Another Look at Open Back Zerodur Primary Mirrors (OBZ PMs) Robert Besuner 12 May 2005

2 Evolution as of 2 weeks ago Mounting is near 2/3 radius rather than peripheral. Offloading schemes are better optimized. Cathedral ribs behind front face sheet to reduce gravity and polishing quilting. Tapered-back option is examined. SAGEM design—basis for early OBZ studies Present tapered-back design

3 This week’s evolution Smaller cell size (294mm  202mm leg length). No cathedral ribs, thinner front face (10mm  8mm). Struts mount to sides of a (thicker) rib instead of to a partial closed cell. Tapered design is less aggressively lightened. Tapered-back design from 2 weeks ago—294mm cells Present tapered-back design—202mm cells

4 Constant-Thickness Meniscus 200mm thick mirror. 8mm front face. 8mm thick outer periphery. 6mm thick ribs. 20mm thick strut-mounting ribs. Glass mass = 245kg (compare to 230kg for prev. OBZ, 248kg PBZ, 204kg CBULE).

5 Tapered-Back Design 200mm thick at bipods. Tapers to 110mm thick at OD, flat back from bipods to ID. 8mm front face. 8mm thick outer periphery. 6mm thick ribs. 20mm thick strut- mounting ribs. Glass mass = 206kg (saves 39kg rel. to meniscus, comparable to CBULE).

6 Performance Comparison

7 Offloader Count Trends

8Conclusion A tapered open back Zerodur design can compete with a closed back ULE design in performance and mass.