Course Grinding Telescope Mirrors


Assuming your tools have been made and your mirror has been curve generated, it is time to start grinding your mirror. The purpose of course grinding is to ensure your mirror and your tool have spherical contact and your radius of curvature is correct. Course grinding starts with 80 grit for small mirrors and 60 grit for large mirrors. The grit sizes in course grinding are 60, 80, 120, 200 and 500 in silicon carbide. A grit size is basically how many particles it would take lined up end to end to make 1 inch. Many mirror makers use more intermediate sizes than this, but the basic rule is to use half the size on the next step than the size you are using now. Any other intermediate sized than what I listed here would be overkill.

Tool on Top Method. Place the mirror atop a barrel and secure it with blocks on the side so the mirror can be freely and easily rotated by hand but not slop around or fall off. Wet the glass, sprinkle lightly your 60 or 80 grit around the mirror evenly distributed, and place the tool atop the mirror onto the grit and water. With some pressure, push the tool forward and backward at [about a 2 Hz rate if the mirror is 10” or less, and about 1 Hz rate if it is 20” or larger]. While doing this, you will move each stroke a bit further left each time, eventually running out of room on the left, at which time you will do each stroke a bit further to the right each time, until you run out of room on the right, and so on. This will create what we call “W Strokes”. Forward and back moving to the right, then forward and back moving to the left, and so on. As you do this, you will be moving slowly around the barrel, which will allow the mirror to be ground from all directions, not just the one you started at. Your grit will get quieter and will be easier to move, at which time the grit has broken itself down to small particles and is no longer doing the work you desire. Remove the tool, wash out the muddy guck, rewet and re-grit, and start in again. Once in a while, rotate the tool in the same direction that you are moving around the barrel, and rotate the mirror in the opposite direction. This will keep the mirror and tool surfaces of revolution, round bowls instead of oblong bowls. When I say oblong, I mean that it will be microscopically oblong. You won’t be able to see this defect visually until you test your telescope mirror and see astigmatism, which will tick you off. So maintain a constancy in your rotation job.

You will be finished with a grit size when you have spherical contact, the mirror has the right radius of curvature, and pits are uniform in size and distribution. Spherical contact occurs when the tool and the mirror mate perfectly no matter where they sit with each other. If they have the same radius of curvature (one concave, the other convex) and they are in exact contact no matter where you move them, you’ve got it. When the mirror and tool are rewetted and fresh grit is applied, you can see through the tool (if it’s glass) or the mirror (if it’s on top) and you will see the grits and water moving around between them at half the rate that the top one is moving, acting like a zillion little ball bearings. This would be spherical contact. The radius of curvature will be right when the sagitta formula is met, . If the sagitta is too large, the focal length is too short, and if too small, the focal length is too long. Finally, the grit leaves pits on the glass. If the pits are uniformly distributed, the same size, and go ALL the way from edge to edge with nothing missing, then you are good to move to a smaller grit. When you go from one size grit to another, such as from 120 to 220, you will examine the glass under microscopic inspection, such as a loupe or an eyepiece used as a magnifier. When the mirror is not yet ready to move to the next grit, you will see a field of pits with a few larger pits distributed amongst them. When those larger pits are gone and the whole field is uniform in pit size, no matter where on the glass you look, then your mirror is ready to go to the next grit. Each mirror, each ATMer, each condition, each problem; they all make changes in how things go. For this reason I am not going to tell you how long on each grit you need to work. Some ATMers push harder, stroke faster, are luckier, and have different temperature conditions (or who knows what else) than other ATMers, so there is no rule for time. I highly recommend not setting deadlines on telescope mirror making.

As you move from grit size to grit size, things get different. The sound gets quieter, the feel gets smoother, but the function and theory are all the same. Between grit sizes, you MUST clean the living crap out of everything. Imagine grinding with 500 grit and somewhere along the way a piece of 120 grit finds its way to your work. It will scratch a path as you move the tool and mirror against each other that is 5 times deeper than the 500 grit that you are using. You will not be able to fix this by continuing with 500 grit. If you get a scratch form a previous sized particle, you will need to go back to that particle size grit and work your way back to where you are. Sorry, that’s the way it is. So clean up the mirror, tool, barrel top, barrel side, room, change clothes, etc. Grit contamination will upset an ATMer. If you are in an area that somebody else is working, you must be carful not to contaminate their process, or if somebody is watching you or “helping” you, don’t let grit contamination occur. When storing grits, separate them, keep their containers tightly shut. And store them big stuff below small stuff, if in the same cabinet at all. When moving toward smaller grits, a tendency to scratch increases. When using 500 grit, the work will try to dry out, and particles will try to clump up, and you will get scratches even if you are contamination free. So the smaller your grit sizes, the more aware of drying out you need to be.

Beginning with 500 grit, you should apply the grit and water and a mixture rather than sprinkling dry grit onto a wet surface. This is true for fine grinding with aluminum oxide as well.

Once you have passed through all these grits, including 500 grit silicon carbide, there are no scratches, the pits are uniform, the radius of curvature is good, then you are ready to go to fine grinding.