Advances in very lightweight composite mirror technology

We report progress in the development of very lightweight (<5 kg/m(2)) mirrors made by replication using graphite fiber cyanate ester resin composites. The replication process is optimized to significantly improve the surface smoothness and figure quality. Achievements include near-diffraction-limited optical performance [<1/20 wave root mean square (rms) at 632.8 nm] in replica flats, fractional wave rms performance in curved mirrors at 90% pupil, and almost exact reproduction of the surface microroughness of the mandrel. The curved mirrors typically show some edge roll off and several waves (rms optical) of astigmatism, coma, and third-order spherical aberration. These are indications of thermal contraction in an inhomogeneous medium. The inhomogeneity is due to a systematic radial variation In density and fiber/ resin ratio induced in composite plies when draped around a small and highly curved mandrel. The figure accuracy is expected to improve with larger size optics and in mirrors with longer radii of curvature. Nevertheless, the present accuracy figure is sufficient for using postfiguring techniques such as ion milling to achieve diffraction-limited performance at optical and UV wavelengths. We demonstrate active figure control using a simple apparatus of low-mass, low-force actuators to correct astigmatism. The optimized replication technique is applied to the fabrication of a 0.6-m-diam mirror with an areal density of 3.2 kg/m(2). Our result demonstrates that the very lightweight, large-aperture construction used in radio telescopes can now be applied to optical telescopes. (C) 2000 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(00)03609-6].