Casting and Polishing 8M Telescope Mirrors

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This award supports the development of technology for casting and polishing 8m telescope mirrors. At this size, considerations of stiffness, weight and thermal response strongly favor honeycomb sandwich design for the mirror blank. Over the past few years a casting technique has been developed at the University of Arizona in which glass is melted into a complex mold to form a honeycomb sandwich structure in one piece. Two 3.5m blanks have already been cast. In the funding period the casting furnace will be enlarged, and the first castings made at the 6.5m and 8m size. The project is also concerned with the development of the stressed lap method of mirror polishing. This is a straightforward and direct attack on the problem of polishing highly aspheric surfaces with reasonable speed. The lap's shape is changed as it moves across the mirror, to match the varying shape of the surface under it. The method is important especially for the largest telescopes, which benefit in stability and economy if the primary has a short focal length, and is thus strongly asperic. Speed of polishing is also critical for economical production of very large surfaces. Both the technologies being developed can be expected to find application to large optics in general, not just in astronomy.