Chromatic Effects on the PSF and Shear Measurement for the Roman Space Telescope

Weak gravitational lensing is a key cosmological probe that requires precise measurement of galaxy images to infer shape distortions, or shear, and thereby constrain cosmology. Accurate estimation of the Point Spread Function (PSF) is crucial for shear measurement, but the wavelength dependence of the PSF introduces chromatic biases that can systematically impact shear inference. One such effect arises from spectral energy distribution (SED) differences between stars, used for PSF modeling, and galaxies, used for shear measurement. We investigate these biases for Roman’s weak lensing (Y106, J129, H158, F184) and wide (W146) filters. We find that these biases exceed the mission’s tolerance limits, but we demonstrate that first-order corrections can mitigate biases in the weak lensing bands, with higher-order corrections needed for the wide filter. We demonstrate that both analytical color-based and machine-learning methods effectively reduce biases, providing opportunities to ensure precise weak lensing measurements with Roman. We also discuss the impact of different survey strategies, assumptions about galaxy spectral energy distributions, and the coaddition process on these chromatic biases and correction methods.

Speaker: Federico Berlfein (Carnegie Mellon University)

The Roman Lecture Series is a monthly virtual lecture series focused on the scientific capabilities and technology of the Nancy Grace Roman Space Telescope, organized by Roman mission partners.