Pixellated Lenses and Estimates of H_0 from Time-delay Quasars

The largest source of uncertainty in the determination of H_0 from a multiply-imaged QSO system is the unknown mass distribution in the lensing galaxy. Parametric models severely restrict the shape of the galaxy thereby underestimating errors and biasing the best estimate of H_0. We present a method that explores the whole of the model space allowed by the image observables and a few general properties of galaxies. We describe blind test of the method and then apply it to PG1115+080 and B1608+656 which yield H_0 between 45 and 80 km/s/Mpc at 90% confidence level, with the best estimate of 60 km/s/Mpc.