A method based on exact constraint for supporting space-based large mirror with a diameter of 2.8m

The larger the aperture of the mirror, the more sensitive it was to the disturbance of temperature and gravity load. So that it was more difficult to guarantee the accuracy of the surface shape. Therefore, the large aperture mirror needs exact constraint and positioning especially. In this paper, the exact constraint theory was applied. The constraint line diagram of 2.8m diameter mirror was drawn by dual line rule. Parallel mechanical connections add constraints and series mechanical connections add the degrees of freedom. According to this principle, the tangential flexible support structure and the axial flexible support structure were designed synthetically by the constraint line diagram, by which the 6 degrees of freedom of the mirror was constrained. There is neither overconstraint nor underconstraint. By means of finite element analysis calculation, under temperature rise 3 working conditions, the surface shape accuracy RMS value of the mirror was 7.5nm. Under self-gravity load on the mirror with gravity compensation device, the surface shape accuracy RMS value of the mirror was 6.4nm. The results showed, this support method finally realized the exact constraint of this large aperture mirror, and sensitivity to temperature and gravity load disturbances was within acceptable range.