The Shack interferometer is a simple and effective device to test optical surfaces in reflection and optical systems in transmission. An essential setup on a reduced scale with a minimum number of components is presented, suited to gain familiarity and practice with optical testing in a laboratory course for undergraduate students. The basic layout and the optics principles are discussed, fringe processing is detailed and the calculation of the point spread function in the presence of typical aberrations is demonstrated. (Contains 6 figures and 8 footnotes.)