Active optics system implemented in the primary mirror support of the Daniel K. Inouye Solar Telescope (DKIST) solar telescope : design, analysis and tests

AMOS has developed a hybrid active optics system that combines hydraulic and pneumatic properties of actuators to support a 4-m primary mirror. The mirror is part of the Daniel K. Inouye Solar Telescope (DKIST, formerly the Advanced Technology Solar Telescope) that is installed on top of the Haleakala volcano in Hawaii. The mirror support design is driven by the needs of (1) minimizing the support-induced mirror distortions under telescope operating conditions, (2) shaping the mirror surface to the desired profile, and (3) providing a high stiffness against wind loads. In order to fulfill these requirements, AMOS proposes an innovative support design that consist of 118 axial actuators and 24 lateral actuators. The axial support is based on coupled hydraulic and pneumatic actuators. The hydraulic part is a passive system whose main function is to support the mirror weight with a high stiffness. The pneumatic part is actively controlled so as to compensate for low-order pupil aberrations that are generated by the mirror support itself or by any other elements in the telescope optical chain. The lateral actuators have only a pneumatic part whose supporting force is adjusted to compensate for the mirror lateral weight. The performances of the support and its adequacy with the requirements are assessed with the support of a comprehensive analysis loop involving finite-element, thermal and optical modellings, and finally validated with a dedicated test campaign.