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Abstract: We analyse the KSB method to estimate gravitational shear fromsurface-brightness moments of small and noisy galaxy images. We identify threepotentially problematic assumptions. These are: 1 While gravitational shearmust be estimated from averaged galaxy images, KSB derives a shear estimatefrom each individual image and then takes the average. Since the two operationsdo not commute, KSB gives biased results. 2 KSB implicitly assumes thatgalaxy ellipticities are small, while weak gravitational lensing assures onlythat the change in ellipticity due to the shear is small. 3 KSB does notinvert the convolution with the point-spread function, but gives an approximatePSF correction which - even for a circular PSF - holds only in the limit ofcircular sources. The effects of assumptions 2 and 3 partially counter-actin a way dependent on the width of the weight function and of the PSF. Wequantitatively demonstrate the biases due to all assumptions, extend the KSBapproach consistently to third order in the shear and ellipticity and show thatthis extension lowers the biases substantially. The issue of proper PSFdeconvolution will be addressed in a forthcoming paper.



Autor: M. Viola, P. Melchior, M. Bartelmann

Fuente: https://arxiv.org/







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