A variant of the Cassegrain telescope, with a concave ellipsoidal primary mirror and a convex spheroidal secondary mirror. This combination produces an image free from astigmatism, although with greater coma (see Coma, Optical) off-axis. It is particularly suited to observations where good resolving power is more important than wide field of view, such as planetary observation. It was invented independently by the English amateur telescope-maker Horace Edward Stafford Dall (1901–86) and the American Alan R. Kirkham (1909–68).
Subjects: Astronomy and Astrophysics.