  We quantify the uncertainty in weak lensing mass estimates of clusters of
galaxies, caused by distant (uncorrelated) large scale structure along the line
of sight. We find that the effect is fairly small for deep observations
(20<R<26) of massive clusters (sigma=1000 km/s) at intermediate redshifts,
where the bulk of the sources are at high redshifts compared to the cluster
redshift. If the lensing signal is measured out to 1.5 h_{50}^{-1} Mpc the
typical 1sigma relative uncertainty in the mass is about 6%. However, in other
situations the induced uncertainty can be larger. For instance, in the case of
nearby clusters, such as the Coma cluster, background structures introduce a
considerable uncertainty in the mass, limiting the maximum achievable S/N-ratio
to \sim 7, even for deep observations. The noise in the cluster mass estimate
caused by the large scale structure increases with increasing aperture size,
which will also complicate attempts to constrain cluster mass profiles at large
distances from the cluster centre. However, the distant large scale structure
studied here can be considered an additional (statistical) source of error, and
by averaging the results of several clusters the noise is decreased.

Astrophysics