The Hartmann-Shack wavefront sensor is a device used in adaptive optics systems to measure the wavefront distortions of incoming light. It consists of a lenslet array that creates a grid of small spots on a detector. The displacement of these spots from their ideal positions provides information about the wavefront distortions, which can then be corrected by the adaptive optics system. The Hartmann-Shack wavefront sensor is named after its inventors, Robert Shack and Brenton Hartmann.