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Mirror grinding

scopemaking.net

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Re: Mirror grinding

#11

Post by digler999 »

chanderson0 wrote: post #6

My parents run a business making telescope mirrors, and I grew up around this exact process. Measurement is the biggest difference between this and what they do. Take a look at https://en.wikipedia.org/wiki/Interferometry#Engineering_and... if you want to know more about how it's done in industry. The best metaphor I got from them about how precise a shape they make in glass is that if you took a typical 1 meter mirr…

I'd imagine the math to be off-the-charts complicated, but I'm interested in the signal processing software people have created for correcting optical aberration (and how effective it is). I believe they pioneered this technology for the hubble's mirror defect. If it's effective, then manufacturers could reduce costs by not even needing to try for perfection, just staying within the limits of what software can fix.

Re: Mirror grinding

#14

Post by jonathansizz »

I know a few keen amateur astronomers who make extra cash by producing astronomical equipment for other amateurs. Some of them are very skilled, and could probably start a fully-fledged business if they wanted to, but they prefer to keep it as a hobby.

I paid one guy $400 to build me an equatorial platform for my 12-inch Dob. The value it provides me is way higher than the financial cost, and he could probably charge more for his work, but I think his main motivation comes from the satisfaction of building things he's proud of and that others can enjoy. Certainly, when I occasionally donate my time and knowledge at public astronomy outreach events, I find it very rewarding, especially if I can inspire children's interest.

N.B. By far the biggest weakness of Dobsonian (or any alt-azimuth mounted) telescopes is the lack of tracking, and thus having continuously to nudge the tube to keep objects centred. An equatorial platform eliminates this weakness, and I strongly recommend one to anyone who's frustrated with their alt-az scope.

Re: Mirror grinding

#15

Post by NegativeK »

chanderson0 wrote: post #6

My parents run a business making telescope mirrors, and I grew up around this exact process. Measurement is the biggest difference between this and what they do. Take a look at https://en.wikipedia.org/wiki/Interferometry#Engineering_and... if you want to know more about how it's done in industry. The best metaphor I got from them about how precise a shape they make in glass is that if you took a typical 1 meter mirr…

Hand techniques are still used for reference surfaces in machining. Granite surface plates (highly accurate planes) are hand lapped[1], and metal bearing surfaces on machine tools and cast iron reference tools are sometimes hand scraped[2].

[1] On location calibration and lapping of granite surface plates: https://www.youtube.com/watch?v=EWqThb9Z1jk [2] Hand scraping (effectively chiseling) of reference cast iron surfaces: https://www.youtube.com/watch?v=nOJrhrne80s

Re: Mirror grinding

#16

Post by dreem »

chanderson0 wrote: post #6

My parents run a business making telescope mirrors, and I grew up around this exact process. Measurement is the biggest difference between this and what they do. Take a look at https://en.wikipedia.org/wiki/Interferometry#Engineering_and... if you want to know more about how it's done in industry. The best metaphor I got from them about how precise a shape they make in glass is that if you took a typical 1 meter mirr…

Even currently, many world class high-precision mirrors are ground and polished by hand. I believe the machines you're describing are just copying the same procedure that is done by hand (a robot or swivel arm imitates the motion that a hand would make), and this process produces surfaces that have an average roughness of less than 1 Angstrom across the optic.

However, there have been other technologies that will polish out tiny non-uniformities and 'bumps' on the order of nanometers tall across a few square microns. These technologies, such as MRF finishing https://qedmrf.com/en/mrfpolishing/mrf-technology/how-it-wor... are the current state of the art to get the best surface finish.

Re: Mirror grinding

#17

Post by digler999 »

chillydawg wrote: post #3

I don't really understand how a perfect shape can come out of that process. Can someone explain how it's a product of the grinding technique?

Depends on what you mean by perfect. its very precise, but I'm sure if you examined the surface with an electron microscope you'd still see imperfections.

Edit: was incorrect about what I said below. Leaving it for posterity

It's important to remember that the final surface - the one that actually reflects photons - is created chemically by releasing a gas inside a vacuum that very evenly coats the surface with reflective metal atoms. I'd wager that process fills in the ~last nanometers~ or so of imperfections. Or at least averages them out enough to not affect the optical performance.

Re: Mirror grinding

#18

Post by conistonwater »

jacquesm wrote: post #7
chillydawg wrote: post #3

I don't really understand how a perfect shape can come out of that process. Can someone explain how it's a product of the grinding technique?

The grit only works where it supports the tool. So as long as you rotate frequently it will just hit new 'high' spots while not affecting the low spots at all. By hitting the interior more often than the edge that's where you'll go deepest, it's more statistics than actual aiming for a some high spot or difference. The tool-on-top or mirror-on-top swap can help to adjust if you've gone too deep by favoring supporting…

> bounce steel blocks on a vibrating drum

Wow, is that real? Do you have a link? I want to see.

Re: Mirror grinding

#19

Post by jacquesm »

conistonwater wrote: post #18
jacquesm wrote: post #7

Earlier quoted context omitted.

The grit only works where it supports the tool. So as long as you rotate frequently it will just hit new 'high' spots while not affecting the low spots at all. By hitting the interior more often than the edge that's where you'll go deepest, it's more statistics than actual aiming for a some high spot or difference. The tool-on-top or mirror-on-top swap can help to adjust if you've gone too deep by favoring supporting…

> bounce steel blocks on a vibrating drum Wow, is that real? Do you have a link? I want to see.

Yes, it's real. I don't have a video and I don't have a metal workshop any more but you can easily try this yourself if you feel like it. For added speed you could add some sand, and if you're just interested in the effect mix some rocks with some wooden blocks, that will speed things up quite a bit.

It's similar to how river beds tend to round the stones that get moved around by the water. Those stones start out as sharp bits of rock.

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