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The cloudy part is where the grit has sanded the glass. Clearly part of the glass is higher and making less contact. I bet it’s because the furnace cement mold was slightly curved because the wooden mold was slightly curved thanks to the grain of the wood
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My grinding tool is making cool patterns!
You’re looking at water trapped under my 12" meniscus mirroe, and flowing between the channels in the home depot porcelain tiles on my grinding tool. Those tiles will grind flat eventually.
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I got a table for $5 at a yard sale. It looks perfect for a mirror grinding setup after putting some weights on the bottom rack.
Now that I’ve completed my 8" mirror polishing, it’s time to try my bigger telescope mirror project. Just making this blank took a lot of effort - now, to grind it.
12" meniscus mirror: Grinding start!
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Someone brought an extremely accurate “double pass autocollimator” tester to an amateur telescope maker meetup… and it showed an uniform shade of gray across the whole mirror, meaning no defects (ignore the circular reflections from the test stand). That means my mirror is an extremely smooth parabola! I’m done mirror grinding!
🎉 First mirror: complete! 🎉 That’s so cool! I thought I was slightly overparabolized based on my Foucalt testing.
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Unable to find image 7164387-8-mirror-actually-image.png Unable to find image 7164387-8-mirror-actually-image2.png Someone brought an extremely accurate “double pass autocollimator” tester to an amateur telescope maker meetup… and it showed an uniform shade of gray across the whole mirror, meaning no defects (ignore the circular reflections from the test stand). That means my mirror is an extremely smooth parabola! I’m done mirror grinding!
🎉 First mirror: complete! 🎉 That’s so cool! I thought I was slightly overparabolized based on my Foucalt testing.
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8" mount v1 complete!
After several days of designing, 3D printing, and experiencing the joys of my favorite telescope related activity, hacksawing through metal, I managed to assemble a complete mount! Design-wise, I had to replace 0.5" screws with 0.75" screws in a few places because the redesigned EMT tubes were too close together. The telescope uses only 0.5" screws, so it would have been nice to match, but it’s fine for an one-off.
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Every telescope needs a mount, to hold it up and let it rotate. 6 months ago, I thought that I was close to the finish line, since all I had to do was make a mirror and a mount! Then the mirror took six months. Now I need to sit down and actually finish this mount. My 4.5" scope uses a truss of EMT steel tubes and the scope sits on a wide C-shaped part.
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I did a tiny bit more parabolizing, then re-measured my mirror. Last time, my spreadsheet and measurements told me my outer zone needed to focus tiny bit farther and my lower zone needed to be a tiny bit closer. To dig the inner zone out, I did some W strokes with lots of mirror movement on the tool edge, and then used long center over center strokes with a tiny offset to dig everywhere but a tiny strip of outside.
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Unable to find image image2_1.jpeg Unable to find image image2_1.jpeg I did a tiny bit more parabolizing, then re-measured my mirror. Last time, my spreadsheet and measurements told me my outer zone needed to focus tiny bit farther and my lower zone needed to be a tiny bit closer. To dig the inner zone out, I did some W strokes with lots of mirror movement on the tool edge, and then used long center over center strokes with a tiny offset to dig everywhere but a tiny strip of outside.
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I’ve switched to using the Foucalt test and a Couder mask for parabolizing. The Foucalt test is more precise (but more annoying) than the Ronchi test, but my local astronomy club has a foucalt testing rig with a micrometer that lets me measure very precise details of where different regions of the mirror focus light, so it’s more accurately moveable than my handheld Ronchi tester. I built a cardboard “Couder mask” to block off regions of my mirror to see where the edge and center are focusing light to separately.
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