494 research outputs found
Spillover and diffraction sidelobe contamination in a double-shielded experiment for mapping Galactic synchrotron emission
We have analyzed observations from a radioastronomical experiment to survey
the sky at decimetric wavelengths along with feed pattern measurements in order
to account for the level of ground contamination entering the sidelobes. A
major asset of the experiment is the use of a wire mesh fence around the
rim-halo shielded antenna with the purpose of levelling out and reducing this
source of stray radiation for zenith-centered 1-rpm circular scans. We
investigate the shielding performance of the experiment by means of a geometric
diffraction model in order to predict the level of the spillover and
diffraction sidelobes in the direction of the ground. Using 408 MHz and 1465
MHz feed measurements, the model shows how a weakly-diffracting and unshielded
antenna configuration becomes strongly-diffracting and double-shielded as
far-field diffraction effects give way to near-field ones. Due to the
asymmetric response of the feeds, the orientation of their radiation fields
with respect to the secondary must be known a priori before comparing model
predictions with observational data. By adjusting the attenuation coefficient
of the wire mesh the model is able to reproduce the amount of differential
ground pick-up observed during test measurements at 1465 MHz.Comment: 14 pages, 17 eps + 1 gif figures and 4 Tables. Accepted for
publication in A&AS. Fig.7 available at full resolution from
http://www.das.inpe.br/~tello/publications.ht
Evidence for a Very Large-Scale Fractal Structure in the Universe from Cobe Measurements
In this work, we analyse the temperature fluctuations of the cosmic microwave
background radiation observed by COBE and show that the distribution can be
fitted by a fractal distribution with a fractal dimension .
This value is in close agreement with the fractal dimension obtained by Coleman
and Pietronero (1992) and Luo and Schramm (1992) from galaxy-galaxy and
cluster-cluster correlations up to . The fact that the
observed temperature fluctuations correspond to scales much larger than and are signatures of the primordial density fluctuations at the
recombination layer suggests that the structure of the matter at the early
universe was already fractal and thus non-homogeneous on those scales. This
result may have important consequences for the theoretical framework that
describes the universe.Comment: 11 pages, postscript file, 2 figures available upon request. To
appear in ApJ Letter
Levantamento dos dados de passaporte e caracterização de acessos do banco ativo de germoplasma de capsicum da Embrapa clima temperado.
O Brasil é um importante centro secundário de espécies domesticadas de pimentas do gênero Capsicum. Muitos agricultores cultivam as variedades crioulas de Capsicum, que são resultados de vários ciclos de seleção por eles realizada. Estes recursos genéticos vêm sofrendo ação da erosão genética, devido à substituição de culturas ou abandono da atividade agrícola. Em função da grande importância de conservar esse germoplasma, a Embrapa Clima Temperado (Pelotas, RS) vem desde 2002 realizando o resgate de variedades locais de Capsicum, as quais são conservadas no Banco Ativo de Germoplasma (BAG) de Capsicum. O presente trabalho tem como objetivo apresentar a situaçãoatual do Banco Ativo de Germoplasma de Capsicum da Embrapa Clima Temperado, com base no levantamento dos dados de passaporte e de caracterização dos acessos. O BAG de Capsicum da Embrapa Clima Temperado mantém em seu acervo 324 acessos, a maioria deles proveniente de coletas realizadas no Rio Grande do Sul. Fazem parte do acervo do banco acessos de C. annuum, C. baccatum, C. frutescens, C. chinense, C. pubescens, C. galapagoense e C. lanceolatum. Foram submetidos à caracterização morfológica 51,2 % dos acessos, à caracterização química 14,8% e à caracterização molecular 6,1%. Foram avaliados agronomicamente 5,5% dos acessos. Os resultados obtidos demonstram que o BAG de Capsicum da Embrapa Clima Temperado reúne importante diversidade genética de variedades crioulas de pimentas, o que evidencia a importância desse banco
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