Deformable membrane mirror with high actuator density and distributed control

Progress on the construction of a deformable mirror with 427 actuators is presented. The mirror consists of a thin continuous membrane on which actuators impose the out-of-plane displacements. Low voltage electro-magnetical push pull actuators are used. The stiffness of the actuator is chosen high enough to avoid mechanical resonances below 1 kHz and to avoid large coupling, but still low enough to be called a soft actuator and to limit the power dissipation to a few mWs per actuator. The actuators are arranged in 7 hexagonal grid modules of 61 actuators. With a pitch of 6 mm, this results in a 48 mm wide hexagonal plate. Each grid has its own 61 channel, 14 bits driving electronics. The drive electronics are designed for low power dissipation and make use of Pulse Width Modulation. A multi-drop LVDS connection provides up to 16 modules with a 1 kHz update rate. Both communication and power is provided through this single cable. In the prototype 7 of these modules are placed on a stable and stiff reference support structure. The modular design makes it possible to extend the mirror to very large numbers of actuators. The deformable mirror by its local influence functions suits the use of distributed control algorithms. A companion paper will report on the control algorithm developments. The 427-channel prototype is to be implemented in the existing breadboard setup at TNO Science and Industry together with a wavefront emulator, wavefront sensor and distributed control algorithm.