A fresh look at diffractive J/psi photoproduction at HERA, with predictions for THERA.

We quantify perturbative and non-perturbative QCD effects in the exclusive J/psi -photoproduction cross section, and in the shrinkage of the different ial cross section with respect to momentum transfer, t. We predict that in the high energy THERA region there will always be a significant contributio n to this process that rises quickly with energy. This implies that the tam ing of the rise of the cross section with energy, due to both the expansion of spatially-small fluctuations in the photon and to higher twist effects, is rather gradual.