3,197 research outputs found
Real-time video correlator
Device provides two-dimensional correlation of video data. Operation is reliable, accurate, and predictable
Revisiting the radio interferometer measurement equation. I. A full-sky Jones formalism
Since its formulation by Hamaker et al., the radio interferometer measurement
equation (RIME) has provided a rigorous mathematical basis for the development
of novel calibration methods and techniques, including various approaches to
the problem of direction-dependent effects (DDEs). This series of papers aims
to place recent developments in the treatment of DDEs into one RIME-based
mathematical framework, and to demonstrate the ease with which the various
effects can be described and understood. It also aims to show the benefits of a
RIME-based approach to calibration.
Paper I re-derives the RIME from first principles, extends the formalism to
the full-sky case, and incorporates DDEs. Paper II then uses the formalism to
describe self-calibration, both with a full RIME, and with the approximate
equations of older software packages, and shows how this is affected by DDEs.
It also gives an overview of real-life DDEs and proposed methods of dealing
with them. Applying this to WSRT data (Paper III) results in a noise-limited
image of the field around 3C 147 with a very high dynamic range (1.6 million),
and none of the off-axis artifacts that plague regular selfcal. The resulting
differential gain solutions contain significant information on DDEs, and can be
used for iterative improvements of sky models.
Perhaps most importantly, sources as faint as 2 mJy have been shown to yield
meaningful differential gain solutions, and thus can be used as potential
calibration beacons in other DDE-related schemes.Comment: 12 pages, no figures, published in A&
Mosaicking with cosmic microwave background interferometers
Measurements of cosmic microwave background (CMB) anisotropies by
interferometers offer several advantages over single-dish observations. The
formalism for analyzing interferometer CMB data is well developed in the
flat-sky approximation, valid for small fields of view. As the area of sky is
increased to obtain finer spectral resolution, this approximation needs to be
relaxed. We extend the formalism for CMB interferometry, including both
temperature and polarization, to mosaics of observations covering arbitrarily
large areas of the sky, with each individual pointing lying within the flat-sky
approximation. We present a method for computing the correlation between
visibilities with arbitrary pointing centers and baselines and illustrate the
effects of sky curvature on the l-space resolution that can be obtained from a
mosaic.Comment: 9 pages; submitted to Ap
Advances in Calibration and Imaging Techniques in Radio Interferometry
This paper summarizes some of the major calibration and image reconstruction
techniques used in radio interferometry and describes them in a common
mathematical framework. The use of this framework has a number of benefits,
ranging from clarification of the fundamentals, use of standard numerical
optimization techniques, and generalization or specialization to new
algorithms
The Enrollment Effects of Merit-Based Financial Aid: Evidence from Georgia's HOPE Scholarship
This paper examines the effects of Georgia's merit-based HOPE Scholarship on college enrollment. Introduced in 1993, the HOPE Scholarship covers tuition, fees, and book expenses for students attending Georgia public colleges, and provides a subsidy of comparable value to students attending in-state private colleges, without any income restrictions. Treating HOPE as a natural experiment, we contrast college enrollment in Georgia with those in the other member states of the Southern Regional Educational Board using IPEDS data for the period 1988-97. We estimate that the HOPE increased total freshmen enrollment by 5.9 percent, with the gains concentrated in 4-year schools. For freshmen recently graduated from high school attending 4-year colleges, two-thirds of the program effect is explained by a decrease in students leaving the state. Both white and black enrollments increased because of HOPE, with the state's historically-black institutions playing an important role. Finally, the total HOPE-induced enrollment increase represents only 15 percent freshmen scholarship recipients.Higher Education, Enrollment, HOPE, Merit-based Aid
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
Cultural Play at the Crazy Horse Colossus: Narrative
This thesis explores the Crazy Horse Memorial orientation film and its rhetorical claim to represent Lakota values in the rhetorically contested Black Hills of South Dakota. Walter Fisher’s concept of narrative rationality is used to analyze the informal logic of the memorial film narrative. The Crazy Horse Memorial is seen as a response to Mt. Rushmore’s colonialist legacy. Analysis shows that the Crazy Horse Memorial actually has much in common with Rushmore’s legacy of Euro-American colonialism. This thesis discusses the effects of this redefinition of Lakota cultural values on the rhetorical sphere of the contested Black Hills
Student Responses to Merit Retention Rules
A common justification for HOPE-style merit-aid programs is to promote and reward academic achievement, thereby inducing greater investments in human capital. However, grade-based eligibility and retention rules encourage other behavioral responses. Using the longitudinal records of all undergraduates who enrolled at the University of Georgia (UGA) between 1989 and 1997, we estimate the effects of HOPE on course enrollment, withdrawal and completion, and the diversion of course taking from the academic year to the summer, treating non-residents as a control group. First, we find that HOPE decreased full-load enrollments and increased course withdrawals among resident freshmen. The combination of these responses results in an 11\% lower probability of full-load completion and an annual average reduction in credits completed of 1.0. The latter implies that between 1993 and 1997 Georgia-resident freshmen completed 15,710 fewer credit hours or 3,142 individual course enrollments than non-residents. Second, the scholarship's influence on course-taking behavior is concentrated on students with GPAs on or below the scholarship-retention margin. Third, the effect increased as the income cap was lifted and more students became eligible for the award. Fourth, these freshmen credit-hour reductions represent an intertemporal substitution, not a general slowdown in academic progress. Finally, residents diverted an average of 1.65 more credits from the regular academic year to the first summer term after their matriculation, which amounts to a 72\% rise in summer course taking.Education, Merit-based aid, Education Finance, HOPE Scholarship
Entangling characterization of (SWAP)1/m and Controlled unitary gates
We study the entangling power and perfect entangler nature of (SWAP)1/m, for
m>=1, and controlled unitary (CU) gates. It is shown that (SWAP)1/2 is the only
perfect entangler in the family. On the other hand, a subset of CU which is
locally equivalent to CNOT is identified. It is shown that the subset, which is
a perfect entangler, must necessarily possess the maximum entangling power.Comment: 12 pages, 1 figure, One more paragraph added in Introductio
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