Atmospheric turbulence distorts the wavefront of the incoming light from an astronomical object and so limits the ability of a telescope to form perfect images. Adaptive optics is a combination of technologies that enable the correction of the wavefront distortion in real time. Conventional adaptive optics operate like auxiliary instruments and use additional relay optics, which reduce total throughput and introduce extra IR emissivity and polarization. Adaptive secondary mirrors avoid additional optical surfaces by providing the optical correction at an existing telescope surface (the secondary mirror). Previous studies have demonstrated the optical efficacy and mechanical feasibility of performing the adaptive correction in this way. A technique demonstrator is being developed to explore features and capabilities applicable to a large adaptive secondary mirror and to explore manufacturing, assembly/disassembly, calibration, and measurement techniques. The paper describes the design of the demonstrator and its predicted performance.