  ACS observations of massive lensing clusters permit an order of magnitude
increase in the numbers of multiply-lensed background galaxies identified
behind a given cluster. We have developed a code to take the pixels belonging
to any given image and generate counter-images with full resolution, so that
multiple systems are convincingly and exhaustively identified. Over 130 images
of 35 multiply lensed galaxies are found behind A1689, including many radial
arcs and also tiny counter-images projected on the center of mass. The derived
mass profile is found to flatten steadily towards the center, like an NFW
profile, with a mean slope d(log(Sigma))/d(log(r)) ~ -0.55 +/- 0.1, over the
range r<250 kpc/h, which is somewhat steeper than predicted for such a massive
halo. We also clearly see the expected geometric increase of bend angles with
redshift, however, given the low redshift of A1689, z=0.18, the dependence on
cosmological parameters is weak, but using higher redshift clusters from our
GTO program we may derive a more competitive constraint.
