  Here are presented the experimental results obtained with a ferrofluidic
deformable mirror controlled by electro-magnet actuators. Using a step input
through a single actuator we obtain a steady-state settling time of 100ms;
however, different combinations of overdrive inputs can be used to decrease it
to 25ms. Also, a new technique which consists in laying down an elastomer
membrane, coated with an aluminum film, on the ferrofluid is discussed. By
adding the membrane on the ferrofluid, it further decreases the time response
by a factor of 2. Furthermore, the thin aluminum layer improves the
reflectivity of the mirror. Finally, using the membrane and the overdrive
techniques combined, the time response is improved by a factor of 20. Numerical
simulations show that ferrofluidic mirrors using membranes and improved
electronics should reach settling times of the order of a millisecond.
Presumably even lower settling times could be possible.
