English: Schematic illustration of a Shack Hartmann wavefront sensor using a so-called lenslet array. The perturbed wavefront coming in at the top (black solid lines) is imaged by an array of small lenses. Each of these lenses only samples a small portion of the full wavefront. The position of the image created by each of these lenses on the camera depends on the local slope of the wavefront (red dashed lines). Thus, by measuring this image displacement Δx for all lenslets, the local wavefront slopes can be obtained and the full wavefront can be reconstructed.