Gravity Deflections Of Lightweighted Mirrors

Two 20-in, diameter, f/4 fused silica mirrors were lightweighted by contouring the rear sides to near parabolic cross sections. One mirror was lightweighted into a single-arch, thick-hub design. The other mirror took the form of a cantilevered double arch. Details of the fabrication are outlined. Following fabrication, these mirrors were interferometrically tested at their centers of curvature in both a face-up and face-down support mode. By subtracting the wavefront errors in these two support modes, the gravity deflections were determined in spite of some residual figure error. Details of the data analysis and the resulting gravity-induced deflections are reported. These empirical results are compared with finite element analyses with favorable results.