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Olivier Lardière Rodolphe Conan Colin Bradley Kate Jackson Peter Hampton 《Monthly notices of the Royal Astronomical Society》2009,398(3):1461-1467
Sodium laser guide stars (LGSs) are elongated sources due to the thickness and the finite distance of the sodium layer. The fluctuations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots, and generate spurious optical aberrations in closed-loop adaptive optics (AO) systems. According to an analytical model and experimental results obtained with the University of Victoria LGS bench demonstrator, one of the main origins of these aberrations, referred to as LGS aberrations, is not the centre-of-gravity (CoG) algorithm itself, but the thresholding applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed 'radial thresholding', is presented here, cancelling out most of the LGS aberrations without altering the centroid measurement accuracy. 相似文献
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Activities that manipulate ecosystems to support economic activities provide major introduction pathways for non-native species. As such, substantial differences in the socioeconomic conditions between countries could influence how ecosystems are manipulated and thus impact the composition of their communities of non-native species. Here, we compared the influence of freshwater fish aquaculture production and macro-socioeconomic drivers on the freshwater fish allodiversity of Europe between 1970 and 2009. A divergence in the socio-economic conditions of Europe prevailed during much of the latter half of the 20th Century as a result of the Cold War. For example, GDP and GDP per capita were significantly higher in Western bloc countries compared to the Eastern bloc. In this 39 year period, aquaculture production in Eastern bloc countries was dominated by Asian cyprinid fish whereas in Western bloc countries it was dominated by the North American rainbow trout Oncorhynchus mykiss. Analysis of a European database on introduced fish into the wild from aquaculture revealed that in entirety, there were 279 separate freshwater fish introductions in Europe associated with aquaculture (Eastern bloc 118, Western bloc 161), involving 117 species from 32 families. There was relatively low homogeneity in these introduced fishes between the two blocs; only 28 species were introduced into both. Western bloc countries also had significantly more introduced fishes and more introduction events, and less similarity in the introduced fishes between their countries. Aquaculture production was a significant predictor of the number of non-native freshwater fish across all the countries, although additional factors, especially human population size and GDP per capita, were also significant predictors. Thus, aquaculture has been a strong introduction pressure in Europe and provides a reliable predictor of fish allodiversity. 相似文献
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