  We describe the concept of splitting spatial frequency perturbations into
some kind of pupil planes wavefront sensors. Further to the existing approach
of dropping higher spatial frequency to suppress aliasing effects (the
so-called spatial filtered Shack-Hartmann), we point out that spatial
frequencies splitting and mixing of these in a proper manner, could be handled
in order to exhibit some practical or fundamental advantages. In this framework
we describe the idea behind such class of concepts and we derive the
relationship useful to determine if, by which extent, and under what kind of
merit function, these devices can overperform existing conventional sensors.
