A vector representation of the formation of fringe profiles in a Fizeau (wedge) interferometer shows that, under certain conditions, off-axis illumination may lead to fringe sharpening. The incident angle is such that the beam is first reflected toward the apex of the wedge and, following a certain controllable number of reflections, is reflected away from the apex. A typical example is shown.
P. Langenbeck, "Fizeau Interferometer—Fringe Sharpening," Appl. Opt. 9, 2053-2058 (1970)