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 ∼15µm systematic and a ∼5µm statistical error.
Funder
National Aeronautics and Space
Administration