Active optics methods for exoplanet direct imaging Stress polishing of supersmooth aspherics for VLT-SPHERE planet finder

Context. The next generation of exoplanet hunters will be targeting hot Jupiter-like exoplanets orbiting around nearby stars through direct imaging. The high contrast needed for such planet finders requires optical surfaces free of high spatial frequency ripples that might remain in the post-coronagraphic image as quasi-static speckles. Aims. We report results on the manufacturing of three supersmooth aspherical mirrors for the ESO/VLT-SPHERE instrument. The excellent optical quality obtained will allow the future planet hunter to increase the level of achievable contrast by a strong reduction of the noise level and residual quasi-static speckles on the image plane. Methods. The stress polishing method used on these mirrors is well suited to superpolishing aspheric components for astronomy. The main advantage of this technique is the very high optical quality obtained either on the form errors or on the high spatial frequency errors. Furthermore, the roughness can be decreased to a few angstroms, thanks to the classical polishing with a large pitch tool. Results. Interferograms obtained during polishing and in the final stage show the supersmooth quality of each mirror. Errors in the high spatial frequency range are lower than 4 nm rms WF, thereby avoiding the degradation of the post-coronagraphic image. A comparison of the power spectral density (PSD) obtained by stress polishing and the PSD of the VLT-UT3 primary mirror demonstrates that the contrast capabilities will not be limited by the aspherical mirrors.