Fine
Grinding Telescope Mirrors
Baldwin
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, you’ll 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 don’t get that
extreme. But if I get a scratch, it will be my own fault. It’s 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!
Don’t get rid of your tool, you’ll 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
you’ll have to return to some grinding, so keep your tool.
OK,
click here to go to Pitch Laps.