A method of finding the focal point or figure of a large lens or mirror. In the basic test, a mask with two holes, equally spaced on either side of the centre, is placed over the objective. A star image is photographed inside and outside the focus position, resulting in two pairs of images. Measurement of these images then allows the focal point to be calculated. Repeating this for different zones around the lens or mirror provides information on its figure. It is named after the German astronomer Johannes Franz Hartmann (1865–1936), who devised it in 1900. An improved version, known as the Shack–Hartmann test, replaces the perforated mask with an array of tiny lenses (termed lenslets). These lenslets focus collimated light from the optical system under test onto a CCD detector, creating an array of spots; any aberrations in the optical system will cause the focused spots to be displaced from their expected reference positions. The Shack–Hartmann test is more precise than the Hartmann test and can be used for much fainter sources. It was devised in 1971 by the American optical engineer Roland Vincent Shack (1927– ).
Subjects: Astronomy and Astrophysics.