The Parabolizing Stroke
This is an F6.5, 12 1/2 inch diameter mirror. The radius of the curve on this mirror is about 162 inches. An F6 mirror is not considered difficult to parabolize. We drew a 6 inch circle on the mirror back. We use a 'W' stroke as shown in the multi frame picture above. As we polish, the circle we drew is to go to the pitch lap edge but not beyond. We began with a five inch circle on the mirror back and went across and back once at each of the six positions of the lazy Susan and repeated for about 10 minutes. Then repeated with this larger 6 inch circle shown in the picture. The smaller the circle on the mirror's back the smaller and deeper the center depression will become. Since this old Corning Pyrex mirror blank is tapered we put double stick rug tape around the edge so we can lift up on the blank as we polish. We learned not to rotate the mirror centered on the lap at this late stage of polishing. Instead we rotate the mirror a bit during the end of each push stroke. Go slow to keep the lap from warming and grabbing. We have done this final polish on this same mirror several times while learning. If the mirror started spherical this bit of polishing will bring it about half way to parabolic so we should test the mirror before doing more.
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