2,819 research outputs found

    An Algorithm for Precise Aperture Photometry of Critically Sampled Images

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    We present an algorithm for performing precise aperture photometry on critically sampled astrophysical images. The method is intended to overcome the small-aperture limitations imposed by point-sampling. Aperture fluxes are numerically integrated over the desired aperture, with sinc-interpolation used to reconstruct values between pixel centers. Direct integration over the aperture is computationally intensive, but the integrals in question are shown to be convolution integrals and can be computed ~10000x faster as products in the wave-number domain. The method works equally well for annular and elliptical apertures and could be adapted for any geometry. A sample of code is provided to demonstrate the method.Comment: Accepted MNRA

    River Habitat Mapping: are Surface Flow Type Habitats Biologically Distinct?

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    Current river habitat mapping uses several methods, many relying on descriptions of habitat units based on depth, velocity, substrate and water surface patterns. Water surface patterns are controlled by local geomorphology and hydraulics and can be remotely sensed, if surface flow type habitats are physically and biologically distinctive this may provide a faster surveying method. Six UK lowland rivers were investigated, surface flow types were mapped and the physical characteristics of each habitat unit recorded. Samples of benthic macroinvertebrates were taken from representative units and quantified. The results show that habitat mapping, using surface flow types in small lowland streams, is viable and that those habitats have some degree of physical distinctiveness. Analysis of benthic macroinvertebrate communities shows that there is some association with mapped habitats, and therefore are potentially biologically relevant
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