This paper proposes an optimal-choice criterion for the relative position of the stop and the optical element during electron-beam evaporation to provide a given reflectance distribution over the surface of an optical element.
E´ . S. Putilin, L. A. Gubanova, V. A. Dmitrenko, and L. M. Studenikin, "Synthesizing the deposition conditions of gradient laser mirrors," J. Opt. Technol. 70, 570-572 (2003)