Manufacturing Precise, Lightweight Paraboloidal Mirrors
- Created on Sunday, 01 January 2006
Success depends on the proper selection of materials and process conditions.
A process for fabricating a precise, diffraction- limited, ultralightweight, composite-material (matrix/fiber) paraboloidal telescope mirror has been devised. Unlike the traditional process of fabrication of heavier glass-based mirrors, this process involves a minimum of manual steps and subjective judgment. Instead, this process involves objectively controllable, repeatable steps; hence, this process is better suited for mass production.
Other processes that have been investigated for fabrication of precise composite-material lightweight mirrors have resulted in “print-through” of fiber patterns onto reflecting surfaces, and have not provided adequate structural support for maintenance of stable, diffraction-limited surface figures. In contrast, this process does not result in “print-through” of the fiber pattern onto the reflecting surface and does provide a lightweight, rigid structure capable of maintaining a diffraction-limited surface figure in the face of changing temperature, humidity, and air pressure.
The process consists mainly of the following steps:
- A precise glass mandrel is fabricated by conventional optical grinding and polishing.
- The mandrel is coated with a release agent and covered with layers of a carbon-fiber composite material.
- The outer surface of the outer layer of the carbon-fiber composite material is coated with a surfactant chosen to provide for the proper flow of an epoxy resin to be applied subsequently.
- The mandrel as thus covered is mounted on a temperaturecontrolled spin table.
- The table is heated to a suitable temperature and spun at a suitable speed as the epoxy resin is poured onto the coated carbon-fiber composite material.
- The surface figure of the optic is monitored and adjusted by use of traditional Ronchi, Focault, and interferometric optical measurement techniques while the speed of rotation and the temperature are adjusted to obtain the desired figure. The proper selection of surfactant, speed or rotation, viscosity of the epoxy, and temperature make it possible to obtain the desired diffraction-limited, smooth (1/50th wave) parabolic outer surface, suitable for reflective coating.
- A reflective coat is applied by use of conventional coating techniques.
- Once the final figure is set, a lightweight structural foam is applied to the rear of the optic to ensure stability of the figure.
This work was done by Frederick Thomas Herrmann of Marshall Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Manufacturing category.