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Designing a 1.7-5.3 μm athermalized lens

In the framework of a research program a broad band lens (1.7÷5.3 μm) has been designed to be inserted in an optoelectronic system of the Italian Navy. The optoelectronic system operates over full spectral band or over one of the seven sub-bands that are selected inserting spectral filters mounted on a wheel. The optoelectronic system FOV is 20°x15°, aperture F/# 2.4 and it is optically athermalized in the range -30°÷+70° C. Color aberrations correction and athermalization was obtained by means an appropriate choice of optical materials (Cleartran, Sapphire, BaF2 and CaF2). The optical athermalization has been achieved for an all aluminum mechanical mounting. The lens mechanical mounting is quite cheap as no moving parts are present while focusing is obtained regardless the objective temperature. The optical layout is composed by five lenses. The design was driven to have a low sensitive manufacturing and mounting tolerances. A diffractive lens free solution was selected because of the unacceptable radiation losses in the higher diffractive orders due to the wide spectral band. Dedicated BBAR coatings for Sapphire and Cleartran have been developed.