Characterization of an f/2 freeform active mirror

The construction of the next generation of 40 m-class astronomical telescopes poses an enormous challenge for the design of their instruments and the manufacture of their optics. Optical elements typically increase in both size and number, placing ever more demands on the system manufacturing and alignment tolerances. This challenge can be met by using the wider design space offered by freeform optics, by for instance allowing highly aspherical surfaces. Optical designs incorporating freeform optics can achieve a better performance with fewer components. This also leads to savings in volume and mass and, potentially, cost. This paper describes the characterization of the FAME system (freeform active mirror experiment). The system consists of a thin hydroformed face sheet that is produced to be close to the required surface shape, a highly controllable active array that provides support and the ability to set local curvature of the optical surface and the actuator layout with control electronics that drives the active array. A detailed characterisation of the fully-assembled freeform mirror was carried out with the physical and optical properties determined by coordinate measurements (CMM), laser scanning, spherometry and Fizeau interferometry. The numerical model of the mirror was refined to match the as-built features and to predict the performance more accurately. Each of the 18 actuators was tested individually and the results allow the generation of look-up tables providing the force on the mirror for each actuator setting. The actuators were modelled with finite element analysis and compared to the detailed measurements to develop a closed-loop system simulation. After assembling the actuators in an array, the mirror surface was measured again using interferometry. The influence functions and Eigen-modes were also determined by interferometry and compared to the FEA results.