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