Abstract: A five stage background filter circuit 10 is disclosed which is capable of responding to background radiation changes at speeds which are from one to two orders of magnitude faster than conventional prior filters. The invention utilizes a cross scan multiplexing technique in combination with filter circuitry depicted in schematic overview in FIG. 1. Circuit 10 includes a differential amplifier 12, a sample and hold device 14, a responsivity corrector 16, a recursive filter 18, and a background subtractor 20. Output signals 11 from a focal plane array of detectors (shown in FIG. 3) are passed through impedance matching input resistors 12a,b to an amplifier. A sample and hold circuit 14a cyclically selects and stores a signal received from first stage 12. The detector outputs are then normalized by responsivity corrector stage 16 which employs a digital-to-analog converter 16a and random access memory 16c.

Abstract: A folded optical system is provided in the form of a telescope (20) for viewing distant objects. At least two optical elements (22, 24) have curved surfaces (32, 34) for imaging rays of radiation from a distant object. The two surfaces are positioned so as to permit a series of reflections therebetween for magnification of the resulting focal length of the optical system and diminution in the overall viewing angle so as to attenuate any light which may emanate from sources of radiation disposed off an optical axis which intercepts the object being viewed. One or more of the optical elements may be provided with apertures (30) through which rays of radiation are directed to a sensor (26) of the radiation, the sensor being positioned on a image plane to receive an image of the objects being viewed. First and second ones of the optical surface may be concave and convex, respectively, or alternatively, each optical surface may be concave with a folding mirror being utilized in the reflection of rays therebetween.