Development of lightweight mirror elements for a very large astronomical adaptive optic primary mirror

Abstract

New very large telescopes with apertures as large as 100 meters are being proposed. They will be made up of mirror segments only a meter or two in diameter and phased together. The diffraction-limited resolution of a mirror is directly proportional to its diameter, and the light-gathering-power is proportional to the square of the diameter. Near-diffraction-limited performance using adaptive optics would make such large mirrors very exciting. We have built two small prototype composite adaptive optic mirrors of graphite fiber impregnated cyanate ester resin driven by actuators spaced 4 cm apart and with a faceplate influence function of 5 cm. The second mirror assembly also makes possible a 2 cm actuator spacing. The overall figure is not yet as good as desired, but we believe that much of this problem can be corrected by mechanical adjustment of the actuator rest positions and use of low expansion mandrels. This mirror concept, when realized in primary mirror segments a meter or more in diameter, should make correction possible for atmospheric turbulence under almost any observatory seeing conditions. The composite optical faceplate in the most recent prototype had a roughness of 0.6 to 0.8 nm. Two centrifugal elutriation super-polishers, each over 1.2 meters in diameter, are in place to produce superpolished mandrels on which to form superpolished faceplates over a meter in diameter. Scattered light from such a mirror surface will be reduced by as much as a factor of ten, as compared to conventional fresh feed polishing. The name "transfer mirrors" rather than the widely recognized but poorer quality "replica mirrors" is given to such faceplates. They have an expansion coefficient comparable to ULE quartz or Zerodur, and are lightweight with 10-20, an aerial density of 17 kg/m² for the mirror with a 4 cm actuator spacing or 34 kg/m² for the mirror with 2 cm actuator spacing. In both cases the effect of gravitational sag is minimized. A 60 volt potential results in actuator displacement of 5 mm as measured inter