Fine Grinding

The rough grinding over, it's time to start the fine grinding.  The rough grinding left the mirrors with a better surface than the curve generator, but the surface was extremely pitted.  Pits are formed by the action of the grit during grinding.  Some pits may be the result of defects in the original blank.  In fine grinding you are simply using a sequence of increasingly fine grinding grit (actually the finer grades are fine powders), with each (hopefully) removing the pits from the previous stage.  It will not be optically smooth at the end of this process, but it will be smooth enough for the polishing phase.

George is looking at the surface of the mirror.  At this point, we have started the first stage (#120 grit) of fine grinding.


In this stage of the grinding, the material is being removed by the grit 'rolling' over the surface of the mirror. This results in the grit fracturing the glass and causing pits in the glass. To help document this process, John Mahony used an "eyepiece projection minus telescope" set-up as a photomicroscope and took a close-up of the surface of the mirror.  The dark spot in the center of the image is a pit left from the previous grade of grit.  The rest of the image shows the finer surface texture resulting from the newer finer grit.


His setup...


Of course, we all have to document the documenters....


Once the fine grinding was done, a pre-polish step was taken...

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