
How do you turn a chunk of glass into a telescope mirror? - nkurz
http://www.scopemaking.net/mirror/mirror.htm
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qwerta
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.

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

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

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Gravityloss
[http://allarscopes.com/telescopes/488mm_f5/index2_eng.htm](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.

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Renaud
What an impressive amount of work! Absolutely beautiful though.

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hrjet
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.

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rm445
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?

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

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amock
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](https://en.wiktionary.org/wiki/ground)
for details.

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pstuart
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](http://www.chabotspace.org/telescope-makers-workshop.htm)

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

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fnordfnordfnord
A few more resources here:
[http://www.telescopelab.com/index.php?page=testing](http://www.telescopelab.com/index.php?page=testing)

How to make blanks:
[http://www.mdpub.com/scopeworks/blanks/index.html](http://www.mdpub.com/scopeworks/blanks/index.html)

~~~
Someone
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.

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

How to make a telescope by Jean Texereau

[http://archive.org/details/HowToMakeATelescope](http://archive.org/details/HowToMakeATelescope)

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

Can anyone explain?

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vonskippy
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.

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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?

~~~
DanBC
> \- 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.

~~~
hfsktr
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](http://www.youtube.com/watch?v=FVbACw0jhmU)

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

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

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fernly
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](https://en.wikibooks.org/wiki/Telescope_Making)

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drnooo
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...](http://fringe.davesource.com/Fringe/Entertainment/Scripts/Real_Genius.html)

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Wingman4l7
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.

