Low-Cost, Very Large Diamond-Turned Metal Mirror
- Sunday, 01 June 2014
Reliable plating and diamond-turning technologies produce visible quality mirrors for applications such as semiconductor manufacturing.
Marshall Space Flight Center, Alabama
This innovation is a method for fabricating a low-cost, lightweight, large-aperture mirror by constructing only the mirror substrate by electroforming on a master form machined from plastic foam. Electroformed tubes of the same NiP alloy are installed in the foam mirror substrate master. Installing electroformed NiP tubes in the plastic mirror master before plating on the plastic foam mirror substrate allows the mirror faceplate and the back surface of the mirror to be plated onto the ends of the connecting tubes in the foam plastic. Removal of the foam after plating is complete results in a very stiff and lightweight mirror substrate made only of a single material. The low cost of the electroformed mirror substrate is made possible by very fast production of a master surface made of plastic foam that can be rapidly machined with modern, high-speed machining technology to very good mechanical tolerances in only a few hours.
No expensive and laboriously produced master surface is required for replication as in traditional electroforming of optical mirrors. After the plastic form has been encapsulated with a suitably thick NiP deposit, the plastic is dissolved away with a solvent. This leaves a hollow, electroformed mirror substrate. The excellent diamond machinability of the high-phosphorus nickel allows the electroformed surface to be diamond turned to produce a highly accurate and very smooth optical surface.
The extremely low cutting forces of diamond turning can produce visible wavelength quality optical surfaces without applying any heavy forces to the mirror face plate, as would be the case with conventional optical lapping and polishing processes. The plated-in array of tubular structures allows the interior of the hollow mirror to be open and easily vented in a vacuum. The very low cutting force of diamond turning allows the mirror faceplate to be made very thin to lower the areal density of the mirror. The low cutting force also minimizes the tendency for print-through of the internal support points of the tubular sections connecting the mirror face plate to the mirror back plate.
Excellent visible quality optical figure and surface finish can be produced by diamond turning. Surface finishes on diamond- turned NiP alloy as smooth as 0.6 nm rms have been obtained without any post-polishing of the diamond turned surface. The fast and flexible machining of a very-low-cost, expendable master form made of plastic foam, combined with a proven electroplating process to produce a thick deposit of diamond machinable NiP, results in a process for producing mirror substrates at a cost as small as one-thousandth the cost of comparable one-square-meter beryllium mirror substrate.