A microelectromechanical deformable mirror device that is optically addressed and designed to actuate at extremely low-light levels for wavefront error correction is fabricated and theoretically described. The device consists of an optically transparent substrate, a photoconductive detector, a thin-film resistor, and insulating posts that support a mirror. The mirror is suspended over the detector by the insulating posts and is deformed when the detector is illuminated through the substrate. The actuation of the device is theoretically modeled as a capacitor in series with a photoconductor under an external dc bias. Under an external 6.3 V dc bias and when back-illuminated with light at 539 nm, a total mirror deformation of 474.3 nm was obtained and substantiated by numerical modeling. This represents the highest actuation sensitivity to date that results in mirror deflection values in hundreds of nanometers.