We extract the S-matrix element for dipole-proton scattering using the data
on diffractive electroproduction of vector mesons at HERA. By considering the
full t dependence of this process we are able to reliably unfold the profile of
the S-matrix for impact parameter values b>0.3 fm. We show that the results
depend only weakly on the choice of the form for the vector meson wave
function. We relate this result to the discussion about possible saturation
effects at HERA.Comment: 15 pages, 6 figure
