Completing Observations of the Highest-Redshift Planck SZ Clusters

Measurements of the growth of cosmic structure, based on the number density andmass distribution of galaxy clusters as a function of redshift, place powerfulconstraints on cosmological models. As the only allsky SunyaevZel.dovich (SZ)selected cluster sample, Planck.s PSZ2 sample has a uniquely powerful role.Despite this, followup Xray observations of the z>0.4 PSZ2 sample areincomplete, with archival coverage biased towards the Xray brightest systems,which is problematic for most cosmological studies. Completing the Xrayfollowup coverage, and thus providing lowscatter Xray massproxies for every cluster at z>0.4, will enable significantly improved,more robust cosmological constraints to be obtained. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]