Inhomogeneous systematic signals in cosmic shear observations

We calculate the systematic errors in the weak gravitational lensing power spectrum which would be caused by spatially varying calibration (i.e. multiplicative) errors, such as might arise from uncorrected seeing or extinction variations. The systematic error is fully described by the angular two-point correlation function of the systematic in the case of the 2D lensing that we consider here. We investigate three specific cases: Gaussian, "patchy," and exponential correlation functions. In order to keep systematic errors below statistical errors in future LSST-like surveys, the spatial variation of calibration should not exceed 3% rms. This conclusion is independently true for all forms of correlation function we consider. The relative size of the E- and B-mode power spectrum errors does, however, depend upon the form of the correlation function, indicating that one cannot repair the E-mode power spectrum systematics by means of the B-mode measurements.