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Mirror grinding

scopemaking.net

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Re: Mirror grinding

#31

Post by jakobegger »

digler999 wrote: post #17
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 refle…

You don't even need nanometer precision, since light has a wavelength of a few hundred nanometers. Random imperfections on a nanometer scale will be literally invisible.

Re: Mirror grinding

#32

Post by semi-extrinsic »

siavosh wrote: post #30
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 love to grind my own mirror, but are there health concerns with the fine powder for the skin and respiratory systems?

I'm guessing these are done with wet sand paper for the coarser grits (Edit: from TFA:

"Always work wet! Sprinkle some water on the grit before you start grinding! Glass dust is very dangerous and can cause silicosis, a serious lung disease if inhaled!"

Re: Mirror grinding

#33

Post by reneherse »

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…

>if you took a typical 1 meter mirror and scaled it up to the size of the United States, the biggest deformity from a perfect curve would be less than you are tall

My father, who worked on the Hubble Space Telescope and its servicing missions, used virtually that exact wording when describing the precision of its mirror to me as a child. So that's evidently a popular metaphor for optically oriented parents :)

Re: Mirror grinding

#34

Post by digler999 »

Florin_Andrei wrote: post #24
digler999 wrote: post #17

Earlier quoted context omitted.

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 refle…

No, it's not needed. Nanometers don't matter, that's the atomic level, far below what visible light could resolve. The metallic coating doesn't change the shape. Once you're below 20 nanometers it basically doesn't matter. The wavelength of visible light is on the order of 400 nanometers. In a different universe where the wavelength of visible light would be comparable to, or smaller than, the size of atoms, it would…

Thanks, I corrected my response. Was I partially correct about the gaseous reflective coating helping to smooth over any micro-imperfections?

Re: Mirror grinding

#35

Post by erikpukinskis »

Random semi-related question I was thinking about last night:

Could you:

1) make a coarse "random" mirror that was bumpy at the large scale

2) buff out all the fine texture so you get clear fragments of an image all across the lens at random focal planes

3) put it on a track with a LCD screen on one side and a camera on the other

4) use some sort of gradient descent algorithm to move the image and camera around and reverse engineer the normal map for the lens

5) Use that displacement map to generate a light field from an image

6) Use the light field to generate any (clear, in focus) image you want in the light field volume.

In other words, can you make a Very Bad but Smooth lens, and then make up for it in software?

My guess: you could, but for any given focal plane you'd recover very little resolution because probabalistically few of the microlens fragments are focused there.

Re: Mirror grinding

#36

Post by Avshalom »

erikpukinskis wrote: post #35

Random semi-related question I was thinking about last night: Could you: 1) make a coarse "random" mirror that was bumpy at the large scale 2) buff out all the fine texture so you get clear fragments of an image all across the lens at random focal planes 3) put it on a track with a LCD screen on one side and a camera on the other 4) use some sort of gradient descent algorithm to move the image and camera around and r…

I don't know if it's exactly what you're thinking but many modern research scopes use adaptive optics which is both software and hardware

https://en.m.wikipedia.org/wiki/Laser_guide_star

Re: Mirror grinding

#38

Post by dekhn »

digler999 wrote: post #11
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.

The approach most advanced people use now is deformable mirrors. They work in concert with a laser that emits from the detector, bounces off the atmosphere, and produces a real time map for the deformation (you have to solve an inverse problem here IIRC). You can also use a wavefront sensor.

For small problems, you can just buy these things off the shelf: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=32... "only" $17.5K

Re: Mirror grinding

#39

Post by dekhn »

jonathansizz wrote: post #14

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…

you can track with an alt-az mount, no problem. It doesn't rotate, but as long as you know an alt az pointing, and have feedback servos, you can track any celestial location. Tracking at very high azimuth is hard, though.

Re: Mirror grinding

#40

Post by Balgair »

erikpukinskis wrote: post #35

Random semi-related question I was thinking about last night: Could you: 1) make a coarse "random" mirror that was bumpy at the large scale 2) buff out all the fine texture so you get clear fragments of an image all across the lens at random focal planes 3) put it on a track with a LCD screen on one side and a camera on the other 4) use some sort of gradient descent algorithm to move the image and camera around and r…

1) Yes, ThorLabs sells them: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=11...

2) You mean like a fun house mirror?

3) So you mean have a screen shine through a wonky lens at a camera?

4) Yes, this is commonly done with interferometers in many junior physics lab classes, though the set-up is different. https://en.wikipedia.org/wiki/Optical_coherence_tomography

5) So, try to figure out how the surface deforms the light image.

6) Yes, we have been doing this for many years now: https://www.edmundoptics.com/resources/application-notes/opt...

Yes, that is considered a 'solved' problem, but the costs and market for such devices limits their wide-spread use. Typically they are used for nanoscopes and telescopes. There are a few purveyors, but add 2 0s to the end of whatever you think the cost should be.

The limiting factor is the light coming in and having a 'known' value to measure your noise against. In telescopes, this is typically a 'guide' star that has a very well calibrated light spectrum and positions that you can measure the noise values against.

Also, another cool thing youmay want to look into is electro-wetting lenses, basically a digital lens: https://en.wikipedia.org/wiki/Electrowetting

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