With the development of the thin-disk laser, thermal lensing issues have been drastically reduced. However, fundamental mode operation at high output powers is still limited by severe efficiency reductions due to diffraction losses introduced by the temperature gradient at the boundary of the pump spot. To countervail these aspherical wavefront deformations, high-power laser mirrors featuring deformable surfaces have been developed. The surface of these mirrors resembles the shape of the thermally induced change of optical path length in the pumped laser crystal. The magnitude of the surface deformation can be actively controlled to provide optimal compensation at different power levels.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.