  We demonstrate that the measurement of an azimuthal angle asymmetry in deeply
virtual Compton scattering (DVCS) at HERA energies, is experimentally feasible
and allows one to determine for the first time the ratio $\eta$, of the real to
imaginary part of the DIS amplitude. We further show that such a measurement
would discriminate between different scenarios of the energy dependence of
$F_2(x,Q^2)$ at energies beyond those reachable at HERA.
