Focus Optimization of a Cryogenic Collimater Using Interferometric Measurements and Optical Modeling

Space Dynamics Laboratory at Utah State University (SDL/IJSU) optimized the focus of an off-axis, cryogenically cooled infrared collimator for cryogenic operating temperatures. Historically, collimator focus was optimized at ambient temperatures where interactive focus adjustment and testing coulà be performed. The focus shift that occurred when the optics were cooled was minimized by collimator design, and the change was negligible compared to the spatial resolution of the IR sensor measuring the collimator's simulated point source. However, the focus determined at ambient temperature does not meet the image quality requirements of state-of-the-art sensors. The method used by SDL to determine optimal focus at cryogenic temperatures applies classical optical techniques to the cryogenically cooled environment. System level interferometric measurements are first made to characterize the system wavefront error. These measurements are then applied to an aberration-free optical model to evaluate system focus for a wavelength of 12 tim. The method also uses a knife edge test to refer the interferometric measurements to the aperture located near the focal point of the collimator. This paper discusses the physical test setup, outlines the optical model and analysis procedure, and presents results before and after focus optimization of a multifunction infrared calibrator.