Mirror grinding
scopemaking.net
Mirror grinding
1–10 of 66 posts
Re: Mirror grinding
#2This appears to be incomplete. It explains coarse and fine grinding but that's not enough to make a usable mirror. You need to continue with polishing and figuring before getting it aluminized. Here's an explanation of the whole process:
Re: Mirror grinding
#3I 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?
Re: Mirror grinding
#4There is also a 90 minutes film »Telescope building with John Dobson« [1], inventor of the Dobsonian telescope [2], going through the entire process of building a 16" telescope from scratch.
Re: Mirror grinding
#5I 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?
I'm not sure in this specific case, but in general, grinding mechanisms to obtain flat planes, angles, and spheres are pretty well-established. For flat planes, the term is "automatic generation of gages" and is a subtle and clever way to grind three faces in pairs. For curves, you'll need a rotor. Interestingly, you can use a fairly coarse system to methodically generate a very precise curved surface.
Re: Mirror grinding
#6My 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 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.
Re: Mirror grinding
#7I 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 the tool at the edge or supporting the tool in the center causing greater amounts of material being taken away at the edge.
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).
Re: Mirror grinding
#8My 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…
Over a decade ago I worked at one of the big semiconductor manufacturing R&D outfits (there's very few of those in the world) and the mirror making for EUV lithography R&D was done by polishing by hand at the end by two guys with metallurgy PhDs. Good chuckles were had.
Re: Mirror grinding
#9I 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?
I'm not sure I can explain it fully, but my understanding is that it is related to the probability distribution of the abrasive technique.
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
Re: Mirror grinding
#10How is the curvature of the mirror measured during manufacturing? It's obvious that the mirrors require extreme precision in order to function correctly.