This paper presents a unimorph deformable mirror intended to be used as secondary corrector in space telescopes. The deformable mirror consists of a single-crystal silicon wafer (76.2 mm diameter, 500 μm thickness) covered with an optical coating on the front side and an array of 25 independent piezoelectric transducer (PZT) actuators acting in d31 mode on the back side. The mirror is mounted on an isostatic support with three position linear actuators controlling the rigid-body motion. The first part of the paper presents the experimental results obtained with the manufactured prototype. The mirror was tested in terms of root mean square (RMS) wavefront error, open-loop long-term stability, voltage budget for active control, rigid-body actuation, reflectivity, and dynamic response. The prototype is fully compliant with the requirements set by the European Space Agency (ESA). The second part of the paper, purely based on numerical simulations, presents a robust way to face thermal distortion, inherent to unimorph architecture.