How do you turn a chunk of glass into a telescope mirror? | Hacker News Reader

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

scopemaking.net

64 pointsnkurz33 comments

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 :-(

But I would recommend building ligh dobsonian to everyone. Mirrors are cheap and rest is just wood or aluminium. My 10" has 20 pounds and is flyable.

One thing the article doesn't mention - how long does it take? How many hours total would be spent grinding?

It took me 50 hours to do 6" mirror with lambda 1/6, I was 14 and totally left handed.

Why did you eat the grinding powder?

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

In the final stages, the powder becomes < 20 micron and the mirror becomes sensitive to even the heat from your fingers. Dust that is invisible to the naked eye can leave deep scratches on the mirror.

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 :-)

http://allarscopes.com/telescopes/488mm_f5/index2_eng.htm

This is a hardcore version. It is interesting that such extremely low technology methods have so precise results.

What an impressive amount of work! Absolutely beautiful though.

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 reference is the Stellafane ATM site and books suggested by them.

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 used 'shined' instead of 'shone' and it was oddly distracting. I wonder how long it will be until all verbs are regular?

http://grammarist.com/usage/grinded/:

"Grinded might be considered incorrect in some contexts, but it has grown more common over the past several decades. It’s especially common in American sports commentary and writing, where grind means to overcome adversity by playing hard."

And no, I don't think all verbs will become regular. Ew verbs tend to be, and it is easier for non-native speakers to have regular verbs, but verbs like 'to be' or 'to have' will keep aiming for conciseness (even though we have "me be hungry" as a sort-of counterexample)

I didn't consciously use 'grinded'; my mind just blurted it out based on what I may have read before. Thanks for pointing it out; though I do wish for a more regular English grammar.

Edit: I also just realized I typed "an year". This one was just sloppy.

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.

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

If interested in this and in the San Francisco East Bay, there's an ongoing workshop for this:

http://www.chabotspace.org/telescope-makers-workshop.htm

(I'm interested but haven't done it yet...)

A few more resources here: http://www.telescopelab.com/index.php?page=testing

How to make blanks: http://www.mdpub.com/scopeworks/blanks/index.html

Probably more for inspiration than for tips: Opticks by Isaac Newton. He does go into detail as to how he ground his lenses, but that advice may be a bit outdated.

Best classical reference (yet somewhat more modern than Newton) is:

How to make a telescope by Jean Texereau

http://archive.org/details/HowToMakeATelescope

Gordon Waite's cheap DIY spherometer tutorial: https://www.youtube.com/watch?v=htBx5SRFeMI&feature=related

I don't understand how randomly moving the grinding tool ends up with a perfectly spherical result.

Can anyone explain?

Brings back memories - I ground a 30 cm reflector back in my teens. Was interesting to read how they no longer use two glass blanks, instead use one blank and one tile grinding tool. Took me most of the summer to get the mirror ready for coating, and that was working on it most every day for at least an hour. All very worth it when everything was finished and assembled and we pointed it up into the night sky. Nothing like a clear cool fall night and a decent (if amateurish) scope to contemplate extremely large numbers.

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.

> - 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.

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

This reminds me very much of grinding limestone slabs in the printmaking studio: http://instagram.com/p/fGcGwdwAT1/

(Printmaking studios make good hackerspaces, and lithography has fun Chemistry involved. Etching has a little too.)

Here's another account of how to do it, in a very authentic voice of somebody who's done it more than once:

https://en.wikibooks.org/wiki/Telescope_Making

Yes but can you make a mirror like Kent did in Real Genius?

"KENT: See! Do you have any idea how hard it is to make a film virtually 100 per cent reflective, one micron thick and apply it to a mirror this shape?"

http://fringe.davesource.com/Fringe/Entertainment/Scripts/Re...

That may be the line in that script but it wasn't in the actual film. He just says something about making the "phase conjugate tracking system mirror" or something to that effect. That must have been some sort of "working script" as I recognize many other lines that were different or omitted in the final film.

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