We describe a method for computing the biases that systematic signals introduce in parameter estimation using a simple extension of the Fisher matrix formalism. This allows us to calculate the offset of the best-fitting parameters relative to the fiducial model, in addition to the usual statistical error ellipse. As an application, we study the impact of residual systematics in tomographic weak lensing measurements. In particular, we explore three different types of shape measurement systematics: (i) additive systematic with no redshift evolution; (ii) additive systematic with redshift evolution and (iii) multiplicative systematic. In each case, we consider a wide range of scale dependence and redshift evolution of the systematics signal. For a future DUNE-like full sky survey, we find that, for cases with mild redshift evolution, the variance of the additive systematic signal should be kept below 10 -7 to ensure biases on cosmological parameters that are subdominant to the statistical errors. For the multiplicative systematics, which depend on the lensing signal, we find the multiplicative calibration m0 needs to be controlled to an accuracy better than 10-3. We find that the impact of systematics can be underestimated if their assumed redshift dependence is too simplistic. We provide simple scaling relations to extend these requirements to any survey geometry and discuss the impact of our results for current and future weak lensing surveys. © 2008 RAS.
