Hartmann test

A way of checking the accuracy of the shape of a large mirror (or lens). Light from a point source near the center of curvature is reflected from the mirror (or transmitted through then lens), over which a screen pierced with hundreds of holes is placed. The light then reconverges and is intercepted by a photographic plate located just inside or outside the focus. The image records aberrations and can be analyzed to yield the objective's figure. The test is named after the German astronomer Johannes Hartmann (1865–1936).

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