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I’m using an advanced technique: cutting out pieces of paper and putting them under the tool while pressing to avoid certain areas of the tool touching the mirror. I know I have a hole in the middle, so by blocking the middle from wearing down I can concentrate my wearing on the outer zones without making the center even deeper.
Looks like after a few hours I’ve managed to get the outer zone almost completely spherical!
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Unable to find image IMG_20240306_124746726_1.jpg Unable to find image IMG_20240306_160753289~2.jpg Unable to find image IMG_20240306_160449695~2.jpg I’m using an advanced technique: cutting out pieces of paper and putting them under the tool while pressing to avoid certain areas of the tool touching the mirror. I know I have a hole in the middle, so by blocking the middle from wearing down I can concentrate my wearing on the outer zones without making the center even deeper.
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Check out this infographic of how to read the Ronchi tests of my telescope mirror! I’ve been posting lots of pictures like this as I grind, and this tells you how to interpret what the pictures say about a mirror’s shape. The Ronchi test can be used to roughly measure a mirror’s shape and see any turned down edge (“TDE”), and with a computer program to analyze them, even give some quantitative measurements.
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Unable to find image ronchi_infographic.png Unable to find image 4890338-a-ronchi-test-infogr-image.png Check out this infographic of how to read the Ronchi tests of my telescope mirror! I’ve been posting lots of pictures like this as I grind, and this tells you how to interpret what the pictures say about a mirror’s shape. The Ronchi test can be used to roughly measure a mirror’s shape and see any turned down edge (“TDE”), and with a computer program to analyze them, even give some quantitative measurements.
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Two hours of mirror grinding brought me from the first pic to the second! The center zone got so big! It’s so much straighter! I’m much closer to a spherical mirror!
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Unable to find image IMG_20240301_215655700~2.jpg Unable to find image IMG_20240304_154953737~2.jpg Two hours of mirror grinding brought me from the first pic to the second! The center zone got so big! It’s so much straighter! I’m much closer to a spherical mirror!
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The center zone, visible in the first pic as the inner area with red on the left and black on the right, is a bit more smooth and has grown from around 50% diameter to around 80% diameter. The outer zone is still a gradual slope but it focuses light to around a centimeter or so further than the inner zone. Previously that outer zone was extremely too tall, so my goal was to reduce it, and compared to a few weeks ago it looks reduced but not gone.
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Unable to find image IMG_20240301_215405266~2.jpg Unable to find image IMG_20240301_215655700~2.jpg The center zone, visible in the first pic as the inner area with red on the left and black on the right, is a bit more smooth and has grown from around 50% diameter to around 80% diameter. The outer zone is still a gradual slope but it focuses light to around a centimeter or so further than the inner zone.
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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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