7,508 research outputs found
Theory and Simulations of Refractive Substructure in Resolved Scatter-Broadened Images
At radio wavelengths, scattering in the interstellar medium distorts the
appearance of astronomical sources. Averaged over a scattering ensemble, the
result is a blurred image of the source. However, Narayan & Goodman (1989) and
Goodman & Narayan (1989) showed that for an incomplete average, scattering
introduces refractive substructure in the image of a point source that is both
persistent and wideband. We show that this substructure is quenched but not
smoothed by an extended source. As a result, when the scatter-broadening is
comparable to or exceeds the unscattered source size, the scattering can
introduce spurious compact features into images. In addition, we derive
efficient strategies to numerically compute realistic scattered images, and we
present characteristic examples from simulations. Our results show that
refractive substructure is an important consideration for ongoing missions at
the highest angular resolutions, and we discuss specific implications for
RadioAstron and the Event Horizon Telescope.Comment: Equation numbering in appendix now matches published version. Two
minor typos correcte
Optimal Correlation Estimators for Quantized Signals
Using a maximum-likelihood criterion, we derive optimal correlation
strategies for signals with and without digitization. We assume that the
signals are drawn from zero-mean Gaussian distributions, as is expected in
radio-astronomical applications, and we present correlation estimators both
with and without a priori knowledge of the signal variances. We demonstrate
that traditional estimators of correlation, which rely on averaging products,
exhibit large and paradoxical noise when the correlation is strong. However, we
also show that these estimators are fully optimal in the limit of vanishing
correlation. We calculate the bias and noise in each of these estimators and
discuss their suitability for implementation in modern digital correlators.Comment: 8 Pages, 3 Figures, Submitted to Ap
UPDATING CORN PROGRAM PAYMENT YIELDS: ARE FARM OPERATORS DIFFERENTIALLY AFFECTED?
Crop yields which determine farm income deficiency payments have been frozen at 1981-1985 levels since 1986. Data from a longitudinal survey of Ohio farm operators are analyzed to evaluate whether updating payment yields will differentially affect farm operators. Results of the analysis imply that farm operators who operate larger farms, live in counties with higher yields, and have higher fertilizer and pesticide expenses per acre of corn will benefit more. In addition, low (high) existing payment yields are understated (overstated) relative to updated payment yields.Agricultural Finance,
POTENTIAL EFFECTS OF SUBSIDIZED LIVESTOCK INSURANCE ON LIVESTOCK PRODUCTION
Recent legislation has cleared the way for subsidized livestock price insurance. Such programs could increase production. Expected feeder cattle prices with and without subsidized insurance will be analyzed using E-V and Stochastic Dominance. Results will highlight the potential effects of the program on marketing risk and production decisions.Livestock Production/Industries, Risk and Uncertainty,
New Late Miocene North American dromomerycine artiodactyl from the Amazon Basin: implications for interchange dynamics
In defence of global egalitarianism
This essay argues that David Miller's criticisms of global egalitarianism do not undermine the view where it is stated in one of its stronger, luck egalitarian forms. The claim that global egalitarianism cannot specify a metric of justice which is broad enough to exclude spurious claims for redistribution, but precise enough to appropriately value different kinds of advantage, implicitly assumes that cultural understandings are the only legitimate way of identifying what counts as advantage. But that is an assumption always or almost always rejected by global egalitarianism. The claim that global egalitarianism demands either too little redistribution, leaving the unborn and dissenters burdened with their societies' imprudent choices, or too much redistribution, creating perverse incentives by punishing prudent decisions, only presents a problem for global luck egalitarianism on the assumption that nations can legitimately inherit assets from earlier generations – again, an assumption very much at odds with global egalitarian assumptions
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