Manufacture of large glass honeycomb mirrors

We have begun a program to develop very large lightweight mirrors needed for ground and space based astronomy. A rigid structure of honeycomb sandwiched between faceplates is formed of borosilicate glass by melting it into a complex mold. Thermal gradients in the 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, when the seeing is good, ground based telescopes with honeycomb mirrors will give better images than those with solid mirrors. Even slight temperature differences in the air near a slowly eauilibrating solid mirror can significantly degrade the wavefront at a turbulent boundary. We discuss in this paper materials and techniques and the experience that has been gained making trial mirrors and test castings. Our future program calls for making a 1.8m blank that will be figured and tested in the MMT in 1983. After further experience with 3m blanks, we plan to build a large furnace to make mirror blanks up to 8m or even 10m diameter.