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After milling out the majority of stock on a NC Bridgeport, the hardened stainless steel blank was ground to final contour using an endless abrasive belt. Contour accuracy from the NC grinder was ±0.0005′′.
Figuring, polishing and testing were all done with the parabola mounted on a Moore spin table. After figuring for a time, the surface was shined up, and a repolished steel ball was placed at the parabola focus. A light source and diagonal flat were used to illuminate a test parabola mounted face down above the steel parabola.
The test is shown in schematic form to illustrate its features. The
In the upper portion of the figure, the light source illuminated a glass fiber to produce a 0.002 in. point source at the focus of the 48-in. focal length test parabola. At the bottom, the steel ball was located with its center of curvature coincident with the steel parabola focus. Incident collimated light was reflected toward the ball by the parabola. The light was autoreflected by the ball, and it retracted its path to form an image of the original point source. eyepiece or cut with a knife edge to determine the location and magnitude of the errors remaining in the parabola.
The autoreflecting steel ball was cemented to a bar for ease of handling. The bar was clamped to a three-axis stage to allow precise location at the parabola focus. Guide pins on the table base allowed the whole fixture to be easily removed and replaced through repeated figuring and testing cycles.
This is a typical interferogram of the polished steel ball as taken at