Additive Shear Systematics: Point-Spread-Function Residuals and the Large-Separation Floor

An additive error in measured galaxy shapes, arising mainly from imperfect correction of the point spread function, contributes a constant offset to shear correlation functions. This paper computes the offset for representative residual amplitudes and compares it with a reference correlation function. A residual of 10 to the minus 3 contributes an additive correlation of 10 to the minus 6, which is 0.46 times the reference xi-plus at 128 arcminutes but a negligible fraction at 8 arcminutes; a residual of 10 to the minus 4 contributes 0.005 at 128 arcminutes. Keeping the additive contribution below ten percent of the signal at 128 arcminutes requires the residual to be below 4.67e-04. Additive systematics therefore set a floor that binds first, and hardest, at the wide separations where large-scale cosmological signatures live. All quantities are computed from stated assumptions, not measured.