Mirror grinding
scopemaking.net
scopemaking.net
I will think of him the next time I use it.
Thanks for sharing this! As I get older and try to record as much family history I can, I really enjoy you and others sharing what you can about family/company history for the future. Stories like this from first hand accounts are getting harder to find and eventually we may lose these great stories and historical accounts for generations to come.
Thanks again!
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 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 - far less than your ability to perceive.
The other thing that was impressed me is that up until recently, the last stage of high-precision mirror making was literally done by hand. My dad would literally rub on a mirror with very, very fine grit to take out bumps on the order of microns. Recently, they've switched to machines in that last step to make it faster and more accurate, but for many applications the traditional way worked just fine.
However, my understanding is that you can improve things but you can't "truly" correct it, generally speaking, because the optical aberration causes information to be lost. eg. if point A on your mirror focuses to point A' on the resulting image, and point B on your mirror, due to an aberration, also focuses to A', there's no way to determine from the image which point on the mirror a photon came from.
This is why Hubble eventually needed a hardware fix... from the linked paper: "it is clear that many image restoration methods are highly successful at deriving images that 'look good' from HST data. These restored images may be qualitatively faithful to the true (unknown) image. However, for most astronomical purposes qualitative agreement with reality is not sufficient; we want quantitative agreement as well."
For small problems, you can just buy these things off the shelf: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=32... "only" $17.5K
I can't find the original paper that I read, but here's a bit of history [1].
[1] https://pdfs.semanticscholar.org/d1ed/a97dd2cf70f54f24b85abc...
Were they working with some sort of fluid interface? Maybe layered liquids, or fluids that you don't want to dip an objective lens into? Or maybe temperature gradients?
[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
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.
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!"
In my experience grinding and polishing samples for petrography, Because the grit etc was once wet, it gets caked-on to everything once it dries. So caked-on that it can be hard to clean everything once it is dry, and if you want to remove it you have to wet it all again. Unless you are stirring up the air with a fan, or trying to remove caked-on grit with compressed air, I do not expect much dust will get airborne. So, clean up whilst it is still wet, and there will be no problems.
I love it how you casually throw that out there. For the un-initiated, outside of grinding lenses: grit 3000 is approximately 6 micron particles and very fine indeed but for this purpose (and gem polishing) it is still considered 'coarse'.
The finest polishing grits go to 100,000, ~0.25 u across.
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 :)
I think there's an analogue to test-driven development in software. Many designs that you look at make no sense until you understand what can be measured readily and what can't, while making the parts and assembling them into a system.
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.
> An equatorial platform eliminates this weakness, and I strongly recommend one
Agreed. You own a dob, just get a platform already. It's even useful for astrophotography.
I've got a hot glass shop on my farm in Kentucky (where I will make blanks and other artistic pieces), and I have built a cold shop with some tools (my favorite is a mold that uses pennies instead of the porcelain that they use in the article) and then using a HD projector and a DSLR that I remotely trigger, I get a map on the screen of where the imperfections are relative to the "top" of the mirror (that I indicate with a dot on the side).
Once I've worked the imperfections out, I'll then take it to an observatory in Ohio to get mirrorized -- and generally speaking, I'll donate my last telescope to the observatory (for public use) once I get the one I'm building done.
Lots of simple processes can cause nearly perfect geometrical shapes to appear: bounce steel blocks on a vibrating drum for a while (and against each other) and you end up with steel balls that rival ball-bearings in roundness (but not in precision, they will be all kinds of sizes).
Wow, is that real? Do you have a link? I want to see.
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.
The technique involves random rotation and random motion, using lapping blocks that are the same size or smaller than the mirror. The center of the mirror is the most likely to be contacted by any motion, so it is abraded more frequently and becomes the deepest point, while the edges are worn less. The distribution between "less" and "more" is spherical.
I can at least understand the process of grinding or lapping a surface flat. Here are a couple examples:
https://www.youtube.com/watch?v=jx1D0buRCOY
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.
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 be very hard to make mirrors.
Source: I make telescope mirrors.
(Although that's more because of destructive self interference than imperfections)
If there's a bump (a hill) on the surface, it tends to grind out more quickly than the rest because it's jutting out and it's more exposed to pressure from the grit. If there's a hole, it remains untouched while the surface around it is being ground down. This way everything tends towards the ideal shape. By carefully doing a uniform rotation of the pieces during grinding, the resulting shape is symmetrical - spherical or flat.
Other shapes are also used in mirrors (revolution surfaces generated by a parabola, hyperbola or ellipse), but they are all basically just small corrections to a sphere (which is a revolution surface generated by a circle).
You start with very rough grit to go faster in the beginning and remove most material. Then you just use finer and finer grit as the surface becomes more and more smooth.
In the final stages you switch to polishing. It's a different process where the tool is not hard (glass) but soft (pitch), and the grit is replaced with microscopic powders (such as iron oxide or cerium oxide) which work not only through physical mechanisms but also via surface chemistry.
Also during polishing you use testing procedures which can show surface errors as small as 0.02 microns. The testing process is steering your polishing techniques; you apply corrections, or choose different polishing strokes to deal with various surface errors. Whereas grinding is mostly automatic (with a few basic checks here and there), during polishing your brain is in the loop, a lot.
The minimum standard for good enough is a under a quarter wavelength in the short end of the spectrum of interest . this is because an extra quarter down plus the same quarter back up and you are a half wave out with another part of the mirror which when combined by your eye results in destructive interference of both parts of the mirror.
one last point on the final smoothness, the interplay of glass, water, pitch, metal oxide and mechanical force is imperfectly understood. crudely it may be closer to planing than grinding but that does not explain the oxide particles which are found beneath the surface of a figured mirror. another thought is the an atom in the glass is "stretched" up and snaps back into a lower energy configuration which is more atomically flat
Keep in mind this is a long term project. If you're looking for something to do with your hands, a physical object in the real world, that would keep you busy for months (assuming you're not working on it full time) - you've found it.
Keep at it and you'll make a very high quality instrument. My first parabolic mirror came out at lambda/25 precision (and no ripples, no turned edge), whereas many commercial mirrors are only lambda/4 or 6 and the edge is sometimes questionable.
As for the instrument itself (everything except the optics), it's not much harder than making a simple cabinet, and in some ways it's easier. Go slow, read a lot, think ahead, and you'll succeed.
> I grinded some mirror. Nice experience but at end monotonous and boring. Also grinding powder is lethal, I had my appendix removed since I eat some :-(
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.
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
[Thompson's Rule for First-Time Telescope Makers] It is faster to make a four-inch mirror and then a six-inch mirror than to make a six-inch mirror.
Bill McKeenan
Wang Institute