An electrostatically deformable single crystal mirror comprising a highly conducting thick substrate layer and a highly conducting thin membrane layer separated from the thick layer by an insulator is disclosed. The center of the insulating layer is etched to form a cavity. The outer surface of the membrane layer is polished and coated with a dielectric to form a mirror. A voltage is applied between the membrane and the substrate to cause the membrane to deform. Various embodiments of the invention provide for planar translation of the mirror and for convex/concave deformation of the mirror. Incorporation of a deformation sensor in the deformable mirror allows for deformation measurement. In a particular embodiment of the invention, the deformable mirror is used in a tunable etalon for use in an optical filter.