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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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I’m in the home stretch - my goal is to turn this sphere shaped mirror into a parabola shaped mirror.
The final stage of mirror grinding is called either parabolizing, because you’re making a parabola, or figuring, because you have to measure carefully and that involves numbers which are also called figures. You use a stroke that goes up and down fast and side to side slowly in a big zigzag, called a W stroke, to remove a tiny bit of glass from both the center and the edge, as seen in the first picture (from Mel Bartels’ site).
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This site is my secret weapon: https://www.bbastrodesigns.com/ronchi.html
It’s a slightly janky javascript app made by some old telescope maker in Oregon, and it lets you get quantitative measurements from all those Ronchi pictures. Very important.
Looks like my outer zone focuses light at around 0.066 in from my desired radius of curvature, and inner zone focuses light at around 0.1 in. (There’s probably big error bars on both those numbers).
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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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I did 20 minutes without my center paper cutout and managed to dig a tiny zone in the center. Yaaaaay. Then I spent two hours ignoring it and trying to get the outside zone down.
On the plus side that outer zone at 70%+ diameter looks spherical now.
Now I have a choice: I think I’m close to a parabola. With that deeper center, if I can flatten the inner edge of that transition I’m very close.
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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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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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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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Here’s my mirror in a Ronchi test and a Foucault test. I’ve managed to reduce the turned down edge to a very small area but it’s still there, visible in the ronchi as hooks at the edges and in the Foucault as a slight black zone at the bottom left of the image. I’ve also managed to dig a huge hole in the center of the mirror trying to fix that edge…