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We describe redMaPPer, a new red sequence cluster finder specifically designed to make optimal use of ongoing and near-future large photometric surveys. The algorithm has multiple attractive features: (1) it can iteratively self-train the red sequence model based on a minimal spectroscopic training sample, an important feature for high-redshift surveys. (2) It can handle complex masks with varying depth. (3) It produces cluster-appropriate random points to enable large-scale structure studies. (4) All clusters are assigned a full redshift probability distribution P(z). (5) Similarly, clusters can have multiple candidate central galaxies, each with corresponding centering probabilities. (6) The algorithm is parallel and numerically efficient: it can run a Dark Energy Survey-like catalog in ∼500 CPU hours. (7) The algorithm exhibits excellent photometric redshift performance, the richness estimates are tightly correlated with external mass proxies, and the completeness and purity of the corresponding catalogs are superb. We apply the redMaPPer algorithm to ∼10, 000 deg2 of SDSS DR8 data and present the resulting catalog of ∼25,000 clusters over the redshift range z ∊ [0.08, 0.55]. The redMaPPer photometric redshifts are nearly Gaussian, with a scatter σz ≈ 0.006 at z ≈ 0.1, increasing to σz ≈ 0.02 at z ≈ 0.5 due to increased photometric noise near the survey limit. The median value for |Δz|/(1 + z) for the full sample is 0.006. The incidence of projection effects is low (⩽5%). Detailed performance comparisons of the redMaPPer DR8 cluster catalog to X-ray and Sunyaev-Zel'dovich catalogs are presented in a companion paper. [ABSTRACT FROM AUTHOR]

We present a new measurement of the scaling relation between X-ray luminosity and total mass for 17 000 galaxy clusters in the maxBCG cluster sample. Stacking subsamples within fixed ranges of optical richness, N200, we measure the mean 0.1–2.4 keV X-ray luminosity, , from the ROSAT All-Sky Survey. The mean mass, , is measured from weak gravitational lensing of SDSS background galaxies. For , the data are well fitted by a power law, . The slope agrees to within 10 per cent with previous estimates based on X-ray selected catalogues, implying that the covariance in and N200 at a fixed halo mass is not large. The luminosity intercept is 30 per cent, or 2σ, lower than that determined from the X-ray flux-limited sample of Reiprich & Böhringer, assuming hydrostatic equilibrium. This slight difference could arise from a combination of Malmquist bias and/or systematic error in hydrostatic mass estimates, both of which are expected. The intercept agrees with that derived by Stanek et al. using a model for the statistical correspondence between clusters and haloes in a WMAP3 cosmology with power spectrum normalization . Similar exercises applied to future data sets will allow constraints on the covariance among optical and hot gas properties of clusters at a fixed mass. [ABSTRACT FROM AUTHOR]