Saturn Deformable Mirrors for Adaptive Optic Applications

Saturn is a membrane deformable mirror suitable for adaptive optics. It has an excellent capabilities by virtue of which it can push and pull the conducting membrane in it.

Saturn comprises a thin reflective and conducting membrane that exists between two actuators. One of the actuators is transparent and is placed on top of the membrane. The structure acts as a set of capacitors that have a common membrane. A typical membrane deformable mirror is illustrated in the following figure.

PAN Deformable Mirrors vs. Saturn Deformable Mirrors

The difference between a normal PAN mirror and the Saturn deformable mirror is that the PAN mirror has actuators only behind the membrane while the Saturn mirror has actuators both behind and in the front of the membrane. The actuators in the front of the membrane are transparent, which gives Saturn its unique capability of being able to push and pull the membrane from its rest position.

As the membrane is fixed only at the borders, an electrostatic pressure is created by applying a voltage to the actuators. This pressure is enough to deform the membrane as per the formula:

where
d is the distance between the membrane and the actuator
ε₀ is the dielectric constant.

The pressure created pulls the membrane to the j-th actuator thus resulting in a local curvature of the membrane. The membrane can be deformed so as to produce desired shapes by modulating the actuator voltages and taking into account their disposition above the membrane. The shape created by one single actuator is known as influence function. The membrane shape is influenced by the set of voltages according to the Poisson equation:

where
M(x,y) is the membrane shape
T is the membrane mechanical tension
p is the electrostatic pressure

Advantages

When compared to other common devices for adaptive optics such as bimorph mirrors or thermal mirrors, they have the following advantages:

• Low cost
• Large dynamic behaviour
• Achromaticity
• No hysteresis
• Relatively high optical load
• Good performance in aberrations generation
• Low power consumption.

The drawback of other devices is limited maximum stroke and high correlation within the electrodes. In terms of these drawbacks, the Saturn mirror is an innovative step forward and has the capability to pull or push the membrane.

Optical Performance

The influence functions, that is, the deformation generated by each actuator, are illustrated in the following figure. These when arranged in a matrix will form the influence matrix. The inverted influence matrix can help generate the required wavefront.

Within a Europa P Kit, the Saturn mirror is supplied with a measured influence matrix. However, for better results, the influence matrix should be measured while placing the mirror in the optical setup.

Characterization of ITO Coating

The ITO glass has an ad hoc anti-reflection coating. The graph’s reflection is depicted as follows:

This information has been sourced, reviewed and adapted from materials provided by Adaptica.

For more information on this source, please visit Adaptica.