Abstract
A method to design athermalized infrared two-lens telephoto objectives is proposed to reduce the number of lenses and simply the structure of an athermal optical system. The telephoto structure is composed of a front refractive-diffractive positive single lens and a rear negative single lens. A diffractive surface is used to compensate most of chromatic aberration and a small amount of thermal aberration, and residual aberrations are balanced by the rear negative lens. A long-wavelength infrared objective used for uncooled focal plane arrays is designed, whose focal length is 120 mm, F number is 1.2 and telephoto ratio is 0.9. The designed modulation transfer function (MTF) is close to the diffraction limit and the measured result of MTF is greater than 0.45 at the spatial frequency of 30 lp/mm. The results of temperature tests show that the target image remained sharp in the temperature range from -40 to 60 . The proposed two-lens structure can be applied in airborne and missile-borne electro-optical equipments.
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