The three deformable mirrors of the EST Multi-Conjugated Optics test bench have been characterized in the lab, in preparation for the integration of all the optical elements.
Figure 1. One of the three deformable mirrors of the EST MCAO testbench, manufactured by ALPAO.
The European Solar Telescope (EST) will include Multi-Conjugated Adaptive Optics (MCAO) to correct the blurring in the images introduced by the Earth’s atmosphere. The difference between classical Adaptive Optics and a MCAO system is that the later uses several deformable mirrors (DM) to achieve image correction in a wider field of view (FoV). To provide maximum spatial resolution over a square FoV of 40”x40”, the EST design includes 5 DMs.
There is no MCAO system working regularly in visible wavelengths, nor in solar neither in nighttime telescopes. To study and evaluate in a controlled environment the novel solutions developed for EST, an MCAO bench demonstrator with three DMs is being developed at IAC within SOLARNET WP7. During 2020 most of the testbench hardware has been received, including 3 ALPAO DMs: one with 820 actuators and two with 468 actuators. These DMs are based on a thin silvercoated optical surface with magnetic actuators attached to it. The actuators deform the optical membrane and compensate for the atmospheric aberration, providing a nearly flat wavefront in the science instruments.
Figure 2. Results of flattening measurements with the Zygo interferometer.
The characterization of the DMs was done last November by EST engineers. These tests were performed in the IAC optical laboratory using a Zygo Fizeau interferometer. The functioning and the specifications of the mirrors have been verified including the validation of parameters such as best flat figures (with surface flatness better than 10nm rms), tip-tilt stroke after flattening (larger than 10 μm peak-to-valley), defocus stroke (larger than 8 μm peakto-valley) or linearity (better than 96%). The next step will be integrating all the optical elements in the bench.