Deformable mirrors for the EST testbed: testing and characterization

Resumen

The European Solar Telescope is a 4-m class solar telescope planned to include a multi-conjugate adaptive optics system (MCAO) for high spatial resolution over a circular field of view with 60” diameter. MCAO systems constitute a very complex and still not mature technology in both optical and control terms. To deal with this issue, a downscaled testbed demonstrator of solar MCAO has been designed and implemented to guide the final proposal for the EST MCAO system. The testbed setup allows up to three deformable mirrors: one deformable mirror with 820 actuators conjugated to the ground layer (DM0), and two deformable mirrors of 468 actuators conjugated to altitude layers (DM1 and DM2). The three deformable mirrors were manufactured by ALPAO and delivered to IAC in September 2020. The main objective of this contribution is to present the different tests and results conducted at the IAC laboratories to characterize and verify the requirements of the three DMs. The laboratory tests included the assessment of best flat performance in closed-loop, including the effect of small temperature changes on them. Different measurements to quantify the peak-to-valley stroke, the interactuator stroke and the mechanical coupling in adjacent actuators were performed. The individual influence functions and their homogeneity through the DM surface were also characterized. Relevant features for the closed-loop DM performance such as linearity, hysteresis and sensitivity were also evaluated. A Zygo interferometer was used for the measurements. The functioning and specifications of the mirrors were verified and validated for the EST testbed.