A wavefront sensor is an instrument for determining an EMR
wave's deformation,
used to control a correcting instrument in an adaptive optics (AO) system.
Types include:
Shack-Hartmann sensor - focuses spots and senses their position.
wavefront curvature sensor - focuses spots and measures relative intensity.
pyramid sensor - splits image four ways, each from a quarter of the aperture, so their positions can be compared.
The deformation of interest consists of the wave lacking coherence
of its phase, i.e., over a cross-section of the passing
EMR, some has its wave peak passing earlier and some later.
If the EMR is collimated, the deformation
consists of it being something other than a plane wave,
and if it is focused to or from a point, the problem consists of
the wave's peak varying in its distance from the focal point.