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

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. The vacuum window and cold filter introduce wavefront error which we characterize using an autocollimating microscope. For the $200\text{ mm}$ diameter aperture $f\mathrm{/}3$ space telescope, SPHEREx, we achieve a focus position measurement with a $\sim 15\mu \text{m systematic}$ and a $\sim 5\mu \text{m statistical}$ error.