
17yo guy built 14.7“ f/2.89 Newtonian reflector telescope - saadalem
https://www.reddit.com/r/space/comments/frdd1d/im_17_years_old_and_just_finished_building_this/
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tzs
I think the most interesting part of homemade telescope making is that you can
check the curve of your homemade mirror to an accuracy of a fraction of a
wavelength of light with a handful of common household items. You can do this
using the Foucault knife-edge test [1] [2].

> It measures mirror surface dimensions by reflecting light into a knife edge
> at or near the mirror's centre of curvature. In doing so, it only needs a
> tester which in its most basic 19th century form consists of a light bulb, a
> piece of tinfoil with a pinhole in it, and a razor blade to create the knife
> edge.

Of course you don't need a light bulb. A candle will do. This means that once
you have a level of technology that can make glass discs, candles, something
opaque with a way to make a pinhole in it, something opaque with a thin
straight edge, abrasive grit in a range of sizes, and some way to coat a glass
surface with a thin layer of metal, you can make telescope mirrors.

Much of that could be done in ancient times. The only things on that list I'm
not sure about are abrasive grit and coating the mirror.

You want a series of grits from coarse to very fine, and I have no idea when
grit making became refined enough to have such a range of sizes.

Coating a mirror can be done with silver. Here are some instructions for doing
it at home [3]. You need silver nitrate, ammonium hydroxide, potassium
hydroxide, detergent, acetone, and nitric acid if you follow this
instructions, plus assorted hardware like stirring rods, absorbent cotton,
beakers and bottles, and that kind of thing. I think all of that goes back at
least a few hundred years.

[1] [https://en.wikipedia.org/wiki/Foucault_knife-
edge_test](https://en.wikipedia.org/wiki/Foucault_knife-edge_test)

[2]
[https://en.wikipedia.org/wiki/Amateur_telescope_making](https://en.wikipedia.org/wiki/Amateur_telescope_making)

[3]
[http://www.webstertelescopes.com/silvering.htm](http://www.webstertelescopes.com/silvering.htm)

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tejtm
it is bootstrapping! you use the mirror you are creating to magnify its own
errors tens of thousands of times.

Grit sizes are can be sorted by the time it takes them to settle out of a
column of water. For a given material, finest take the longest (Elutriative
Time).[0]

un-affiliated: commercial source for the parents third link [1]

I will add that building better telescope mounts for pre-existing mirrors can
be very rewarding in itself. and can be an easier way to get into the hobby
than mirror making, especially without access to other experienced mirror
makers.

0
[https://en.wikipedia.org/wiki/Elutriation](https://en.wikipedia.org/wiki/Elutriation)
1 [https://angelgilding.com/](https://angelgilding.com/)

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localhost
When I was a kid, I borrowed Jean Texreau's How to Make a Telescope from the
library and read it from cover-to-cover many times.[1] I never did have the
resources as a child to go ahead and actually build a telescope, but by
reading the instructions I had the opportunity to live out the experience in
my imagination.

This might make for an interesting quarantine project today ...

[1] [https://www.willbell.com/tm/tm3.htm](https://www.willbell.com/tm/tm3.htm)

~~~
kitd
I picked that book up at a second-hand bookshop many years ago on a whim, not
having much interest in telescopes. I was completely hooked! I even started
(but never completed) making the mirror grinder.

I still have wistful thoughts of making my own telescope one day when I have
more time.

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jackfoxy
[https://en.wikipedia.org/wiki/Dobsonian_telescope](https://en.wikipedia.org/wiki/Dobsonian_telescope)

I once worked with a woman who learned from John Dobson about lens grinding
and telescope construction. Watched a Youtube video of John recently. This is
too labor intensive for me, but happy others enjoy it.

~~~
emilga
This 90min video by the Dobson himself is a great explanation of how to build
one:
[https://www.youtube.com/watch?v=snz7JJlSZvw](https://www.youtube.com/watch?v=snz7JJlSZvw)

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Sharlin
Another amateur (these days professional) telescope maker who started in his
teens and has written in length and detail about his projects, Allar Saviauk:
[http://www.allarscopes.com/telescopes/index_eng.htm](http://www.allarscopes.com/telescopes/index_eng.htm)

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zw123456
When I was in high school I spent an entire summer grinding my own mirror out
in the barn for a 6" Newtonian telescope. It was the most fun and satisfying
project a kid can do. I bought the mirror blanks and basic parts from a mail
order company (it was 1972) called Edmunds Scientific. They are still in
business but they sell toys now, they have a serious branch
[https://www.edmundoptics.com/](https://www.edmundoptics.com/) but I don't
think very many people grind their own mirrors any more.

~~~
macintux
The Edmunds Scientific and Barnes & Noble catalogs were two of the highlights
of my youth. I never ordered anything from them, don’t know if my parents ever
did, but it was such a pleasure to peruse them.

~~~
localhost
A slightly more pedestrian catalog was Radio Shack. And while researching this
answer, I just discovered that the Internet has archived all the Radio Shack
catalogs. Here's one page listing capacitors from 1979.[1]

The great folks at archive.org have archived a ridiculous number of these
catalogs (and even books like Lance Leventhal's classic Z80 Assembly Language
Programming! [2])

[1]
[http://www.radioshackcatalogs.com/html/1979/hr098.html](http://www.radioshackcatalogs.com/html/1979/hr098.html)
[2]
[https://archive.org/details/Z-80_Assembly_Language_Programmi...](https://archive.org/details/Z-80_Assembly_Language_Programming_1979_Leventhal)

~~~
jacquesm
Those were much more than just catalogs, they were sources for ideas.

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new_realist
When I tried to do this as a kid, I was stymied by the impressive costs of the
kit involved. Has it become cheaper, or does this kid have enabling parents?

~~~
elcomet
He said in the thread that it costed him $4000, that he earned by himself, by
working (his parents didn't give him money for this).

~~~
GuiA
That's one thing I was definitely envious of American teens of, as a French
teenager. I did odd jobs for family friends etc., but that was a few hundred
euros every few months at most; I was jealous of my internet friends in the US
who were making the same in a week or so by working at the local coffee
shop/tee shirt store.

They could drive too, which I cared less about because buses/subways/trains
seemed much less of a hassle and much safer.

~~~
dwild
At 17 you couldn't work?

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gattr
Dobsonians scale well; here [1] is Dr. Hänssgen's 42" telescope, which can be
de/assembled and un/loaded for transport by one person.

[1]
[http://www.cruxis.com/scope/scope1070.htm](http://www.cruxis.com/scope/scope1070.htm)

~~~
jacquesm
That's some mad building skills. Very impressive.

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siavosh
I'd love to do something like this with my kid when they get a little older,
but am always hesitant on the health risks on grinding the mirror. Can anyone
shed light on the health risks if any?

~~~
tzs
You grind a mirror by hand by taking two glass discs, and rubbing one against
the other with a slurry made of abrasive grit and water between them, with the
length and direction of your strokes determining which disc becomes your
concave mirror.

You start with large grit and work down to smaller grits as you get closer and
closer to the desired final mirror curve. The large grits seem unlikely to
become aerosolized during this. The finer grits are pretty small (a few
microns) so probably could become suspended in air but because you grind wet,
not dry, this probably isn't likely. Just work in a place with good air
circulation and it should be OK.

There's also the glass particles that get removed to consider. I'd expect that
they just end up in the abrasive slurry.

This all assumes you are hand grinding. If you use some kind of power tool
that goes a lot faster than the sedate processes that is hand grinding, all
bets are off.

There might be other ways things could get in the air. You can deal with all
of that by wearing a respirator like you would for painting or sanding (once
they become available again...).

~~~
londons_explore
If I were giving this to my child to do, I would give the child gloves,
instruct them not to eat the slurry, and make sure they wash everything each
day.

If they break any of the rules, the health risks are minor.

~~~
AstralStorm
Well, to remove the final risk you can wear glasses and a dust mask.

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peignoir
Oh brings me back memories :) I did that at his age too. Dobsons are awesome,
would dev a tracking system if I were 17 now (with a raspberry pie) keep on
the good work!

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DrFell
He didn't grind his own mirror, though. If you want to be a real ATM, you
gotta make the mirror.

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OJFord
Why must we have 'smart quotes'? And if we must, if only they would be the
right way around.

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jankotek
He did not make the mirror, this is one weekend in workshop.

~~~
reportingsjr
People rarely make telescope mirrors of this size at home, it just takes way,
way too long. As mirror radius increases the amount of material you have to
(very slowly) grows in a cubic fashion, so a 14.7" mirror requires removing
over 8 times as much as a 7" mirror. It takes too long to do this by hand to
be reasonable for most people.

~~~
TwoBit
Given that all commercial mirrors are made by machines and computers, why not
the same at home?

~~~
AstralStorm
The jig to do it is somewhat expensive it and hard to put together. The
especially hard part is getting the lens to not vibrate at higher speeds,
which is probably done by using pneumatic or hydraulic drive or very strong
magnetic drive.

And if the thing goes off, you could have tens of kilos of broken glass flying
in all directions.

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s800
f/2.89 is a very fast scope. The sagitta or "depth of the parabola" of his
mirror is almost 1cm.

~~~
TwoBit
Seems like it would take a long time to grind. I can imagine targeting
something shallower but not getting it right and keep fixing it and end up
with a deeper lens.

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avalys
That’s great! Certainly better than huffing glue or supporting communism or
whatever most kids are up to these days.

But it would be more correct to say he assembled his own telescope, since it
seems like he purchased all the important components, including the mirror.

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xellisx
Wow, a 17 year old did something... Why do people point out age, like we are
supposed to be impressed. If it was a 2 year old that did this, I would be
impressed.

While the act of what this person did is neat, but pointing out the age seems
like useless info for a title.

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m-p-3
It's still relevant, a 17 years old teenager doesn't have the same resources
and experience than someone in his 40s.

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xellisx
Actually doesn't make sense. I know a lot of 40 year old unskilled (in this
type of activity) people that don't have a lot of resources available to them.
If they did it, it would still be impressive, no matter what age they are.

Teenagers have advantages over 40 year olds, the major ones are free time and
energy.

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econcon
No one will share how actually it's done?

Reddit comment ranking algorithm didn't let me see OPs comment.

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JshWright
You mean other than the detailed comment the reddit OP made with links to all
sorts of resources?

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brandmeyer
Which in turn links to a topic on Cloudy Nights that is his build log!

