Assumptions

• The method outlined above is valid, if the vignetting effect is constant within one PSF (this is OK for XMM) and remains the same for energies differing by the order of (this is OK for XMM except at the very lowest energies).
• The current version ignores the PATTERN value. This results in vignetting being computed for PATTERN=0. This is all right since the pattern distribution is approximately the same for all CCDs.