090051

Measurements of the growth of cosmic structure comma based on the number density and mass distribution of galaxy clusters as a function of redshift comma place powerful constraints on cosmological models. As the only all-sky Sunyaev-Zel.dovich openParSZclosePar selected cluster sample comma Planck.s PSZ2 sample has a uniquely powerful role. Despite this comma follow-up X-ray observations of the z>0.4 PSZ2 sample are incomplete comma with archival coverage biased towards the X-ray brightest systems comma which is problematic for most cosmological studies. Completing the X-ray follow-up coverage comma and thus providing low-scatter X-ray mass proxies for every cluster at z>0.4 comma will enable significantly improved comma more robust cosmological constraints to be obtained.