2.2 Design of an AO system
The task of the AO system is to restore and maintain sufficient phase coherence across the
telescope aperture to enable formation of a diffraction limited core. The goal is to design a well
performing AO system that achieves high Strehl and, hence, approaches the performance of the
ideal telescope. Figure 5 shows arrangement of the basic components of an AO system, which
- a wavefront sensor (WFS) that measures the wavefront aberrations. The wavefront is typically
sensed indirectly by, e.g., measuring wavefront gradients at a number of positions in a pupil
plane. An example of such a WFS is the Shack–Hartmann WFS (SHWFS), which is shown in
Figure 5 and will be explained in detail in Section 4.
- a wavefront corrector, such as a deformable mirror (DM) that corrects phase aberrations by
introducing the correct, compensating optical path difference. The mirror surface is deformed
by actuators located at the back of the thin mirror substrate also referred to as faceplate.
- a reconstructor, or more generally, a processing unit that computes actuator commands (e.g.,
voltages) from the WFS information. The processor unit typically also implements a closed-loop
servo algorithm for driving the DM in the most effective way.
Figure 5: Principle of adaptive optics. The main adaptive optics components are the deformable
mirror, the wavefront sensor and a control system that includes a wavefront reconstructor. A beam
splitter sends a small fraction of the light to the wavefront sensor while most of the light is distributed
to the science instrument(s) (courtesy of Claire Max, Center for Adaptive Optics, UC Santa Cruz).
Different approaches to solar wavefront sensing and different implementations for wavefront correctors
will be discussed briefly in Section 3 in the context of the history of solar AO development. More
information can be found in textbooks and other relevant literature (e.g., Proc. SPIE). As a general
comment it is noted that the desire to achieve high Strehl ratio leads directly to a requirement for high
order correction, meaning that the wavefront aberrations have to be sampled with high density. A similarly
large number of DM actuators is required to fit the incoming wavefront with high fidelity. Furthermore, the
temporal bandwidth of the AO system has to be sufficient with respect to the seeing time constant. The
number of corrective elements or degrees-of-freedom (DOF) of an AO system is roughly . A
more detailed analysis will be performed in the context of developing an wavefront error budget