Program: GS-2013A-Q-2

We are conducting a program to achieve compelling and robust cosmographic measurements from five gravitationally lensed quasars with exquisite time-delays and HST images. We aim with this analysis to measure H_0 with an accuracy better than 3.8%, resulting in constraints on cosmological parameters comparable to the one from current BAO and SN work when combined with CMB data. To reach this goal, we need to track any source of errors. The analysis of the first two systems has shown that H_0 can be derived from a single lens with better than 7% accuracy, and that the main source of uncertainty come from the lens environment. In this proposal, we propose to obtain multi-object spectroscopy of the galaxies in the field of two time-delays lenses in order to measure their redshift (spread over 2013A/B). This will pin down the error from the environment to a few percent, allowing us to deliver state of the art constraints on cosmological parameters.