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Goodbye mars! (Zoom in!)
Taken using my 3D printed telescope, with a crowd of friends invited to watch the occultation of mars by the moon! The phone I was taking photos on ran out of battery, so I hastily threw a friend’s iphone into the holder just in time to capture these.
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8" telescope: COMPLETE!!
It has literally taken all year, but my second telescope ever is done! It’s made of 3D printed parts, metal tubes, nuts and bolts, and a mirror I hand-polished over the course of 8 months! I’m very proud.
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When grinding thin mirrors, there is something called the “twyman effect”: grinding one side adds stress that can slightly curl the mirror’s other side. It can lead to slight astigmatism in thin mirrors that disappears once you grind the back side to relieve that stress. (See https://quinsightspectre.com/16-25-f-3-1-meniscus-mirror/ !)
To avoid any problems, I decided to follow the wisdom and grind my meniscus mirror’s convex back through #220 grit. Plus, it would get rid of any saddle shape, which I could see existed when beginning to grind the front.
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Today’s hour of mirror grinding has completely gotten rid of the low zone! Now I just need to grind down through that one tiny divot.
I noticed it seemed like my tool was pushing the water I sprayed out off the mirror. I used a blade to try to carve notches in between my hexagonal tiles, and afterwards I could feel the tool sliding more smoothly, allowing water to flow between the tool and the mirror.
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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.
Let the grinding start!
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Taken using my 4.5" telescope and phone! Both Mars and Jupiter fit into telescope view!
Very annoying observing session. Tried to take photos of the ring nebula but missed; planet conjunction was extremely slightly out of focus and I didn’t see until morning, and I looked for but still couldn’t see M33. Couldn’t see any detail on mars by eye either; that should change as we get closer to opposition.
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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.
Read More