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5.1 Introduction The progression of an achromat from the basic conventional form to a diamondturned aspheric hybrid for the infrared spectrum will be demonstrated in this Chapter. We start with a cemented doublet for the visible spectrum, then change to the mid-wave and long-wave infrared regions, and proceed with the rest of the improvements relating to aberration reductions in the long-wavelength spectral band. For this exercise, we elect a 100-mm focal length with an f∕4 relative aperture and add a field of ±2 deg. 5.2 Thin Achromat for the VIS Spectrum, On-Axis Performance A conventional achromat consist of two elements. One has positive power and a low relative dispersion (high Abbe number), the other has negative power and a high relative dispersion (low Abbe number). The elements' powers, required for chromatic aberration correction, are ϕ a =V a (V a −V b ) ϕ for the front element, and ϕ b =V b (V b −V a ) ϕ for the rear element. V a andV b are the mentioned Abbe numbers for the two elements.
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