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Another Look at Open Back Zerodur Primary Mirrors (OBZ PMs)

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

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Another Look at Open Back Zerodur Primary Mirrors (OBZ PMs)

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  1. 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. Present tapered-back design SAGEM design—basis for early OBZ studies

  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. Present tapered-back design—202mm cells Tapered-back design from 2 weeks ago—294mm 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

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

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