important and challenging task. Although brain and cerebrospinal fluid samples from patients with different central nervous system (CNS) disorders have been studied, a thorough examination of specific spinal cord proteins and the changes induced by injury or associated to conditions such as neurodegeneration, spasticity and neuropathies has yet to be performed. In the present study, we aimed to describe total protein content in the spinal cord of healthy rats, employing different proteomics tools. Accordingly, we have developed a fast, easy, and reproducible sequential protocol for protein extraction from rat spinal cords. We employed conventional two dimensional electrophoresis (2DE) in different pH ranges (eg. 4–7, 3–11 NL) combined with identification by mass spectrometry (MALDI-TOF/TOF), as well as first dimension protein separation combined with Liquid Chromatography Mass Spectrometry/Mass Spectrometry (LC-MS/MS) to maximise the benefits of this technology. The value of these techniques is demonstrated here by the identification of several proteins known to be associated with neuroglial structures, neurotransmission, cell survival and nerve growth in the central nervous system. Furthermore this study identified many spinal proteins that have not previously been described in the literature and which may play an important role as either sensitive biomarkers of dysfunction or of recovery after Spinal Cord Injury
Low wave-functions of pions and kaons and their parton distribution functions
We study the low wave-functions of pions and kaons as an expansion in
terms of hadron-like Fock state fluctuations. In this formalism, pion and kaon
wave-functions are related one another. Consequently, the knowledge of the pion
structure allows the determination of parton distributions in kaons. In
addition, we show that the intrinsic (low ) sea of pions and kaons are
different due to their different valence quark structure. Finally, we analize
the feasibility of a method to extract kaon's parton distribution functions
within this approach and compare with available experimental data.Comment: 13 pages, 3 postscript figures include
A New Strategy for Deep Wide-Field High Resolution Optical Imaging
We propose a new strategy for obtaining enhanced resolution (FWHM = 0.12
arcsec) deep optical images over a wide field of view. As is well known, this
type of image quality can be obtained in principle simply by fast guiding on a
small (D = 1.5m) telescope at a good site, but only for target objects which
lie within a limited angular distance of a suitably bright guide star. For high
altitude turbulence this 'isokinetic angle' is approximately 1 arcminute. With
a 1 degree field say one would need to track and correct the motions of
thousands of isokinetic patches, yet there are typically too few sufficiently
bright guide stars to provide the necessary guiding information. Our proposed
solution to these problems has two novel features. The first is to use
orthogonal transfer charge-coupled device (OTCCD) technology to effectively
implement a wide field 'rubber focal plane' detector composed of an array of
cells which can be guided independently. The second is to combine measured
motions of a set of guide stars made with an array of telescopes to provide the
extra information needed to fully determine the deflection field. We discuss
the performance, feasibility and design constraints on a system which would
provide the collecting area equivalent to a single 9m telescope, a 1 degree
square field and 0.12 arcsec FWHM image quality.Comment: 46 pages, 22 figures, submitted to PASP, a version with higher
resolution images and other supplementary material can be found at
http://www.ifa.hawaii.edu/~kaiser/wfhr
Total photoproduction cross-section at very high energy
In this paper we apply to photoproduction total cross-section a model we have
proposed for purely hadronic processes and which is based on QCD mini-jets and
soft gluon re-summation. We compare the predictions of our model with the HERA
data as well as with other models. For cosmic rays, our model predicts
substantially higher cross-sections at TeV energies than models based on
factorization but lower than models based on mini-jets alone, without soft
gluons. We discuss the origin of this difference.Comment: 13 pages, 9 figures. Accepted for publication in EPJC. Changes
concern added references, clarifications of the Soft Gluon Resummation method
used in the paper, and other changes requested by the Journal referee which
do not change the results of the original versio
Elliptic and hyperelliptic magnetohydrodynamic equilibria
The present study is a continuation of a previous one on "hyperelliptic"
axisymmetric equilibria started in [Tasso and Throumoulopoulos, Phys. Plasmas
5, 2378 (1998)].
Specifically, some equilibria with incompressible flow nonaligned with the
magnetic field and restricted by appropriate side conditions like "isothermal"
magnetic surfaces, "isodynamicity" or P + B^2/2 constant on magnetic surfaces
are found to be reducible to elliptic integrals. The third class recovers
recent equilibria found in [Schief, Phys. Plasmas 10, 2677 (2003)]. In contrast
to field aligned flows, all solutions found here have nonzero toroidal magnetic
field on and elliptic surfaces near the magnetic axis.Comment: 9 page
How can the Odderon be detected at RHIC and LHC
The Odderon remains an elusive object, 33 years after its invention. The
Odderon is now a fundamental object in QCD and CGC and it has to be found
experimentally if QCD and CGC are right. In the present paper, we show how to
find it at RHIC and LHC. The most spectacular signature of the Odderon is the
predicted difference between the differential cross-sections for proton-proton
and antiproton-proton at high s and moderate t. The experiment can be done by
using the STAR detector at RHIC and by combining these future data with the
already present UA4/2 data. The Odderon could also be found by ATLAS
exeperiment at LHC by performing a high-precision measurement of the real part
of the hadron elastic scattering amplitude at small t.Comment: 14 pages, 16 figures, two typographical errors corrected and
acknowledgments adde
Theoretical expectations for total cross-sections at the large hadron collider
In this note, we summarize and compare various model predictions for
total cross-section , giving an estimate of the range of
predictions for the total cross-section, expected at
the LHC. We concentrate on the results for \sigma_{\tot}^{pp} obtained in a
particular QCD based model of the energy dependence of the total cross-section,
including the effect of soft gluon radiation. We obtain the range of
predictions in this model by exploring the allowed range of model parameters.
We further give a handy parametrisation of these results which incidentally
spans the range of various other available predictions at the LHC as well.Comment: 12 pages, 6 figures, laTeX, requires ias.cls, ias.sty include
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