Background sky obscuration by cluster galaxies as a source of systematic error for weak lensing

Lensing magnification and stacked shear measurements of galaxy clusters rely on measuring the density of background galaxies behind the clusters. The most common ways of measuring this quantity ignore the fact that some fraction of the sky is obscured by the cluster galaxies themselves, reducing the area in which background galaxies can be observed. We discuss the size of this effect in the Sloan Digital Sky Survey (SDSS) and the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), finding a minimum 1 per cent effect at 0.1 h^-1 Mpc from the centres of clusters in SDSS; the effect is an order of magnitude higher in CFHTLenS. The resulting biases on cluster mass and concentration measurements are of the same order as the size of the obscuration effect, which is below the statistical errors for cluster lensing in SDSS but likely exceeds them for CFHTLenS. We also forecast the impact of this systematic error on cluster mass and magnification measurements in several upcoming surveys, and find that it typically exceeds the statistical errors. We conclude that future surveys must account for this effect in stacked lensing and magnification measurements in order to avoid being dominated by systematic error.