A narcissus calculating method for cryogenic infrared imaging systems is proposed in this paper. The accuracy is largely improved compared to the traditional paraxial analysis, as ray blocking of the optical opertures is taken into account and real ray data are used during the calculation. The narcissus distribution on the full focal plane can be obtained via analyzing field by field. Meanwhile, it can be implemented simply and fast as sequential ray tracing is utilized and rays only pass through three surfaces during the cold return statistics for every retro-reflecting surface. According to this method, a general narcissus calculation package was realized using the macro language of optical design software Code V. The performance of the new method was tested by calculating an example system using this package and comparing it with traditional methods. The results showed that the new method produced the same result accuracy and information quantity as the nonsequential ray trace, while the whole analysis took only 5 s, which was significantly shortened compared with the nonsequential ray trace, which took about 30 min.
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