The atmospheric wavefront can be sensed with the Shack-Hartman method (Roggeman et al., 1997), by putting a microlens array into the pupil plane (P1) and a specific imaging lens in the GR wheel, (cf. Fig. 1),
Another technique could be applied to images of the pupil (Avila et al., 1997) to measure the wind speed and the altitude of the turbulence layers.