A simple geometrical method to measure the aberration of astigmatism present in the wavefront that emerges from a telescope is presented. The method is based on the analysis of the external contour of the image of a slightly defocused star. An expression elliptical edge is obtained, which links the rms value of Z22 to the geometric parameters of the ellipse. This expression is tested as a function of introduced defocus and astigmatism aberration in telescopes of the San Pedro Martir Observatory. It is shown that the method gives comparable results to wavefront tests, being capable of measuring astigmatism values of approximately 60 nm and larger without the need for auxiliary optics.