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 5View Image shows arrangement of the basic components of an AO system, which are:
View Image

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 ( ) D- 2 DOF ≈ r0. A more detailed analysis will be performed in the context of developing an wavefront error budget (Section 6).

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