Abstract: A deviation measuring device for localizing an infrared radiation source relative to an optical axis, comprising a focalization lens, a linear scanning device, an infrared detection device in the focal plane of the lens and a processing circuit for the output signals of the detection device for delivering signals characteristic of co-ordinates X and Y of the source in a reference system centered on the optical axis, in which the detection device comprises two threadlike detectors non-parallel to the scanning direction and non-parallel therebetween and the scanning device comprises means for delivering pulses defining respectively a time origin and the end of a scanning period and a reference pulse indicative of the arrangement of the threadlike detectors, said pulses being applied to the processing circuit.