Method to minimize the gravity sag of a facing ground Φ1m flat mirror

A method for minimizing the optical distortion from gravity sag on a suspended and autocollimated flat mirror facing ground has been proposed in the paper. Referring to this method, a mechanism consisting of 18 pulleys and weight sets unloads the gravity of the Φ1m flat mirror the alignment and testing benchmark of the 1m aperture optical system of the Space Solar Telescope (SST). Three positioning points support the mirror, allow the mirror to be held or suspended above an object to be viewed steadily and reliably. 18 pulling points on the back of the mirror unload the mirror's weight. The bonded joints of these 18 points are analyzed and tested to be reliable, and can bear the affection of variation of temperature and inner stress. The manufacturing and assembling precision of the support is analyzed and controlled, the unload forces of these 18 points are assigned. With this method, the surface error of the flat tested by Ritchey-Common arrangement is satisfactory for the alignment of SST of less than 1/50λ (RMS, λ=633nm).