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In order to use strong gravitational lens time delays to measure precise andaccurate cosmological parameters the effects of mass along the line of sightmust be taken into account. We present a method to achieve this by constrainingthe probability distribution function of the effective line of sightconvergence k_ext. The method is based on matching the observed overdensity inthe weighted number of galaxies to that found in mock catalogs with k_extobtained by ray-tracing through structure formation simulations. We exploreweighting schemes based on projected distance, mass, luminosity, and redshift.This additional information reduces the uncertainty of k_ext from sigma_k $0.06to ~0.04 for very overdense lines of sight like that of the system B1608+656.For more common lines of sight, sigma_k is reduced to ~<0.03, corresponding toan uncertainty of ~<3% on distance. This uncertainty has comparable effects oncosmological parameters to that arising from the mass model of the deflectorand its immediate environment. Photometric redshifts based on g, r, i and Kphotometries are sufficient to constrain k_ext almost as well as withspectroscopic redshifts. As an illustration, we apply our method to the systemB1608+656. Our most reliable k_ext estimator gives sigma_k=0.047 down from0.065 using only galaxy counts. Although deeper multi-band observations of thefield of B1608+656 are necessary to obtain a more precise estimate, we concludethat griK photometry, in addition to spectroscopy to characterize the immediateenvironment, is an effective way to increase the precision of time-delaycosmography.

Improving the precision of time-delay cosmography with observations of galaxies along the line of sight