How well can we determine cluster mass profiles from weak lensing?

Weak gravitational lensing provides a direct way to study the mass distribution of clusters of galaxies at large radii. Unfortunately, large-scale structure along the line of sight also contributes to the lensing signal, and consequently affects the measurements. We quantify the effect of distant uncorrelated large-scale structure on the inferred mass profile of clusters as measured from weak lensing. We consider Navarro, Frenk & White profiles, and find that large-scale structure is a major source of uncertainty for most practical situations, when a model, with mass M-200 and concentration parameter c as free parameters, is fitted to the observations. We find that the best constraints are found for clusters at intermediate redshifts (z approximate to 0.3). For a cluster at z = 0.3, optimal results are obtained when the lensing signal is measured out to 10-15 arcmin. Measurements at larger radii do not improve the accuracy with which the profile can be determined, contrary to what is expected when the contribution from large-scale structure is ignored. The true uncertainties in M-200 and the concentration parameter c are similar to2 times larger than when distant large-scale structure is not included in the error budget.