| The spin-casting method has been utilized to produce a finished epoxy optical surface for use in visible light. Since the properties of a polymer are critical for achieving sufficient image formation, the chemistry of the system has been studied and tuned accordingly.; An epoxy resin system should include such characteristics as low viscosity and surface tension, high glass transition temperature, high strength and hardness, low shrinkage, and a slow rate of cure.{09}At the University of South Carolina, a diluent has been added to the system to reduce viscosity and surface tension. Also, dynamic mechanical testing has measured the viscoelastic material properties of several samples revealing a higher glass transition temperature for samples with a high percentage of aromatic epoxy and high crosslink density.; To reduce shrinkage in the mirror, the amine curing agent has been chemically modified by imine formation with benzaldehyde and reduced with lithium aluminum hydride to provide a bifunctional amine. When mixed with the tetrafunctional amine in a 50/50 ratio, crosslinking is reduced, resulting in a smoother mirror surface.; Another way to reduce stress is to prolong the gelation point during the curing reaction. A recent cure study revealed this point could be postponed several days allowing the polymer to relieve any stress that it may incur.; Once the mirrors have been fabricated, optical testing is needed to provide information about surface quality. Both the Foucault and Ronchi tests have been performed providing valuable information for the manufacturing process. Attempts to provide a surface profile through these tests failed, but resulted in a novel ray path test method of optical testing. This hybrid test measures the longitudinal aberrations of a mirror by directly locating rays to and from the mirrors. These measurements can then be compared to the theoretical values and a mirror surface analysis is obtained. When applied to the epoxy mirrors, the analysis revealed a perfectly paraboloidal area on the mirrors. One mirror was fitted with a mask and the first astronomical image with a polymeric mirror was captured. |
