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 theirsurface shapes. The mirrors often should be tested and fabricated more accurately than the referenceoptics against which they are compared. Furthermore, the mirrors require precise measurement tonanometer accuracy in weightless conditions. In order to address these requirements, an absolutetest scheme should be adopted to separate the surface shape of the target mirror from the errors inthe interferometer and the errors due to the gravity effect. In this manuscript, we propose a newpixel-based absolute test method that can extract the surface shape of the mirror without the errorscaused by both the interferometer and the gravity effect. The proposed method is demonstratedwith actual data from a lightweight primary mirror with a diameter of 1 m that has partially closedpockets at the back side and three bosses at the rim for flexure mounting.