Optical design and athermalization analysis of infrared dual band refractive-diffractive telephoto objective

In order to improve the remote target detection ability of infrared (IR) images effectively, an infrared telephoto objective for 3μm∼5μm and 8μm∼12μm dual wave-band is designed for 640 pixel×512 pixel infrared CCD detector. The effects of the surrounding environmental temperature are analyzed and the refractive diffractive hybrid thermal compensation is discussed. The focal length of the system is 200mm, the relative aperture is 1:2.2 and the field of view is 7°. The infrared dual band telephoto system with small volume and compact structure is designed in a large range of temperature. The system is composed of four lenses with only three materials of zinc sulfide, zinc selenide and germanium to compensate for the temperature. The image quality of the system is evaluated by ZEMAX optical design software. The results show that the modulation transfer function (MTF) for each field of view at cut-off frequency of 17 lp/mm are respectively greater than 0.6 and 0.4 which approaches the diffraction limit. The telephoto objective has favorable performance at the working temperature of -40°C∼+60°C. The relative aperture, field of view, and focal length are same for both spectral regions. The system meets the requirements of technical specification.