Optical System Design Of An Infrared Dual-Band Seeker

High-performance missile is the key to carry out a task in the modern warfare. As we all know, seeker is the eye of the whole missile, which has independent capability in target recognition and tracking. It can also show the guided signal. However, the optical system of seeker which is in front of coordinator also has a certain influence on seeker's operating range and tracking ability. Therefore, the imaging quality of optical system plays a vital important role in the seeker.Based on the appearance of dual-band infrared device and the technical parameters of infrared seeker, a new-type Cassegrain optical system is designed. The main advantages of this system are large field-of-view, dual-band common path and compact structure. In the Cassegrain optical system, working wavebands are 3.7μm-4.8μm and 8μm-12μm, the field-of-view 4°and the aperture 240mm. The primary mirror of Cassegrain optical system is ellipsoid and secondary is hyperboloid. In order to balance the aberration caused by aperture and field, three compensating lenses are used in this system at first. Moreover, the system has a good performance of athermalization between negative 40℃and positive 50℃. The design results of this system show that the cut-off frequency is 11 lp/mm when the pixel of infrared CCD is 45μm, the MTF value of each field is greater than 0.7 except for the marginal field. Wavefront aberrations for all the fields are less than 0.25λ.The technical parameters of the optical system are as following:Considering the fabrication process, production cost and other factors, the system structure is improved for further. Aspheric surfaces are used both the compensating lens and the fairing. Therefore, aberrations of the whole system become much better. The aspheric surface of primary and secondary mirrors is replaced by spherical surface, so the problems of high machining accuracy and difficult alignment caused by large aperture and aspheric surface become much easier. Moreover, the amount of compensating lenses has been reduced from three to two. The total length of this system is 170mm, and its ratio of total length to focal length is 0.62. Not only economize the cost and reduce weight, but make the whole structure of system more compact. The design results of the improved system show that the cut-off frequency is 11 lp/mm when the pixel of infrared CCD is 45μm, the MTF value of each field is greater than 0.7, wavefront aberrations for all the fields are less than 0.038λ. The RMS radius of the first field is 63.416μm, the second field 65.228μm, the third field 62.249μm and the fourth field 39.566μm respectively. This system can also meet the requirements of the optical structure and image quality.