Abstract
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 the background thermal infrared loading on the detectors and optics. The vacuum window and cold filter introduce a wavefront error that we characterize using an autocollimating microscope. For the 200 mm diameter aperture $f/3$ space telescope SPHEREx, we achieve a focus position measurement with a ${\sim}15\;{\unicode{x00B5}{\rm m}}$ systematic and a ${\sim}5\;{\unicode{x00B5}{\rm m}}$ statistical error.
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