Abstract: A method for determining a radiation imaging system focus error, that employs a novel focus sensor. In a preferred embodiment, the novel focus sensor comprises two beamsplitters, a spacer, and three detectors comprising CCD arrays. The three detectors are constrained to satisfy radiation path length specifications, with respect to the beamsplitters and spacer. In the method, the focus sensor may be aligned in an unused portion of an imaging system radiation field. The detectors simultaneously image the same scene, sampling it at three different focus positions. The sampling action becomes a basis for generating a parabolic curve; a maximum of the parabolic curve is a measure of the imaging system focus error.