Program: GN-2013A-Q-72

We have recently shown that a single gravitational lens with high-quality data can place constraints on H_0, w, and Omega_k that are comparable to constraints from other cosmological probes. Furthermore, since each lens makes an independent measurement of these quantities, a significant improvement in precision can be obtained by increasing the sample size, while also providing internal checks on systematics. This establishes lensing as a powerful and independent tool for determining cosmography, in preparation for the hundreds of time-delay lenses that will be discovered by future surveys. The largest current contributor to the error budget from lensing is uncertainty about the line-of-sight mass distribution. The proposed observations are critical for reducing these uncertainties by providing redshifts and stellar masses for galaxies in the light cones of the target lens systems, and will do so with only a modest investment of observing time. We have an approved Gemini program (GN-2012B-Q-11) that is providing these observations on three high-quality lens systems observable in the fall semester. This proposal requests time for the one remaining high-quality lens system that is accessible from Gemini North.