Program: GN-2017A-Q-39

Measuring cosmological parameters is a main goal of modern cosmology. The tension between low- redshift measurements of the Hubble constant (H0) and that derived from the CMB under a flat LCDM model either implies new physics or the presence of systematic errors. We are undertaking a comprehensive effort to measure accurate cosmological parameters using time delays in gravitationally lensed quasars. Our published results from three systems achieve 3.8% precision on H0 and cosmological constraints comparable to current BAO/SNe studies. Aiming to achieve 2% precision on H0 (comparable to Cepheids), we are expanding the sample to nine lenses. We have obtained time delays and deep HST imaging for the full sample, but we still need to track systematic uncertainties due to perturbations by mass along the line of sight to four of these lenses (two visible in 2017A). We request wide-field griK imaging to infer photometric redshifts and stellar masses of galaxies in four fields that currently lack such data. Our work has shown that without these data, our H0 measurement will be systematics-limited. This imaging will pin down the lensing effect of the line-of-sight structures, allowing us to reach 2% precision on H0 whilst keeping systematic uncertainty below 1%.