Fine Grinding Telescope Mirrors


Once a telescope mirror has been course ground through 500 grit silicon carbide, and it has uniform pitting and is in spherical contact, then it is ready to be fine ground. We usually do fine grinding using aluminum oxide, and I recommend using 12 micron, 5 micron and 3 micron grit sizes. Some ATMers continue on to 1 micron, which would reduce the time it takes to polish the mirror, but the probability or clumping and scratching is so high that I do not do this myself. For smaller mirrors, maybe this would be proper, but for larger mirrors, it is generally a scratch waiting to happen. Even 5 micron can give you issues in this department.

Fine grinding follows the same cautionary detailing of cleanliness as course grinding, only more anal. Clean, clean, clean. Period. Anything that gets into your work will result in a scratch. If it dries while working it, youíll get a scratch. You will still use a loupe to observe pits and decide to continue based on the same pit differential as before, only they are much smaller in size. Some ATMers will shroud their environment with plastic sheeting and hose their rooms down, but I usually donít get that extreme. But if I get a scratch, it will be my own fault. Itís up to you, your luck, and if your environment is dusty or not.
Also, when you are about to take the tool off after working, add some grit mixture and slop it around. It sounds like a waste of grit, but taking off the tool as the fine grit mixture dries up can make them stick and then scratch.

When your fine grinding is over, you have completed a landmark. Your grits will go away and you will start on the polishing part of your mirror project. Awesome! Donít get rid of your tool, youíll either be putting a pitch lap on it, or if you make a separate pitch lap, it is possible that there will be a goof and youíll have to return to some grinding, so keep your tool.

OK, click here to go to Pitch Laps.