XMM Users' Handbook

   
RGS spectral resolution for extended sources

The RGS spectral resolution depends on the angular extent of the observed source. For an extended source ($\geq30''$), emission lines are broadened according to the relation

$$\Delta \lambda (ext) = 0.124 / m \times\ \theta (ext),$$ (2)

where $\Delta\lambda$(ext) is the resolution (in Å) in case of an extended source, for the spectral order m and a source extent of $\theta$(ext), in arcmin. This illustrates that, for a source extent of order 1', RGS still clearly outperforms EPIC in terms of spectral resolution.