Athermalization design of infrared refractive-diffractive telephoto objective

In order to improve target detection ability of IR images in long range effectively, an infrared telephoto objective for 8 μm~12 μm wave-band is designed for 640 pixel×512 pixel infrared CCD detector. Effects of surrounding environmental temperature are analyzed and refractive diffractive hybrid thermal compensation is discussed. Focal length of the system is 200 mm, relative aperture is 1:2.2 and field of view is 7°. Infrared 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. Image quality of the system is evaluated by ZEMAX optical design software. Results show that modulation transfer function (MTF) for each field of view at cut-off frequency of 17 lp/mm is greater than 0.4 which approaches diffraction limit. Root mean square radius of spot diagram for each field of view is close to Airy disk radius. Telephoto objective has favorable performance at working temperature of -40 ℃~+60 ℃. The system meets the requirements of technical specification.