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 1 \sigma 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 signal-to-noise 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 .