May 1998 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - May 1998 Issue

The Telescope Nut
by Jeff Baldwin

Course-to-Fine Grinding

When making a telescope mirror you work two pieces of glass together. One is concave and the other is convex. The telescope mirror will be the concave glass. They have the same radius of curvature, meaning that the curve is the same on both glasses except that one is concave and one is convex.

During curve-generation, the largest grit is used, generally 60 grit. Sixty grit is made of 1/60^th inch particles of silicon carbide, a very hard abrasive. Once the mirror and tool are of the correct curve, then the two must be made to be co-spherical. This is done by performing 'W' strokes with one on the other with the grit between them. Place the mirror on the barrel and wet it. Sprinkle a little 60 grit on it and place the tool gently upon it. Start grinding the two together in back and forth strokes that also work side to side making a "W" pattern. This will eventually grind the two into a section of a sphere.

An important reminder here: At all stages of grinding and polishing, you should be constantly working your way around the barrel, and frequently turning the mirror in the opposite direction between strokes.

When the charge of grit becomes less noisy and offers less resistance, then it is time to replenish your grit and water. This work is continued until the two glasses are co-spherical. You will know when this happens by the bubble pattern between the two glasses. If the two glasses are perfectly in contact, then the particles between them will roll at half the rate as the tool is moving over the mirror. If the mirror has any low spots in it then a vacancy will occur over that spot that will appear to not move with the other grit particles. If there is a low spot on the tool then a vacancy will occur over that spot and will appear to move with the tool. All particles that are between the mirror and the tool in perfect spherical contact will move at half the rate as the tool leaving no vacancies that are stationary or moving full rate with the tool.

When the mirror is in spherical contact with the tool, then it is time to move to smaller grit. The next grit is 80 grit, which is 1/80^th of an inch across. "W" strokes are performed with the glasses now just as you did with the 60 grit.

Two things must occur before moving on to the next smaller grit: 1. spherical contact must exist, and 2. pits in the glass must be only one size, the size of the current grit. When the pits are of two distinct sizes -- those of the current grit and those of the previous grit -- it is too early to move on to the next grit. When only one distinct size remains, then the mirror has been ground out to that degree of smoothness.

Following 80 grit is 120 grit (1/120^th inch), then 200 grit, 320 grit, 400 grit, 500, 600, then 12 micron grit and 5 micron grit. A micron is 1/1000^th of a millimeter. A millimeter is 1/25.4 of an inch. That makes a micron 1/25400 of an inch, 12 micron grit about 1/2000 of an inch, or 2000 grit, and 5 micron grit would be about 1/5000 inch, or 5000 grit. The exactness of this number is meaningless; we generally jump by factors of 2 or so. Some mirror makers skip 400 grit going from 320 to 500, and some skip even more than that. As long as your tests of spherical contact and single pit sizes pass, you're ok to go on.

At this point, you have a mirror and a tool that are spherical to about 1/5000" with pits about that size. This is very smooth. You need a close-up microscope to see this pitting. Reflection is another great tool to determine how big the pits are. The angle at which light reflects off a pitted surface can also determine if you are ready to go on to the next grit, and sometimes the color of the reflections can be of help. As you make more mirrors you will find ways of telling if you are smooth enough to go on or not.

Last time I spoke about grinding the back of the mirror first. The back of the mirror and the back of the tool could be ground together to become flat to 5 micron smoothness. Some folks even polish the backs. If it isn't a solar scope, I don't polish the backs. If it is a big fat mirror I will usually grind to about 220. Thin mirrors I like to grind the backs to at least 500 grit, so mirrors in the 10" to 16" range I'll grind to 5 micron. If the backs are very flat then there should be a minimization of the transfer of astigmatic curvature from the back to the front. This is important to prevent -- just ask Lloyd Altamirano!

When grinding a mirror to a specific focal length you have previously measured its sagitta. I like to do this between every grit just to make sure that my curvature is still on target. If you check it at 60 grit and never check it again until the mirror is smooth to 5 micron, it may have changed focal length by an inch or more. You can lengthen your focal length by working strokes on the outside region of the mirror, and you can shorten your focal length by overworking the inner regions of your mirror. It is easy to subtly change your focal length with smaller grits, and it may be necessary.

Ok, the mirror is now curved and smooth to 5 micron. It is ready to have a pitch lap poured to begin polishing. See you next time.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.