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Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response To Posted by Robert May on January 14, 2002 19:55:21 UTC |
The last stages of grinding is usually the problem of the edge of the mirror not polishing out properly. What is happening is that the final grit isn't getting all the way out to the edge of the mirror. The edge was getting ground finer but that grinding was not on the ROC that the rest of the mirror was getting. This is why you only use the strong ROC changing chordal strokes when digging the hole in the glass and then go to much shorter and closer to on-center strokes after that. This promotes a good spherical shape to the glass. Please note that the strokes that you tend to use for coarse grinding are similar to those needed to make the paraboloid in the figuring. Needless to say, the overhang is what makes the surface a non-spherical surface and that isn't what you want when you are doing the final grinding part. |
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