August 14, 2019 at 6:37 am #109598
I’m genuinely interested in building my own telescope one day. One idea that came to mind would be to make the parabolic mirror out of a thick acrylic (2″+), and mill the exact contours of the concave surface with a CNC. I’d then mirror coat it, and polish to a super high gloss.
Considering 24″+ Mirrors would run you easily $10,000, or more, this seems like an ideal second option to save enormous amounts of cash.
Anyone into telescopes, and thought of doing this?August 14, 2019 at 8:54 am #109615
I know one of the reasons they are so expensive is because of the insanely precise rigs they use to make them.
In college, we coated a piece of glass to make a mirror finish (Why was this part of the summer session in the Physics department, IDK) and it required a vacuum and putting a large voltage on a piece of metal (aluminum, I think) and the atoms would fly off and coat the glass. It made nice mirrors, but it was hard and we were doing it on like 1/2″ mirrors.
But besides that, just getting it extremely accurately milled is going to be tough. There are different versions of “flat” and this would require a really “flat” setup, where flat in this case means exactly along that sphere.
If it’s interesting to you though, it would definitely be fun to try. There’s a good change you’ll get something that will work, although I don’t know how well. Since it’s so big, it would be able to catch a lot of light, but if it’s not perfectly “flat” it won’t focus well. Either way, it would be neat.August 14, 2019 at 10:05 am #109619
Well, even if it got it close, there’s a lot of hand sanding techniques that can get it the rest of the way to the finish line. There’s tons of videos online that discuss light testing to ensure the mirror is an acceptable f-value, and I’d assume you can do several of these tests throughout the grinding process prior to turning the acrylic into a mirror surface.
Who knows – maybe I’ll look into it down the road when I need to push the limits. I don’t even have my cnc assembled yet, so I need to focus on that first lolAugust 14, 2019 at 10:57 am #109625
I have a project on my “someday” list, to make a corrector plate for a spherical telescope. You can get small spherical mirrors from Amazon for cheap ($20 for an 80 mm mirror). I built a telescope with PVC and a Raspberry Pi and one of these mirrors. The problem is that at high magnification, the spherical aberration is pretty bad.
So I could calculate a thin aspherical corrector element and mill it out of acrylic, but here’s where I had another idea: milling tolerances being what they are, it might not be very good, but I could cheat by using two pieces of acrylic and milling the lens as a cavity between the two pieces, and then filling the cavity with a material with almost the same index of refraction as the acrylic. The small index difference makes the correction much weaker, meaning the optic must be thicker but also much less sensitive to machining errors.
Not quite what you asked but related and maybe inspires you.
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