Pixel-based absolute test of a 1-m lightweight mirror for a space telescope

Large mirrors for spaceborne instruments require high-accuracy interferometry to measure their surface shapes. The mirrors often should be tested and fabricated more accurately than the reference optics against which they are compared. Furthermore, the mirrors require precise measurement to nanometer accuracy in weightless conditions. In order to address these requirements, an absolute test scheme should be adopted to separate the surface shape of the target mirror from the errors in the interferometer and the errors due to the gravity effect. In this manuscript, we propose a new pixel-based absolute test method that can extract the surface shape of the mirror without the errors caused by both the interferometer and the gravity effect. The proposed method is demonstrated with actual data from a lightweight primary mirror with a diameter of 1 m that has partially closed pockets at the back side and three bosses at the rim for flexure mounting.