Hubble, chandra and keck constraints on massive galaxy clusters at z=0.2 and z=0.5

I present recent observations from two Hubble Space Telescope(HST)/ACS programs that target the most X–ray luminous and thus (presumably) most massive galaxy clusters at z=0.5 – the highest redshift at which complete, well–defined samples of such rare systems are available. The first program (GO:9836, PI: R.S. Ellis) exploits a huge mosaic of 41 ACS pointings spanning a 10 Mpc region centered on MS0451-03. This is the largest contiguous space–based image of a cluster to date. I describe a preliminary weak–lensing analysis and a new Keck/DEIMOS redshift catalog of 1000 galaxies in this field. The second program (GO:9722, PI: H. Ebeling) studies the core regions of the twelve most luminous clusters at z≥0.5 from the MAssive Cluster Survey (MACS; Ebeling et al. 2001). Multi–color ACS observations in combination with recent Keck/LRIS spectroscopy of gravitational arcs constrain the distribution of mass in the cluster cores, thus laying the foundation for detailed multi–diagnostic (lensing, X–ray, near–infrared, SZE) investigation of this sample. For example, it is of particular interest to explore how the structure and state of relaxation of massive clusters evolved between this sample at z≥0.5 that measured by Smith et al. (2004, astro–ph/0403588) at z=0.2.