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We describe a technique for measuring focus errors in a cryogenic,wide-field, near-infrared space telescope. The measurements are made with acollimator looking through a large vacuum window, with a reflective cold filterto reduce background thermal infrared loading on the detectors and optics. Forthe $300\textrm{ mm}$ diameter aperture $f/3$ space telescope, SPHEREx, weachieve a focus position measurement with $\sim \! 5\textrm{ }\mu \textrm{mstatistical}$ and $\sim \! 15 \textrm{ }\mu \textrm{m systematic}$ error.

Cryogenic Focus Measurement System for a Wide-Field Infrared Space Telescope