Forming thermal imaging system field of view with afocal lens cap

Method of infrared lenses field of view step variation is discussed in this paper. Simple afocal lens caps based on the classic Kepler or Galilee telescopic schemes are considered. The afocal cap have to provide acceptable vignetting of the extreme rays and moderate transverse dimensions. Mersenne two-mirror system of Galilean-type was selected as the most suitable lens cap. Equations were obtained to calculate dimensional parameters of the cap in respect that vignetting coefficient should be minimal.