Steps toward 8m honeycomb mirror blanks: Ii. experiments with waffleplate and honeycomb casting

Large, inexpensive, lightweight mirrors needed for ground and space based astronomy can be made from borosilicate glass. The thermal gradients in glass that degrade the figure of thick borosilicate mirrors during use can be largely eliminated in a honeycomb structure, by internal ventilation (in air) or careful control of the radiation environment (in space). We anticipate that ground based telescopes with honeycomb mirrors will give better images than those with low expansion solid mirrors, because even slight temperature differences in the air near a slowly equilibrating solid mirror can significantly degrade the wavefront at a turbulent boundary. Two fabrication methods are under investigation, remelting glass into complex molds of vacuum formed ceramic fiber and fusion methods. This paper descibes the former method used to cast in one piece either waffleplate or full honeycomb sandwich blanks. Details are given on the construction of molds and the cycle for melting and annealing. The technique is an extension of the method used nearly 50 years ago for the Palomar 5.1m mirror. In October 1982 we cast a 1.1m square blank with a 2.4cm thick face and 1.6cm ribs, 15cm deep on 15cm squares. A 1.8m circular blank of full honeycomb construction is scheduled for production in April 1983. We anticipate casting blanks up to 8m in diameter. Densities of 200kg/m², like that of the ST mirror, are typical of casting. A summary is given of our development of a new method for making honeycomb structure by fusion bonding.