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Optimization and performance of multi-deformable mirror correction on the THD2 bench

High-contrast imaging (HCI) techniques appear like the best solutions to directly characterize the atmosphere of large orbit planets and planetary environments. In the last 20 years, different HCI solutions have been proposed based on coronagraphs. Some of them have been characterized in the laboratory or even on the sky. The optimized performance of these coronagraphs requires a perfect wavefront unreachable without active control of the complete electrical field (phase and amplitude) at the entrance of the instrument. While the correction of the phase aberrations is straight forward using deformable mirrors (DM), correcting amplitude defects is complex and still under study at the laboratory level. The next generation of HCI instrument either for ground-based (PCS instrument for ELT) or space-based (LUVOIR, HabEx) telescopes will require a practical and operational solution for amplitude corrections. The implementation of a DM located at a finite distance from the pupil is a simple solution that has been chosen by most of the projects. There have been only a few investigations on the optimization of the mirror positions for dedicated optical designs. In this paper, we give an intuitive approach that helps defining the best deformable mirror position in an instrument. Then, we describe its application to the THD2 and the performance in the laboratory that reaches a contrast level below 10-8 at distance larger than 6 λ/D.