Preliminary design of the Adaptive Secondary Mirror for the European Solar Telescope

This paper describes the preliminary design of the Adaptive Secondary Mirror (ASM) for the European Solar Telescope (EST), as designed by TNO. This ASM will contain 1950 actuators over an optical aperture of 080cm. The mirror-shell of this ASM is concave with a radius of 2.156m and has a thickness of 2.5mm. To cope with the high optical heat-load, the mirror shell will be cooled actively by means of conduction through a small (0.3mm) airgap between the mirror shell and the cooled SiC supporting structure. One of the unique characteristics of this ASM are the highly efficient actuators by TNO, delivering 30N Peak-to -Valley of linear force range within a packaging of 015x4Omm. These actuators are based on the proven electromagnetic actuation principle by TNO but have been significantly redesigned to improve manufacturability and integration. The actuators are interfaced to the mirror shell via a stiff carbon fiber strut assembly and are designed to be replaceable during off-telescope maintenance. For overall alignment of the ASM a hexapod is used, while a faster tip -tilt stage is used to compensate for tip -tilt disturbances up to 20Hz bandwidth. The optical performance of the ASM under various environmental conditions has been extensively studied with Finite -Element-Analysis. To verify the performances of the actuator and the cooling systems, an actuator and thermal- breadboard are being constructed.