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Byline: D. H. Lumb (1), A. Finoguenov (2), R. Saxton (3), B. Aschenbach (2), P. Gondoin (1), M. Kirsch (4), I. M. Stewart (3) Keywords: CCDs; X-ray astronomy; X-ray detectors; X-ray mirrors; XMM-Newton Abstract: We describe measurements of the mirror vignetting in the XMM-Newton Observatory made in-orbit, using observations of SNR G21.5-09 and SNR 3C58 with the EPIC imaging cameras. The instrument features that complicate these measurements are briefly described. We show the spatial and energy dependences of measured vignetting, outlining assumptions made in deriving the eventual agreement between simulation and measurement. Alternate methods to confirm these are described, including an assessment of source elongation with off-axis angle, the surface brightness distribution of the diffuse X-ray background, and the consistency of Coma cluster emission at different position angles. A synthesis of these measurements leads to a change in the XMM calibration data base, for the optical axis of two of the three telescopes, by in excess of 1 arcmin. This has a small but measureable effect on the assumed spectral responses of the cameras for on-axis targets. Author Affiliation: (1) Adv. Concepts &amp Science Payloads Office, European Space Agency, ESTEC, 2200AG, Noordwijk, the Netherlands (2) Max-Planck Institut fur Extraterrestrische Physik, Giessenbachstrasse 1, D-85748, Garching, Germany (3) XMM Survey Science Centre, Dept. of Physics &amp Astronomy, Leicester University, Leicester, LE1 7RH, U.K (4) XMM Science Ops. Centre, European Space Agency, Apartado, P.O. Box 50727, 28080, Madrid, Spain Article History: Registration Date: 15/09/2004

Byline: D. H. Lumb (1) Keywords: CCD detectors; CHANDRA; detector simulation; XMM; X-ray astronomy Abstract: Simulations of the XMM EPIC instruments are presented. They suggest the correct operating mode must be chosen to ensure that spectral analysis of the data is not compromised by 'pile-up' effects. The performanceis contrasted with the CHANDRA AXAF CCD Imaging Spectrometer, showing how the higher effective area but better over-sampling of its mirror response function in XMM, impacts this pile-up criteria. Targetsexceeding a flux of a few 10.sup.-12 ergs cm.sup.-2 s.sup.-1 will becompromised for spectral analysis in these CCD instruments, unlesscare is taken in defining correct instrument configurations. Somesimple methods of mitigating the effects are described, but this featureof X-ray pile-up warrants careful attention in calibration. Author Affiliation: (1) Space Science Department, European Space Agency ESTEC, Noordwijk, The Netherlands Article History: Registration Date: 07/10/2004