The structure of a wavefront sensor suitable for adaptive optics is described. The sensor scans rapidly between extra focus images with a vibrating membrane mirror acting as a variable curvature active optical element. The membrane mirror and driver are described. The optical system is coupled to an array detector optimized to match the response of a bimorph mirror and directly produces wavefront error signals. Results are presented and compared with other techniques for curvature sensing. Tip-tilt corrections achieved using the wavefront sensor are discussed.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.