The mirror test: what I learned from grinding my first telescope optic

I bought a glass blank, some grit, and a tube of pitch with the dream of building my own Dobsonian telescope. Everything I read made it sound simple: grind the curve, polish the surface, add a coat of aluminum, and you get a mirror that can pull in galaxies. I learned the hard way that the difference between a good mirror and a great mirror comes down to one thing: how you measure the shape of the glass while you are still grinding. I made that mistake, and it cost me forty hours of work and a blank I had to throw away.

After three weeks of hand grinding I thought my mirror was ready for polishing. The curve looked smooth, the focal length matched my plan, and I was eager to see the first star. But when I set up a simple knife-edge test the image was a mess. Dark bands ran across the surface. I could not tell if the center was too high or too low. I turned to the web and found a set of practical notes on stellamia.org that walked through the Foucault test in plain terms. That site showed me what I had been doing wrong: I had never checked the correction after each grit change. I had been grinding blind.

I do not want you to make the same mistake. Here is what happened and what I changed on the second attempt.

The allure of speed vs. the reality of glass

I started with a motorized turntable to speed up the rough grinding. The tool spun, the grit cut, and the curve deepened fast. That felt like progress. But because I never stopped to measure the curve, I did not realize the center was getting deeper than the edge. By the time I switched to finer grit, the correction was already too steep. I tried to fix it with a shorter stroke, but that only made the surface wavy. I ended up with a mirror that had a turned-down edge and a zone in the middle that looked like a saddle.

• I did not test the radius of curvature after each grit stage
• I used the same stroke length for too long without checking the result
• I let the tool sag under its own weight because the turntable was not perfectly level

The Foucault test – your only friend

The Foucault test sounds intimidating, but it is just a light source, a knife edge, and your eye. You move the knife across the focal point and watch how the shadow moves across the mirror. A perfect sphere shows a smooth, uniform fade. Any dip or hill shows up as a dark band that moves differently. I had seen diagrams of these shadow patterns before, but I never understood what they meant until I read the explanation on stellamia. The key insight is that you do not need a fancy tester. You can build one with a cheap LED, a razor blade, and a measuring tape.

• A stiff cardboard box to hold the light and the knife edge
• A fine-thread micrometer to track the knife movement
• A dark space where no stray light hits the mirror
• Patience – expect to test three times before you trust what you see

When to start over

I spent two more weeks trying to save the first mirror. I regroud, I changed strokes, I even tried a new tool. Nothing fixed the turned-down edge. The glass had lost too much thickness in the center. I finally gave up, ordered a new blank, and started again. The second time I tested after every grit change. I stopped the turntable and checked the curve with the Foucault setup. It took longer – maybe an extra hour per session – but the mirror came out clean. Polishing went smoothly, and the final figure held a sharp null under the test.

That first mirror now sits on a shelf. I use it as a paperweight and a reminder that speed saves time only if you know exactly what you are doing. The telescope I built with the second mirror shows me the rings of Saturn clearly at 200x. That is the result of taking the time to test, not the time to grind fast.