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Predicting The Focus Of Cryogenically-Cooled Optical Systems

Results of an experimental study to ascertain how well the focal-plane location of cryogenically-cooled optical systems can be predicted are reported. These results indicate that if the required low-temperature thermal expansion and index-of-refraction data are available, the focal shift caused by cooling to cryogenic temperatures can be accurately predicted by simply computing the shift in the paraxial focus. In this study, the differences between the measured focal shifts and the computed shift in the paraxial focus were less than the diffraction-limited depth-of-focus tolerance. The results of this study also indicate that for off-the-shelf optical systems ray-tracing analysis may not adequately predict the absolute location of the focal plane. Thus, the following method of predicting the focal-plane location of a cryogenically-cooled optical system is suggested: first measure the focal-plane location with the optics at room temperature, and then add the computed paraxial focal shift to the measured location.