Another look at open back zerodur primary mirrors obz pms
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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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Evolution as of 2 weeks ago
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


This week s evolution
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


Constant thickness meniscus
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).


Tapered back design
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).




Conclusion
Conclusion

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


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