Measuring Projection Effects in Optical Cluster Catalogues

The impact of projection effects on optical, photometric galaxy clusterselection has recently been clarified at z virgul 0.1 using extensive spectroscopy ofputative member galaxies. The projected fraction of photometrically associatedgalaxies has been constrained as a function of richness, along with therelations between photometric and spectroscopicallycorrected richness andvelocity dispersion. We propose XMMNewton observations of a complete subsampleof these clusters at high richness to obtain lowscatter Xray mass proxies.These measurements will provide independent, complementary information onprojection effects and, crucially, will allow the scatter ofspectroscopicallycorrected richness with mass to be quantified. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]