XMM-Newton Users Handbook

3.4.4.4 Wavelength Scale Accuracy

The wavelength scale is determined by the relative alignment of the various instruments and their structural stability. Fig. 67 shows the residuals of the RGS measured versus nominal positions for certain emission lines and a set of observations of coronal sources. Residuals from first order spectra are indicated in the top panel. For an on-axis source, the wavelength scale accuracy is 7 mÅ (1$\sigma$). This accuracy can probably be improved in the future. It has been verified that the wavelength scale is stable over a long timescale.

Figure 67: Comparison of measured and laboratory wavelengths for a number of sources and observations. Top panels show the difference in Å between laboratory and measured wavelengths as a function of wavelength for the first (red) and second (blue) orders; the bottom panels show the accumulated histograms, including a Gaussian fit.