In order to meet the demand of miniaturization of multi-function EO sensor systems design, the compact infrared optical
system is required. Based on the two present design method of compact cooled transmitted infrared optical system,
the paper puts forward a new design idea: using the telephoto structure compress the overall length and re-imaging
or multi-imaging reduce the radial diameter of optical system. A compact cooled transmitted infrared optical system,
which focal length is 150mm (F#=2), is designed using this method. The cold aperture efficiency is 100% and the image
quality is excellent, and the length is less than 120mm, the diameter of the first lens is less than 82mm.