Large adaptive deformable membrane mirror with high actuator density: design and first prototypes

Abstract

A large adaptive deformable mirror with high actuator density is presented. The DM consists of a thin continuous membrane which acts as the correcting element. A grid of low voltage electro-magnetical push-pull actuators, - located in an actuator plate -, impose out-of-plane displacements in the mirror's membrane. To provide a stable and stiff reference plane for the actuators, a mechanically stable and thermally decoupled honeycomb support structure is added. The design is suited for mirrors up to several hundred mm with an actuator pitch of a few mm. One of the key elements in the design is the actuator grid. Each actuator consists of a closed magnetic circuit in which a strong permanent magnet (PM) attracts a ferromagnetic core. Movement of this core is provided by a low stiffness elastic guiding. A coil surrounds the PM. Both the coil and the PM are connected to the fixed world. By applying a current through the coil, the magnetic force acting on the core can be influenced. This force variation will lead to translation of the ferromagnetic core. This movement is transferred to the reflective mirror surface in a piston-free manner. The design allows for a long total stroke and a large inter actuator stroke. The actuators are produced in arrays which make the design modular and easily extendable. The first actuators and an actuator grid are produced and tested in a dedicated test set-up. This paper describes how relevant actuator properties, such as stiffness and efficiency, can be influenced by the design. The power dissipation in the actuator grid is optimized to a few milliwatts per actuator, thereby avoiding active cooling.