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Figures (7)

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1

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′′.

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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.

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The test is shown in schematic form to illustrate its features. The f 10.15 parabola is drawn to scale to show the light coming to focus after a grazing incidence reflection at the rim of the parabola. Nearly 75% of the surface area of the ball is used to autoreflect the rays incident on it. Because the light is reflected twice from the surface of the parabola, this test is very sensitive to surface errors.

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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.

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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.

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This is a typical interferogram of the polished steel ball as taken at f/1.25 with a Tropel SMI. The ball may be placed in the interferometer in any arbitrary position with similarly good results. We feel confident from this that the ball is spherical to at least λ/10. Further confidence of this was gained since the parabola was figured to ½ per inch, and we could see no evidence of error due to the ball. The ball was made by repolishing a hardened steel ball bearing.

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7

The finished stainless steel parabola with a clear aperture of 9 in. The photograph attests to the luster- and zone-free figure of the surface.

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