Most infrared optical materials exhibit a relatively strong dependence on refractive index on temperature. This dependence is about one or two orders of magnitude more severe than for the optical glasses used in the visual spectral range. As a result, important optical properties of an IR-optical system (e.g., focal length, image plane), change with temperature. This is particularly annoying with FLIRs in aircraft because the necessary refocusing increases the pilot's workload. A method to overcome that problem will be described. It makes use of automatic autocollimation, where the radiation of a special thermal target is passing the optical system twice before hitting the infrared detector. A microprocessor evaluates the detector signals and moves an optical element in such a manner that the image plane of the optical systems always remains on the detector focal plane.