Difference between expected and observed wavelengths as a function of expected wavelengths for RGS1 (squares) and RGS2 (triangles). RGS2 data have been slightly (horizontally) displaced for clarity. Error bars are the standard deviations of the statistics of the line fits. The spread of the data points seems somewhat smaller than the error bars indicate. There is a possibility this is due to correlations between spectral bins because of rebinning of the original CCD pixels, which might cause the statistical error to be somewhat overestimated.