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Unable to find image 6966242-8-telescope-mirror-image.png 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.
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Attempt 1 This time I bought some new porcelain tiles from a local store for $5. Apparently glaze on tiles are slightly softer than porcelain, but per advice I just placed the tiles upside down. Mirror sat below, then plastic wrap on top, then tiles, and finally dental stone gets mixed and poured on top. Dental stone is a waterproof plaster that hardens fast; last time it hardened in my bucket after just 15 mins of mixing.
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To grind my 12" blank into a mirror, I need a grinding tool. Since the glass is already curved, I’m going to make a grinding tool for my slumped telescope blank using these ceramic tiles I bought and dental stone, a waterproof plaster! The tiles are hard and will resist the grinding from grit pieces; they’re placed onto the curved glass so the tool takes its curved shape, and dental stone will be poured over this and harden into a convex tool for grinding with hard ceramic pieces sticking out.
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I got the news from my local glass studio in an all-caps email that said nothing but “IT WORKED!”
Look at this! My glass is now clearly curved, around 1/4" taller at the sides than at the center! That’s very close to f/4, my goal! SLUMPING DRAPING: SUCCESS
Weirdly, even though the mirror is the right shape, the cement form seems to have warped. The top face is now flat, and the bottom is now curved - which is weird because originally the bottom was flat and the top was curved.
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I’m at the final step of polishing a telescope mirror: turning a sphere shaped piece of glass into a parabola shaped piece of glass, called “figuring” or “parabolizing”. I messed up, twice, and since then I have been regrinding my glass back to a sphere for figuring attempt #3. Today, after several months, I finally have a spherical enough surface to start figuring attempt #3!
Here’s both a Ronchi test and a Foucalt test.
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Unable to find image verystraight_20240610_150033.jpg Unable to find image 6533527-8-mirror-grinding-image.png I’m at the final step of polishing a telescope mirror: turning a sphere shaped piece of glass into a parabola shaped piece of glass, called “figuring” or “parabolizing”. I messed up, twice, and since then I have been regrinding my glass back to a sphere for figuring attempt #3. Today, after several months, I finally have a spherical enough surface to start figuring attempt #3!
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I’m parabolizing my telescope mirror - I’ve ground it to a sphere shape, and now I’m grinding a tiny bit more to change its shape precisely into a parabola.
I’ve done 3 grinding sessions for a total of 34 minutes of grinding. Here’s a Ronchi picture of my current progress. Mel’s online Ronchi calculator has overlaid some semi-transparent white lines which show the ideal Ronchi grid of a somewhat parabolized mirror.
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Unable to find image image.png Unable to find image texereau-normal I’m parabolizing my telescope mirror - I’ve ground it to a sphere shape, and now I’m grinding a tiny bit more to change its shape precisely into a parabola.
I’ve done 3 grinding sessions for a total of 34 minutes of grinding. Here’s a Ronchi picture of my current progress. Mel’s online Ronchi calculator has overlaid some semi-transparent white lines which show the ideal Ronchi grid of a somewhat parabolized mirror.
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I’M SO CLOSE TO A SPHERE
(You know you’re a sphere when the lines are completely straight. See this infographic for more)
total mirror grinding time: TWENTY GODDAMN HOURS
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Unable to find image IMG_20240309_162343854~2.jpg Unable to find image IMG_20240309_162149283~2.jpg Unable to find image IMG_20240309_161749437~2.jpg I’M SO CLOSE TO A SPHERE
(You know you’re a sphere when the lines are completely straight. See this infographic for more)
total mirror grinding time: TWENTY GODDAMN HOURS