18,475 research outputs found
Resolution-enhanced Mapping Spectrometer
A familiar mapping spectrometer implementation utilizes two dimensional detector arrays with spectral dispersion along one direction and spatial along the other. Spectral images are formed by spatially scanning across the scene (i.e., push-broom scanning). For imaging grating and prism spectrometers, the slit is perpendicular to the spatial scan direction. For spectrometers utilizing linearly variable focal-plane-mounted filters the spatial scan direction is perpendicular to the direction of spectral variation. These spectrometers share the common limitation that the number of spectral resolution elements is given by the number of pixels along the spectral (or dispersive) direction. Resolution enhancement by first passing the light input to the spectrometer through a scanned etalon or Michelson is discussed. Thus, while a detector element is scanned through a spatial resolution element of the scene, it is also temporally sampled. The analysis for all the pixels in the dispersive direction is addressed. Several specific examples are discussed. The alternate use of a Michelson for the same enhancement purpose is also discussed. Suitable for weight constrained deep space missions, hardware systems were developed including actuators, sensor, and electronics such that low-resolution etalons with performance required for implementation would weigh less than one pound
Concatenation of convolutional and block codes Final report
Comparison of concatenated and sequential decoding systems and convolutional code structural propertie
Image Ellipticity from Atmospheric Aberrations
We investigate the ellipticity of the point-spread function (PSF) produced by
imaging an unresolved source with a telescope, subject to the effects of
atmospheric turbulence. It is important to quantify these effects in order to
understand the errors in shape measurements of astronomical objects, such as
those used to study weak gravitational lensing of field galaxies. The PSF
modeling involves either a Fourier transform of the phase information in the
pupil plane or a ray-tracing approach, which has the advantage of requiring
fewer computations than the Fourier transform. Using a standard method,
involving the Gaussian weighted second moments of intensity, we then calculate
the ellipticity of the PSF patterns. We find significant ellipticity for the
instantaneous patterns (up to more than 10%). Longer exposures, which we
approximate by combining multiple (N) images from uncorrelated atmospheric
realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also
verify that the measured ellipticity does not depend on the sampling interval
in the pupil plane using the Fourier method. However, we find that the results
using the ray-tracing technique do depend on the pupil sampling interval,
representing a gradual breakdown of the geometric approximation at high spatial
frequencies. Therefore, ray tracing is generally not an accurate method of
modeling PSF ellipticity induced by atmospheric turbulence unless some
additional procedure is implemented to correctly account for the effects of
high spatial frequency aberrations. The Fourier method, however, can be used
directly to accurately model PSF ellipticity, which can give insights into
errors in the statistics of field galaxy shapes used in studies of weak
gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for
publication in the Astrophysical Journa
Evidence for Proportionate Partition Between the Magnetic Field and Hot Gas in Turbulent Cassiopeia A
We present a deep X-ray observation of the young Galactic supernova remnant
Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range
(232 ks) image reveals low-surface-brightness X-ray structure, which appears
qualitatively similar to corresponding radio features. We consider the
correlation between the X-ray and radio morphologies and its physical
implications. After correcting for the inhomogeneous absorption across the
remnant, we performed a point by point (4" resolution) surface brightness
comparison between the X-ray and radio images. We find a strong (r = 0.75)
log-log correlation, implying an overall relationship of . This is
consistent with proportionate partition (and possibly equipartition) between
the local magnetic field and the hot gas --- implying that Cas A's plasma is
fully turbulent and continuously amplifying the magnetic field.Comment: 8 pages with embedded bitmapped figures, Accepted by ApJ Letters
5/1/9
A 10 GHz Quasi-Optical Grid Amplifier Using Integrated HBT Differential Pairs
We report the fabrication and testing of a 10 GHz grid amplifier utilizing sixteen GaAs chips each
containing an HBT differential pair plus integral bias/feedback resistors. The overall amplifier consists of
a 4x4 array of unit cells on an RT Duroid™ board having a relative permittivity of 2.2. Each unit cell
consists of an emitter-coupled differential pair at the center, an input antenna which extends horizontally
in both directions from the two base leads, an output antenna which extends vertically in both directions
from the two collector leads, and high inductance bias lines. In operation, the active grid array is placed
between a pair of crossed polarizers. The horizontally polarized input wave passes through the input
polarizer and couples to the input leads. An amplified current then flows on the vertical leads, which
radiate a vertically polarized amplified signal through the output polarizer. The polarizers serve dual
functions, providing both input-output isolation as well as independent impedance matching for the input
and output ports. The grid thus functions essentially as a free-space beam amplifier. Calculations indicate
that output powers of several watts per square centimeter of grid area should be attainable with optimized
structures
Derivation and Performance of Standardized Enhanced Liver Fibrosis (ELF) Test Thresholds for the Detection and Prognosis of Liver Fibrosis
INTRODUCTION: Noninvasive tests are increasingly used to assess liver fibrosis and determine prognosis but suggested test thresholds vary. We describe the selection of standardized thresholds for the Enhanced Liver Fibrosis (ELF) test for the detection of liver fibrosis and for prognostication in chronic liver disease. METHODS: A Delphi method was used to identify thresholds for the ELF test to predict histological liver fibrosis stages, including cirrhosis, using data derived from 921 patients in the EUROGOLF cohort. These thresholds were then used to determine the prognostic performance of ELF in a subset of 457 patients followed for a mean of 5 years. RESULTS: The Delphi panel selected sensitivity of 85% for the detection of fibrosis and >95% specificity for cirrhosis. The corresponding thresholds were 7.7, 9.8, and 11.3. Eighty-five percent of patients with mild or worse fibrosis had an ELF score ≥7.7. The sensitivity for cirrhosis of ELF ≥9.8 was 76%. ELF ≥11.3 was 97% specific for cirrhosis. ELF scores show a near-linear relationship with Ishak fibrosis stages. Relative to the <7.7 group, the hazard ratios for a liver-related outcome at 5 years were 21.00 (95% CI, 2.68-164.65) and 71.04 (95% CI, 9.4-536.7) in the 9.8 to <11.3 and ≥11.3 subgroups, respectively. CONCLUSION: The selection of standard thresholds for detection and prognosis of liver fibrosis is described and their performance reported. These thresholds should prove useful in both interpreting and explaining test results and when considering the relationship of ELF score to Ishak stage in the context of monitoring
Induced Ge Spin Polarization at the Fe/Ge Interface
We report direct experimental evidence showing induced magnetic moments on Ge
at the interface in an Fe/Ge system. Details of the x-ray magnetic circular
dichroism and resonant magnetic scattering at the Ge L edge demonstrate the
presence of spin-polarized {\it s} states at the Fermi level, as well as {\it
d} character moments at higher energy, which are both oriented antiparallel to
the moment of the Fe layer. Use of the sum rules enables extraction of the L/S
ratio, which is zero for the {\it s} part and for the {\it d}
component. These results are consistent with layer-resolved electronic
structure calculations, which estimate the {\it s} and {\it d} components of
the Ge moment are anti-parallel to the Fe {\it 3d} moment and have a magnitude
of .Comment: 4 pages, 5 figures, submitted to Phys. Rev. Let
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