To achieve its full diffraction limit in the infrared, the primary mirror of the Keck telescope (now telescopes) must be properly phased: The steps or piston errors between the individual mirror segments must be reduced to less than 100 nm. We accomplish this with a wave optics variation of the Shack-Hartmann test, in which the signal is not the centroid but rather the degree of coherence of the individual subimages, Using filters with a variety of coherence lengths, we can capture segments with initial piston errors as large as +/- 30 mu m and reduce these to 30 nm-a dynamic range of 3 orders of magnitude. Segment aberrations contribute substantially to the residual errors of similar to 75 nm. (C) 1998 Optical Society of America. [References: 17]
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