Absolute Characterization of Gravity Sag

G-release - defined as the knowledge uncertainty of a mirror's gravity-sag or the residual error between the mirror's predicted and actual on-orbit shape - is a critical error budget element for space telescopes. For a UV/Optical telescope, G-release needs to be less than 4 nm rms. Absolute metrology techniques developed for high-accuracy optical surface testing can be used for characterizing gravity-sag of space optics. This paper presents the importance of gravity sag for space optics and reviews methods for its compensation, characterization or correction.