Steps toward 8 m honeycomb mirror blanks: III. 1.8 m honeycomb sandwich blanks cast from borosilicate glass.

As part of a program leading to the production of 8m honeycomb mirrors, we have recently made two 1.8m blanks. These have honeycomb sandwich form, with hexagonal honeycomb ribs sandwiched between front and back plates. Each is cast in one piece from borosilicate glass, using techniques that can be extended to larger sizes. Pieces of the glass are melted together in a circular container made of hard ceramic tiles, held together against hydrostatic pressure by bands of nickel alloy. Voids in the glass to give the honeycomb structure are formed by hexagonal blocks of ceramic fiber, held down against flotation with silicon carbide bolts. Liquid glass runs over the blocks to form the face sheet, and under the blocks, which are spaced above the base tiles, to form a back sheet with holes. After the casting has been annealed and cooled, the base tiles are unbolted and the ceramic fiber blocks removed from the glass honeycomb by water blasting. Both blanks are of high quality, free from cracks and voids, and with an adequately low bubble content. The second and better blank, made of Ohara's E6 glass, is now to be figured to high precision, 0.25 arcsecond images, and is to be tested for an extended period in the Multiple Mirror Telescope.