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We describe measurements of the mirror vignetting in the XMM-NewtonObservatory made in-orbit, using observations of SNR G21.5-09 and SNR  3C58 with the EPIC imaging cameras. The instrument features that complicatethese measurements are briefly described. We show the spatial and energydependences of measured vignetting, outlining assumptions made in deriving theeventual agreement between simulation and measurement. Alternate methods toconfirm these are described, including an assessment of source elongation withoff-axis angle, the surface brightness distribution of the diffuse X-raybackground, and the consistency of Coma cluster emission at different positionangles. A synthesis of these measurements leads to a change in the XMMcalibration data base, for the optical axis of two of the three telescopes, byin excess of 1 arcminute. This has a small but measureable effect on theassumed spectral responses of the cameras for on-axis targets.Comment: Accepted by Experimental Astronomy. 26 pages, 18 figure

In-Orbit Vignetting Calibrations of XMM-Newton Telescopes