To measure my progress on rough-grinding the mirror, I mounted a dial indicator in a piece of high-density particle board. I figured that the smooth back of my mirror is the flattest surface I have, so I calibrated my zero point on that. The two markings indicate my target depths, exactly 0.1 inches for an F-5, and about 0.01 inch less than that for an F-5.5.
The rest of these pictures are taken at about 1-2 hour intervals. i.e. This whole page represents about seven or eight hours of grinding the mirror. I started including the tool in the pictures to show how the tiles have worn down from grinding the mirror. As the glass becomes concave, the tile tool becomes convex and by the end of rough grinding they should both hopefully be spherical.
Before I had reached 0.05 inches of depth, I was using the "chordal stroke" to grind. This stroke will aggressively dig out the center of the glass, removing the bulk of the glass rather quickly. The tile tool shows how this stroke wears on the sides of the tool much more quickly than the center. For the mirror it is the opposite, and the 0.05 mark was reached within a few hours of grinding.
Next I moved to the "W stroke," which should quickly bring the shape into a sphere after the center has been ground out. If done for too long, though, the W stroke will go beyond spherical and create a parabola of the wrong focal length. Notice how the tile tool is being worn more and more toward the center.
I had hoped to have every part of the tile tool ground before I reached my target depth of 0.1 inches, but as the final picture shows that didn't happen. I'll have to switch from the W stroke to the "normal stroke," which is meant to maintain a spherical shape but continue digging deeper into the glass and tool. When the entire tile tool is worn through, I can be sure that both the tool and the glass have spherical curvature. By alternating between grinding with the tool on the bottom and the glass on the bottom, I can maintain that curvature at a proper radius.
I love that tool. I'm going to have to steal the idea from you. I imagine it's incredibly accurate and easy as anything to use! Good job coming up with it.
ReplyDeleteOne of the great things about building telescopes is that you're always making stuff in the process. Build a pattern to build a mold to build a jig to build a tool to build a telescope...
You can see the couple of tiles that were slightly uneven when I poured the dental stone. They should work down to come in contact before too long, though. It's looking really good.
ReplyDeleteRun one wet extra long so your slurry gets to be real fine, and you will get a good look at the contact between the tool and the glass.
Also, another test for contact is the sharpie test. Use a sharpie marker to draw a grid on your glass, and watch how it grinds away to see how good your contact is. My experience, however, is that even with good contact it will still grind off of the center first.