The Study Of Off-axis Catadioptic MWIR Continuous Zoom Optical System

With the rapid development of the detection technology, new requirement isproposed to the optical detection system, especially the large diameter and long focallength continuous zoom optical system. But the existing design type can’ t fully meetthe requirements, so the solution of large aperture and long focal length is the linchpinof the infrared continuous zoom system.In allusion to the infrared continuous zoom system’s design requirements, theoff-axis catadioptric design method is bring forward in this dissertation. Theathermalization design and the stray lights’ effect to the signal-to-noise ratio areemphatically analyzed. Specific studies of this paper are as follows:1、Through analyzing existing infrared continuous zoom system design type andcomparing the advantages and disadvantages of various types, bring forward off-axiscatadioptric design method. The infrared continuous zoom system is divided into theoff-axis reflective non-power system and the latter transmission continuous zoomsystem. The two parts are optimized separately. Then combinate the two partstogether according to matching the pupil and cold shield. At last, unified optimizationis carried out to the overall system. A off-axis infrared catadioptric continuous zoomsystem with a large-diameter, long focal length and meeting the cold apertureefficiency of100%is designed.2、The infrared optical systems are sensitive to temperature changes. And thesystem’s imaging quality deteriorate with temperature change. So the hybridrefractive/diffractive athermal design is proposed to infrared continuous zoom system.According to the continuous zoom system’s optical power formula and hybridrefractive/diffractive system’s athermal formula, derive the continuous zoom system’sathermal formula. And on this basis, athermal design in the temperature range of-30～+50degree is carried out to the transmission continuous zoom part. By theathermal design, the zoom system in full focal length range have a good imaging quality with the temperature change and meet the requirement of the infrared zoomsystem being used in the harsh environment.3、Analyzed the stray radiation sources and characteristics. The methods andsteps of the stray radiation reduction are studied. And the stray radiation suppressingis put into practice to the infrared zoom system designed in this paper. The strayradiation analysis inhibition models and the model of the stray radiation effect to thesignal-to-noise ratio are established. Get the equivalent number of photon of thevariety sources of stray radiation, such as the sun, sky background and the systemthermal radiation. Accordingly calculate the equivalent photon signal-to-noise ratio.4、The model of ghost imaging’s effect is established. Analyzed the ratio of themain image illumination with the illumination in the image plane of the bright spotsource in the extend source. For the narcissus of the infrared system, the noiseequivalent temperature difference (NETD) is calculated. Then the ghost and thenarcissus of the zoom system designed in this paper are analyzed. The results of theanalysis are that the ghost illumination is less than one ten-thousand of the mainimage illumination, and the NETD of the narcissus of each surface less than detectorminimum detectable temperature difference.The final study results show that: the large aperture and long focal length MWIRcontinuous zoom system have a good imaging quality and simple mechanicalstructure. The stray radiation’s effect to the signal-to-noise ratio is small. The resultmeet the design requirements.