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 mm diameter aperture f/3 space telescope, SPHEREx, we achieve a focus position measurement with a ∼15μm systematic and a ∼5μm statistical error.
@article{arxiv.2311.18043,
title = {Cryogenic Focus Measurement System for a Wide-Field Infrared Space Telescope},
author = {Samuel S. Condon and Stephen Padin and James Bock and Howard Hui and Phillip Korngut and Chi Nguyen and Jordan Otsby},
journal= {arXiv preprint arXiv:2311.18043},
year = {2024}
}