090183

The impact of projection effects on optical photometric galaxy cluster selection has recently been clarified at z virgul 0.1 using extensive spectroscopy of putative member galaxies. The projected fraction of photometrically associated galaxies has been constrained as a function of richness along with the relations between photometric and spectroscopically-corrected richness and velocity dispersion. We propose XMM-Newton observations of a complete sub-sample of these clusters at high richness to obtain low-scatter X-ray mass proxies. These measurements will provide independent complementary information on projection effects and crucially will allow the scatter of spectroscopically-corrected richness with mass to be quantified. [truncated! Please see actual data for full text]