11,194 research outputs found
High Dynamic-Range Radio-Interferometric Images at 327 MHz
Radio astronomical imaging using aperture synthesis telescopes requires
deconvolution of the point spread function as well as calibration of the
instrumental characteristics (primary beam) and foreground
(ionospheric/atmospheric) effects. These effects vary in time and also across
the field of view, resulting in directionally-dependent (DD), time-varying
gains. The primary beam will deviate from the theoretical estimate in real
cases at levels that will limit the dynamic range of images if left
uncorrected. Ionospheric electron density variations cause time and position
variable refraction of sources. At low frequencies and sufficiently high
dynamic range this will also defocus the images producing error patterns that
vary with position and also with frequency due to the chromatic aberration of
synthesis telescopes. Superposition of such residual sidelobes can lead to
spurious spectral signals. Field-based ionospheric calibration as well as
"peeling" calibration of strong sources leads to images with higher dynamic
range and lower spurious signals but will be limited by sensitivity on the
necessary short-time scales. The results are improved images although some
artifacts remain.Comment: to appear in Comptes Rendus Physique (2011
Understanding the gender and ethnicity attainment gap in UK higher education
In recent years the success rates of different groups of students in higher education (HE), have come under considerable scrutiny, with gender and ethnicity identified as key attributes predicting differential achievement of ‘good degrees’. A review of previous studies highlights the need for research which looks beyond ‘the deficit model’ to explain the attainment gap. This research used a mixed-methods approach to explore the academic and social experiences of students, as well as lecturers’ views on student achievement, in one UK University. Findings suggest that there are significant differences in motivation and confidence speaking English for different ethnic groups in this study, and a divergence in attendance and study time by gender – both of which may go some way to helping understand the gaps in attainment. In addition, male and BME students tended to over-estimate their likelihood of achieving a good degree outcome, compared to other groups
Beam squint and Stokes V with off-axis feeds
Radio telescopes with off-axis feeds, such as the (E)VLA, suffer from "beam
squint" in which the two orthogonal circular polarizations sampled have
different pointing centers on the sky. Its effects are weak near the beam
center but become increasingly important towards the edge of the antenna power
pattern where gains in the two polarizations at a given sky position are
significantly different. This effect has limited VLA measurements of circular
polarization (Stokes V) and introduced dynamic range limiting, wide-field
artifacts in images made in Stokes I. We present an adaptation of the
visibility-based deconvolution CLEAN method that can correct this defect "on
the fly" while imaging, correcting as well the associated self-calibration. We
present two examples of this technique using the procedure "Squint" within the
Obit package which allows wide-field imaging in Stokes V and reduced artifacts
in Stokes I. We discuss the residual errors in these examples as well as a
scheme for future correction of some of these errors. This technique can be
generalized to implement temporally- and spatially-variable corrections, such
as pointing and cross-polarization leakage errors.Comment: 9 pages, 6 figures (five of them double), to appear in Astronomy &
Astrophysics (accepted: May 9, 2008). High-resolution versions of the figures
(gzipped, tar,gzipped) can be downloaded from
http://www.cv.nrao.edu/~juson/technical/squint/squint_figures.g
Revolutions and Second-Best Solutions: Education for Sustainable Development in Higher Education
PT-symmetry broken by point-group symmetry
We discuss a PT-symmetric Hamiltonian with complex eigenvalues. It is based
on the dimensionless Schr\"{o}dinger equation for a particle in a square box
with the PT-symmetric potential . Perturbation theory clearly
shows that some of the eigenvalues are complex for sufficiently small values of
. Point-group symmetry proves useful to guess if some of the eigenvalues
may already be complex for all values of the coupling constant. We confirm
those conclusions by means of an accurate numerical calculation based on the
diagonalization method. On the other hand, the Schr\"odinger equation with the
potential exhibits real eigenvalues for sufficiently small
values of . Point group symmetry suggests that PT-symmetry may be broken
in the former case and unbroken in the latter one
Treatment of Young Children with HIV Infection: Using Evidence to Inform Policymakers
PMCID: PMC3404108This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
A New Group of Leaders; The Assistant Principal Academy For Instructional Leadership and Development
In order to develop the skills needed for the role of today\u27s school leader, providing quality professional development and learning opportunities to administrators is essential. The district created systems of administrative support to provide multiple learning experiences to adequately prepare leaders to serve at the school level. Riverwoods Public Schools invested in professional development that provided instructional leadership support. Professional development sessions provided new assistant principals with opportunities to network and engage in dialogue that provided examples of best practice in instruction
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