Optical and Opto-Mechanical Design of an F/2.5 Broadband Infrared Objective Lens Using Multiple Cold Stops and Cooled Detectors

Abstract

This paper discusses the design and production of a f/2.5, 25mm focal length lens that images portions of the SWIR and MWIR spectrums from 1 – 5.5µm. Challenges associated with imaging in such a wide spectrum are discussed, including the need to correct for chromatic errors when a limited number of materials are available. The lens is designed to be used on three different cryogenically cooled detectors, each with differing stop locations relative to the lens. The paper discusses the effect that a varied stop has upon the third-order aberrations and covers specifics of an opto-mechanical design which meets the performance requirements for each detector. Cooled detectors require additional analysis to ensure that narcissus effects will not dominate the image. A method for analyzing the narcissus effects is highlighted. The evolution of the design as the lens is transitioned from theory into prototype is discussed. Images captured with the lens assembly are presented along with performance data to compare theoretical to measured performance.