Many of Roman's key science objectives require precise point spread function (PSF) calibration. In particular, weak gravitational lensing studies depend on measurements of very small distortions in galaxy shapes, placing stringent requirements on the characterization of detector effects that will systematically distort Roman images. One such effect is the brighter-fatter effect (BFE), which causes brighter sources to appear larger. Since the PSF of bright stars are used to correct the images of relatively faint galaxies for weak lensing measurements, the BFE must be calibrated to prevent overcorrection of galaxy shapes and thus biases in resulting cosmological parameter estimates. Roman’s primary instrument contains 18 near-infrared detectors which are known to exhibit BFE, as well as other nonlinear effects. We present a characterization of the BFE observed by projecting a grid of bright spots onto a non-flight Roman detector. The BFE causes decreasing flux in bright pixels and increasing flux in neighboring pixels over the course of a single exposure, resulting in an effective PSF that increases in size over the exposure as well. We also present results from applying a BFE correction to the spot data, which partially mitigates the effect, and discuss future work required to improve BFE mitigation in Roman images.

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