Replication of Wolter I x-ray mirrors by electroforming techniques

Cylindrical (hyperbolic-parabolic Wolter I) mirrors have been electroformed from nickel over an electroless nickel-phosphorous (NiP) plated aluminum mandrel in support of the NASA AXAF-S x-ray spectrometer program. The electroless nickel was diamond turned and polished to achieve a surface finish of 10 angstroms rms or better. Gold was then plated on the nickel alloy after an electrochemical passivation step. Next a heavy layer of pure nickel was plated one millimeter thick with computer controlled stress at zero using a commercial PID program to form the actual mirror. This shell was removed from the NiP alloy coated mandrel by cryogenic cooling and contraction of the aluminum to release the mirror. It is required that the gold not adhere well to the NiP but all other plated coatings must exhibit good adherence. Four mirrors were fabricated from two mandrels prepared by this method. Two mirrors were made from each mandrel. Electrolytically deposited gold was used on three parts and vacuum deposited gold (1500 angstroms) on the fourth. The mandrel surface finish was about 10 angstroms rms at the time of plating in each case. The area of each part is 0.7 square meters (7.5 square feet).