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…the cement form’s curvature isn’t right. I measured the sagitta (how deep the curve is in the middle compared to the edge) and it’s 1/16". it should be around 1/4". This would make a telescope that’s 12 feet long.
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAARGH
I CNCed mold’s shape wrong. I have two choices: I can either grind/sand down the edges of my cement form into a deeper curve (probably releasing tons of bad to breathe in dust), or start the process all over again with new wood.
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In addition to grinding an 8" mirror, I’m trying a technique to make big thin 12" mirrors. See the #meniscusmirror tag for more. Previously, setting things on fire didn’t improve the problem.
Now I have a $35 tub of castable furnace cement (amazon had it for $14 cheaper than home depot). It’s dry sand that needs water and throws up clouds of dust, glad I have a respirator from covid.
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After a few weeks of drying, the wood was very warped and I couldn’t get the furnace cement out; perhaps it had bonded to the wood. What’s the difference between wood and furnace cement? Cement is heat resistant, and wood is very flammable.
So I started a fire. I grabbed some sticks and some newspaper as kindling and piled it on, then lit it on fire inside a barbecue. Turns out I suck at starting a fire - I didn’t have enough small sticks to sustain a fire for a while and catch the large wooden mold on fire.
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To make a meniscus mirror I need a precisely-shaped thing that will stay the same shape even at 600C. On Jan 31, I machined out a new piece of wood, sprayed it with polyurethane, and then poured in furnace cement!
…but several days of drying later, I couldn’t get the cement out of the mold. It stuck to the wood too well. In attempt #1 I used 3 coats of polyurethane, but in this I only used one.
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I have many projects in mind for telescope upgrades. Since the last list I’ve completed one and added one. Here’s what I want to do this year telescope wise:
#1: Sliced Pifinder (complete!) Complete! I built a Pifinder for 1/5 of the list price by using a different cheaper camera, secondhand older pi and battery pack, and printing and soldering parts myself. It’s been very helpful when it works, and let me take pictures of M33 even without seeing it!
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Poured some cement into my CNCed mold! Very fitting that my experimental telescope making technique involves a black hole
I noticed my mold had warped before putting cement on it - maybe from the polyurethane spray I used, or from waiting a month in between cncing and pouring? I put some weights on it because of the warping, walked away… And the next day I found my wood split down the middle of a boundary between planks.
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One of my telescope shenanigans is trying to make a meniscus mirror by slumping glass in a kiln. To do that, I need to make a precisely shaped form that can withstand kiln temperatures. I CNCed a wood mold, and next I need to put some furnace cement into that wooden mold so it becomes the right cement shape. I’ve never used furnace cement before. Wish me luck