Filter-substrate refraction causes dominant lateral shifts yielding 0.3-0.4% PSF size and ellipticity residuals across most Roman bands that exceed weak lensing requirements by an order of magnitude, while longitudinal defocus shifts remain negligible.
Shapes and Shears, Stars and Smears: Optimal Measurements for Weak Lensing
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abstract
We present the theoretical and analytical bases of optimal techniques to measure weak gravitational shear from images of galaxies. We first characterize the geometric space of shears and ellipticity, then use this geometric interpretation to analyse images. The steps of this analysis include: measurement of object shapes on images, combining measurements of a given galaxy on different images, estimating the underlying shear from an ensemble of galaxy shapes, and compensating for the systematic effects of image distortion, bias from PSF asymmetries, and `"dilution" of the signal by the seeing. These methods minimize the ellipticity measurement noise, provide calculable shear uncertainty estimates, and allow removal of systematic contamination by PSF effects to arbitrary precision. Galaxy images and PSFs are decomposed into a family of orthogonal 2d Gaussian-based functions, making the PSF correction and shape measurement relatively straightforward and computationally efficient. We also discuss sources of noise-induced bias in weak lensing measurements and provide a solution for these and previously identified biases.
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The parity-odd intrinsic alignment power spectrum probes the collapsed limit of the parity-odd primordial trispectrum and can tighten constraints on parity-violating PNG when bias parameters are calibrated from N-body simulations.
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Modeling the impact of filter-substrate refraction in the Roman point spread function
Filter-substrate refraction causes dominant lateral shifts yielding 0.3-0.4% PSF size and ellipticity residuals across most Roman bands that exceed weak lensing requirements by an order of magnitude, while longitudinal defocus shifts remain negligible.
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Parity Violation in Galaxy Shapes: Primordial Non-Gaussianity
The parity-odd intrinsic alignment power spectrum probes the collapsed limit of the parity-odd primordial trispectrum and can tighten constraints on parity-violating PNG when bias parameters are calibrated from N-body simulations.