A new prototype adaptive deformable mirror for future AO-systems is presented that consists of a thin continuous
membrane on which push-pull actuators impose out-of-plane displacements. Each actuator has ±10μm stroke,
nanometer resolution and only mW's heat dissipation. The mirror's modular design makes the mechanics,
electronics and control system extendable towards large numbers of actuators. Models of the mirror are derived
that are validated using influence and transfer function measurements. First results of a prototype with 427
actuators are also presented.
Future large optical telescopes require adaptive optics (AO) systems whose deformable mirrors (DM) have ever
more degrees of freedom. This paper describes advances that are made in a project aimed to design a new AO
system that is extendible to meet tomorrow's specifications. Advances on the mechanical design are reported in
a companion paper [6272-75], whereas this paper discusses the controller design aspects.
The numerical complexity of controller designs often used for AO scales with the fourth power in the diameter
of the telescope's primary mirror. For future large telescopes this will undoubtedly become a critical aspect.
This paper demonstrates the feasibility of solving this issue with a distributed controller design. A distributed
framework will be introduced in which each actuator has a separate processor that can communicate with a few
direct neighbors. First, the DM will be modeled and shown to be compatible with the framework. Then, adaptive
turbulence models that fit the framework will be shown to adequately capture the spatio-temporal behavior of
the atmospheric disturbance, constituting a first step towards a distributed optimal control. Finally, the wavefront
reconstruction step is fitted into the distributed framework such that the computational complexity for
each processor increases only linearly with the telescope diameter.
In the design of a large adaptive deformable membrane mirror, variable reluctance actuators are used. These consist of a
closed magnetic circuit in which a strong permanent magnet provides a static magnetic force on a ferromagnetic core
which is suspended in a membrane. By applying a current through the coil which is situated around the magnet, this force
is influenced, providing movement of the ferromagnetic core. This movement is transferred via a rod imposing the out-of-plane
displacements in the reflective deformable membrane. In the actuator design a match is made between the negative
stiffness of the magnet and the positive stiffness of the membrane suspension. If the locality of the influence functions,
mirror modes as well as force and power dissipation are taken into account, a resonance frequency of 1500 Hz and an
overall stiffness of 1000 N/m for the actuators is needed. The actuators are fabricated and the dynamic response tested in a
dedicated setup. The Bode diagram shows a first eigenfrequency of 950 Hz. This is due to a lower magnetic force than
expected. A Helmholtz coil setup was designed to measure the differences in a large set of permanent magnets. With the
same setup the 2nd quadrant of the B-H curve is reconstructed by stacking of the magnets and using the demagnetization
factor. It is shown that the values for Hc and Br of the magnets are indeed lower than the values used for the initial design.
New actuators, with increased magnet thickness, are designed and currently fabricated.