Development of optically flat mirrors using graphite/epoxy laminates

The purpose of this project is to investigate the feasibility of using composite materials to reduce the weight of high quality optical mirrors for space flight applications. The approach that is used to fabricate test optics uses the same optical quality glass material (Zerodur) as the mold used to form the composite. The mold is the actual material that is ground into the finished polished surface, and it is bonded onto the composite during the normal curing of the composite. No secondary bonding is necessary. The mold is now ground away leaving a thin layer of Zerodur that becomes the polished surface. The composite material used in this experiment is P75/ERL1962 laid up in a quasi-isotropic laminate. Its properties will be tailored such that its coefficient of thermal expansion is close to that of the mold in the plane of the laminate. One flat optic has been made and tested. A second flat optic is close to completion, but it has not been tested. The first optic was tested in a cryogenic vacuum chamber using a Zygo interferometer to measure the distortion of the surface of the optic while the chamber was cooled down to 200 K. The measured change in flatness was 0.4 lambda using a test wavelength of 633 nm. The average fiber volume was calculated to be 56.5% with a change in fiber volume from one side to the other of 2.5%.