Velocity Dispersion of the Gravitational Lens 0957+561

0957+561 is the first gravitational lens system to be discovered and firstfor which a time delay was measured. Because the system is unusually rich inobservables such as image positions, fluxes, VLBI structure, and polarizationit has been modelled quite extensively. However, since it resides in a clusterproviding substantial convergence, the velocity dispersion of the lensinggalaxy is an important component to models. We have measured this dispersion tobe 288+/-9 kms (1-sigma), which implies a Hubble constant of 72+/-7 kms/Mpc(1-sigma) according to the preferred FGS model of Grogin and Narayan, and70+/-7 kms/Mpc (1-sigma) using the SPLS model. The unknown velocity dispersionanisotropy of the central galaxy produces an additional uncertainty of perhaps15% or more. We see no variation in dispersion with position to a radius of+/-3". In addition to a galaxy from the background cluster at z=0.5, our slitserendiptously picked up a galaxy at z = 0.448, and two galaxies which may bepart of a background cluster at a redshift of z=0.91.Comment: 13 pages, AASTeX Latex, including 5 Postscript figures, submitted to Astrophysical Journal, June 25, 199