The RGS spectral resolution depends on the angular extent of the observed source. For extended sources, emission lines are broadened according to the relation
where (ext) is the resolution in Å in case of an extended source, for the spectral order m and a source extent of (ext), in arcmin. This illustrates that, for a source extent of order 1, RGS still clearly outperforms EPIC in terms of spectral resolution. This is demonstrated for example by the observed spectra of a moderately extended source like 1E 0102.2-7219. This target is a supernova remnant in the Small Magellanic Cloud with an angular extent of 2. The first and second order spectra of 1E 0102.2-7219 are shown in Figs. 93, 94 and 95.