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The performance of an image-stabilization system is commonly evaluated by measuring the difference or residual error between the corrected and the true wavefront. The residual error is a combination of the errors introduced by the individual components that make up the complete system. In evaluating the performance of an image-stabilization system, it is convenient to assume that each of the individual component errors is random and uncorrelated, so that they can be combined as a simple summation. This is not precisely correct; however, it is common to treat them this way to avoid the additional complications from correlation. The tool most commonly used to evaluate the system performance is the Strehl ratio, first used by K. Strehl in 1895. The Strehl ratio measures the height of the Airy function compared to its ideal height. This is a very sensitive measure of the performance of the optical system, because even small changes cause a degradation of the Strehl ratio. For comparison, an optical system operating at the Rayleigh limit will have a Strehl ratio of 0.8 (Smith 2000). This chapter provides an introduction to image structure, the Strehl ratio, and how it applies to image stabilization and comparison of system performance.
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