It is generally assumed that when a plane wave enters an afocal system, the wave comes out plane from the system. For instruments, like high-precision interferometers using afocal combinations, the wavefront after the second lens has opposite curvature to that which would be produced by Fresnel diffraction, if only a stop is placed at the appropiate place near the first lens.
DANIEL MALACARA, "Diffraction of a Plane Wavefront in an Afocal System," J. Opt. Soc. Am. 56, 14-14 (1966)