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Casting a Large, Light-Weight Telescope Mirror from Recycled Glass

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Step 12: Bonus Material: Rough and Fine Grinding the Mirror

Photo #1 shows the rough grinding process. The mirror blank is ground against a ceramic tile tool with #60 carbide grit in between. Using the correct grinding stroke results in the mirror becoming concave as the tool becomes convex. This process is continued until the mirror reaches the desired curve. The process was all done by hand, and took several weeks of Saturdays, (and raised a few blisters).

Photo #2 shows the mirror at the end of rough grinding. I was aiming for an f/4.5 mirror, so the required depth of the curve is 0.175 inch. Once the required depth is reached, the rough grinding is complete. Amazingly, the rough grinding has removed over a pound of glass. The mirror now weighs in at only only 8 pounds 15 ounces. Now the fine grinding can begin to clean up the rough surface left by the rough grinding.

Photo #3 shows the sequence of abrasive grits that must be used to get from rough grinding, through fine grinding, to polishing.  Successively finer grits are used to clean up the pits left by the previous stage. Eventually the mirror reaches the stage where polishing with Cerium Oxide can begin.

Photo #4 shows the mirror at the end of fine grinding. The surface is not quite transparent. It still has a slight frosted look to it. Rough and fine grinding are done outside to prevent contaminating the polishing room with large grits that could cause scratches on a mirror. Once it is time to polish, the mirror gets thoroughly cleaned and I go into the polishing room. See the next step for polishing.
 
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