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How do you turn a chunk of glass into a telescope mirror?

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Re: How do you turn a chunk of glass into a telescope mirror?

#21

Post by qwerta »

rm445 wrote: post #14
hrjet wrote: post #8

I have grinded two 8" mirrors. The first one took three years (calendar time) and was a disaster. Lots of scratches on the mirror and astigmatism. I didn't give up though. Built a second one, applying all tricks that I learnt during the first experience. It turned out very good and only took an year to make. The linked page doesn't talk about the most painstaking part: the figuring and polishing of the mirror. A good…

OT: You are the second poster in this discussion to use 'grinded', instead of 'ground', as the participle of grind. I'm just curious whether that's telescope lingo or just your word choice? I don't want to call it an error, exactly, because it seems to be part of a trend of verbs being regularised in American English. Lit => lighted etc. I read a novel the other day (Brandon Sanderson's new effort) that repeatedly us…

I am Czech and I would not understand what "mirror grounding" means.

Re: How do you turn a chunk of glass into a telescope mirror?

#22

Post by fnordfnordfnord »

hrjet wrote: post #13
qwerta wrote: post #10

Earlier quoted context omitted.

It is sticky, very hard to wash and gets everywhere.

It is also dangerous to inhale these powders. They are very fine and very abrasive. One needs to maintain a clean-room environment, both for the protection of the mirror from dust, and of yourself from the abrasives.

>One needs to maintain a clean-room environment, both for the protection of the mirror from dust, and of yourself from the abrasives.

That's crazy talk, and an impossibility. The stuff gets everywhere. It is a messy process. Here is a particularly messy example. https://www.youtube.com/watch?v=cukjxWCPVeg

Re: How do you turn a chunk of glass into a telescope mirror?

#23

Post by hrjet »

fnordfnordfnord wrote: post #22
hrjet wrote: post #13

Earlier quoted context omitted.

It is also dangerous to inhale these powders. They are very fine and very abrasive. One needs to maintain a clean-room environment, both for the protection of the mirror from dust, and of yourself from the abrasives.

>One needs to maintain a clean-room environment, both for the protection of the mirror from dust, and of yourself from the abrasives. That's crazy talk, and an impossibility. The stuff gets everywhere. It is a messy process. Here is a particularly messy example. https://www.youtube.com/watch?v=cukjxWCPVeg

In the final stages, the powder becomes That video shows the kind of clean-room I meant. Gloves to reduce contact, container to avoid spills. I also used a cloth over my nose and mouth when dealing with the finer grits (in dry form).

It need not be as clean as a fab, but it sure needs to be cleaner than a typical workshop or garage.

Re: How do you turn a chunk of glass into a telescope mirror?

#24

Post by amock »

qwerta wrote: post #21
rm445 wrote: post #14

Earlier quoted context omitted.

OT: You are the second poster in this discussion to use 'grinded', instead of 'ground', as the participle of grind. I'm just curious whether that's telescope lingo or just your word choice? I don't want to call it an error, exactly, because it seems to be part of a trend of verbs being regularised in American English. Lit => lighted etc. I read a novel the other day (Brandon Sanderson's new effort) that repeatedly us…

I am Czech and I would not understand what "mirror grounding" means.

The words "grounding" and "grinding" are not related in meaning: "grounding" refers to "ground", meaing the surface of the Earth, and "grinding" is a participle of "grind". See https://en.wiktionary.org/wiki/ground for details.

Re: How do you turn a chunk of glass into a telescope mirror?

#25

Post by sp332 »

qwerta wrote: post #21
rm445 wrote: post #14

Earlier quoted context omitted.

OT: You are the second poster in this discussion to use 'grinded', instead of 'ground', as the participle of grind. I'm just curious whether that's telescope lingo or just your word choice? I don't want to call it an error, exactly, because it seems to be part of a trend of verbs being regularised in American English. Lit => lighted etc. I read a novel the other day (Brandon Sanderson's new effort) that repeatedly us…

I am Czech and I would not understand what "mirror grounding" means.

"Ground" is the past participle of "grind" So: I grind, I am grinding, I had ground, I was grinding.

Re: How do you turn a chunk of glass into a telescope mirror?

#26

Post by foobarian »

Some questions that come to mind:

- Why is this a manual process? Aren't there jigs or tools that can automate out the grinding and get a more precise result?

- Why does the mirror have to be made out of glass? Lens I can understand, but a mirror just needs to be sturdy enough and chemically compatible with the metal layer. Could a 3d printer produce a precise enough substrate?

Re: How do you turn a chunk of glass into a telescope mirror?

#27

Post by icegreentea »

foobarian wrote: post #26

Some questions that come to mind: - Why is this a manual process? Aren't there jigs or tools that can automate out the grinding and get a more precise result? - Why does the mirror have to be made out of glass? Lens I can understand, but a mirror just needs to be sturdy enough and chemically compatible with the metal layer. Could a 3d printer produce a precise enough substrate?

Industrial lens and mirror making is surely not a purely manual process. The method described is clearly meant for use with minimal infrastructure. The lack of jigging and tooling then is actually a feature in this instance.

The substrate requires sufficient hardness and dimensional stability to hold its shape. While glass is certainly not the only material that fits the bill, it's probably the cheapest.

High-end (read Industrial) 3D printers can probably -not- get a precise enough substrate, at least as designed right now.

Re: How do you turn a chunk of glass into a telescope mirror?

#28

Post by qwerta »

hrjet wrote: post #23
fnordfnordfnord wrote: post #22

Earlier quoted context omitted.

>One needs to maintain a clean-room environment, both for the protection of the mirror from dust, and of yourself from the abrasives. That's crazy talk, and an impossibility. The stuff gets everywhere. It is a messy process. Here is a particularly messy example. https://www.youtube.com/watch?v=cukjxWCPVeg

In the final stages, the powder becomes That video shows the kind of clean-room I meant. Gloves to reduce contact, container to avoid spills. I also used a cloth over my nose and mouth when dealing with the finer grits (in dry form). It need not be as clean as a fab, but it sure needs to be cleaner than a typical workshop or garage.

That is different level. I was very proud for dropping the mirror only twice during the grinding :-)

Re: How do you turn a chunk of glass into a telescope mirror?

#29

Post by DanBC »

foobarian wrote: post #26

Some questions that come to mind: - Why is this a manual process? Aren't there jigs or tools that can automate out the grinding and get a more precise result? - Why does the mirror have to be made out of glass? Lens I can understand, but a mirror just needs to be sturdy enough and chemically compatible with the metal layer. Could a 3d printer produce a precise enough substrate?

> - Why is this a manual process?

Partly it's a tradition handed down over the years. Many years ago it was much cheaper to get glass blanks and grind them yourself to get a good mirror than it was to buy the mirrors. So there are many documents from old astronomers telling how to grind a mirror. You learn a lot about lens terminology.

For smaller mirrors you don't really need jigs or automation.

For bigger mirrors there is plenty of automation.

As I understand it it's a self-calibrating process. The friction and movement of the tool create the shape, which is why they talk about moving the tool around a lot.

YouTube has some interesting videos of home made contraptions to polish rocks into spheres - you have 3 cones with a ring of abrasive, and some abrasive slurry, with a lump of rock in the middle of the cones, and you just spin the cones and end up with a nice sphere.

Re: How do you turn a chunk of glass into a telescope mirror?

#30

Post by hfsktr »

DanBC wrote: post #29
foobarian wrote: post #26

Some questions that come to mind: - Why is this a manual process? Aren't there jigs or tools that can automate out the grinding and get a more precise result? - Why does the mirror have to be made out of glass? Lens I can understand, but a mirror just needs to be sturdy enough and chemically compatible with the metal layer. Could a 3d printer produce a precise enough substrate?

> - Why is this a manual process? Partly it's a tradition handed down over the years. Many years ago it was much cheaper to get glass blanks and grind them yourself to get a good mirror than it was to buy the mirrors. So there are many documents from old astronomers telling how to grind a mirror. You learn a lot about lens terminology. For smaller mirrors you don't really need jigs or automation. For bigger mirrors t…

I had no idea about the spheres so I looked up one of those videos. This one seemed to show some of the stages and the machine going. Really cool.

http://www.youtube.com/watch?v=FVbACw0jhmU

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