How to bias the MMDM?

In the simplest case biasing can be achieved by applying a constant voltage to all actuators of the MMDM. Theoretically under the constant bias the MMDM should take a parabolic shape. In practice, MMDM biased with a constant voltage has figure error of up to 300nm RMS from the nearest perfect parabola (sphere). The bias voltages should be adjusted to make the biased surface perfect. The theoretical value of the bias voltage equals to 0.71 V_max where V_max is the maximum voltage that can be applied to the mirror. This value provides for equal mirror stroke in positive and negative direction from the biased position. In the case of digital control (one byte 0 to 255 per channel) all electrodes should be set to 180. These values should be adjusted to obtain a perfect spherical figure.