Abstract: A twin polygon thermal imager is provided with a compensator for compensating for horizontal distortion in the image produced on a CRT. The distortion compensator applies an amount of correction which is determined, in each horizontal line, to compensate for the particular magnitude of the distortion in that line. The amount of correction needed is determined as a function of the angle between the rotational axis of the polygons and the particular facets thereof producing the particular video line. Compensation is achieved by adjusting the timing and the frequency at which digital video data is read-out from a signal store.