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Composite Materials

The ideal material for a telescope mirror is one which has a low coefficient of thermal expansion (CTE), a low specific heat and density, or both. Materials with these thermal properties will come to thermal equilibrium faster and in such a way that the figure of the surface will remain as unchanged as possible - although the focal length may change. It is also important for the material at the surface to be homogeneous and isotropic to prevent disfiguring. 2

The material used as the composite matrix for this experiment was an industrial cement called Ultracal 30 manufactured by U.S. Gypsum. While Ultracal had a nice viscosity for spin-casting and was both cheap and easy to work with, its surface hardness was far too low to polish into a mirror surface. Also, its CTE is not ideal at $8.3 \times 10^{-6} in/in/\ensuremath{^{\circ}}C$.

Epoxy and Alvin LabMetal were also tested as surfacing materials. Like Ultracal, LabMetal's hardness was too low for it to be easily fine ground or polished. The epoxy was also difficult to grind since bits of the grinding compound became lodged in the surface and could not be removed easily with water. Epoxy is also unsatisfactory as a surface material because of its poor thermal properties with a CTE of $100\mu m/m/\ensuremath{^{\circ}}C$.

A benefit of using carbon fiber as the composite fiber is that it has a negative CTE of $-0.6\mu m/m/ \ensuremath{^{\circ}}C$ at $20 \ensuremath{^{\circ}}C$. 3 A composite using carbon fiber might have a cancelling effect that reduces the CTE of the composite, similar to how black vitrified ceramic (BVC) achieves its low CTE. 4


Footnotes

... disfiguring.2
For more information about the thermal behaviour of telescope mirrors see http://astron.berkeley.edu/~jrg/MATERIALS/node6.html
.... 3
See http://matweb.com/search/SpecificMaterial.asp?bassnum=ETHOR6 for the material properties of the Thornel T-300 carbon fiber used in the project.
... CTE.4
For the thermal properties of BVC, see http://pages.infinit.net/asmprod/techinfo.html

next up previous
Next: Spin-Casting Equipment Up: RAB 2004 Report: Construction Previous: Basics of Mirror Grinding
Mark Howison 2004-09-10