Design of dual-band shared-aperture Co-zoom optical system

•Optical system, fusing visible/mid-IR light in the same light path, is designed.•A method is obtained to compensate zoom ratio difference between visible and mid-IR.•Volume of dual-band continuous zoom optical system is reduced.•This optical system can conduct synchronous observation, tracking and measurement with dual-band.

An optical system that features visible plus mid-infrared light, shared aperture, synchronous and continuous zoom is designed with a 10× zoom ratio. Analysis is performed to differentiate visible plus mid-infrared light focal length and zoom ratio during zooming, and the change law of this difference. Upon combination with two-group and three-group zoom theories and upon derivation of the conditions for compensating zoom ratio difference, a method has been obtained to directly compensate this difference. The focal length and the zoom ratio for visible/mid-infrared light at any zoom location are similar with this method, thereby conducting synchronous observation, tracking, and measurement on the target. Design results have shown that the system is small, has a fast response, has an excellent in overall image quality, and is athermal for temperatures between −40 °C and 60 °C.