EXAFS means Extended X-ray Absorption Fine Structure. This CCF describes the detailed absorption lengths of different layers of CCDs in the vicinity of absoption edges to facilitate accurate calculation of quantum efficiencies. Away from the edges, the well-known Henke coefficients are used. There are two notable RGS instrumental absorption features by oxygen and fluorine. Of these oxygen is more important and clearly visible in RGS spectra. Its absorption is modelled as due to a layer of water ice.