6,538 research outputs found
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
Evaluation and development of satellite inferences of convective storm intensity using combined case study analysis and thunderstorm model simulations
Major research accomplishments which were achieved during the first year of the grant are summarized. The research concentrated in the following areas: (1) an examination of observational requirements for predicting convective storm development and intensity as suggested by recent numerical experiments; (2) interpretation of recent 3D numerical experiments with regard to the relationship between overshooting tops and surface wind gusts; (3) the development of software for emulating satellite-inferred cloud properties using 3D cloud model predicted data; and (4) the development of a conceptual/semi-quantitative model of eastward propagating, mesoscale convective complexes forming to the lee of the Rocky Mountains
Supporting the use of learning objects in the K-12 environment: A design-based research project
This paper outlines the methodology behind a Design- Based Research project conducted at the University of Wollongong. The focus of the research project was on the systematic development of a support system to aid K-12 teachers as they integrated learning designs with learning objects. The foundations of the support system were based on a combination of existing design theories and on an analysis of the practical problems faced by teachers as they attempted to create effective pedagogical experiences which incorporate learning objects. The focal point of this paper is the journey through the Design-Based Research process rather than outcomes of the project
The Position of Sgr A at the Galactic Center
The absolute position of the compact radio source at the dynamical center of
the Galaxy, Sgr A, was known only to an accuracy of in spite of its
accurate location with respect to near-IR stellar sources to within 30
milliarcsecond (mas). To remedy this poor positional accuracy, we have selected
15 high-resolution, high-frequency VLA observations of Sgr A carried out in
the last 13 years and determined the weighted average position with the average
epoch 1992.4 to be at , [1950] =
29\dsec3076, , or
, [2000] = 40\dsec0383,
which agrees with earlier
published values to within the error bars of the earlier measurements.
An accurate absolute position of Sgr A can be useful for its identification
with sources at other wavelengths, particularly, in soft and hard X-rays with
implications for the models of a massive black hole at the Galactic center.Comment: 11 pages, one figure and one table. ApJL (in press
Oral History Interview: Mr. and Mrs. W. C. Cotton
This interview is one of a series conducted concerning rural life in West Virginia and surrounding areas. At the time of the interview, Mr. and Mrs. Cotton were residing in Gallipolis, Ohio. Topics discussed in the interview include food preservation, religion, trading, entertainment, law, farming, and doctors.https://mds.marshall.edu/oral_history/1167/thumbnail.jp
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