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Chapter 4:
Low-Order Wavefront Compensation
A wavefront sensor is used to evaluate the optical-path-length differences or phase variations in an aberrated wavefront compared to a reference wavefront. Wavefront compensation requires an optical element that can change the effective optical path length over the wavefront. In general, wavefront sensors measure the wavefront variation with little regard for how the information is used; however, the optical element used for compensation needs to be well matched to the aberrated wavefront shape, or at least to the aberration being corrected. Image-stabilization systems compensate for low-order aberrations, typically tip and tilt and sometimes piston and defocus. To restore an aberrated wavefront to its pristine form, all the optical-path-length variations in the wavefront must be compensated; both high- and low-order modes. Wavefront compensation devices are often designed specifically for either high- or low-order correction, so the choice of the optical device defines the application of the particular system. Mirrors are the most common optical devices that can compensate for the tilt in a wavefront, or even change its direction for beam-steering applications. Some static mirrors can also be used to distort images, by placing on them a static shape or figure, and so providing a single phase change over the image. Such a mirror can be used to compensate for static aberrations by inducing a fixed correction. The shape of wavefronts passing through a turbulent medium will continue to evolve and change over time and distance. Low-order, slow varying aberrations are the most easily compensated; many optical devices can be used for this purpose. In general, these devices are mirrors that can be changed in angle and follow changes in the optical path length over the wavefront. The most challenging problems in optical compensation are related to wavefronts that undergo rapid changes, such as those introduced by a very turbulent atmosphere. These wavefronts can have wide variation in the shape of the wavefront over time on scales of a few milliseconds or less. The wavefront sensor and wavefront compensator must be able to operate at high speeds and accurately adjust to the changing wavefront shapes.
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