  We calculate in the framework of collinear factorization the amplitude for
the photoproduction of a near forward large mass diphoton at leading twist and
next to leading order (NLO) in $\alpha_s$. We demonstrate the validity of
factorization at this order, which was never achieved for such a reaction where
the coefficient function describes a $2 \to 3$ hard process. While the Born
order amplitude was purely imaginary and only probed the $x=\pm \xi$ cross-over
line of generalized parton distributions (GPD) domain, the NLO result contains
both a real and an imaginary part and probes the whole domain of definition of
quark GPDs. The phenomenology of our results for medium (JLab) and higher
energy (EIC) experiments will be developed in a future study.
