A Hartmann Shack (HS) wavefront sensor is an optical instrument used to characterise the optical quality of an optical system, and more specifically its amount of wavefront aberrations. In the case of the eye, it samples a point source on the retina through a set of lenslets, creating an equal number of spots on a camera sensor. The wavefront aberrations are then calculated through the deviation of the spots from the ideal spots, located on the lenslet camera axis.