17yo guy built 14.7“ f/2.89 Newtonian reflector telescope | Hacker News Reader

17yo guy built 14.7“ f/2.89 Newtonian reflector telescope

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131 pointssaadalem50 comments

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

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

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

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 1 https://angelgilding.com/

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

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.

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.

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

Dobson founded the San Francisco Sidewalk Astronomers public outreach group:

http://sfsidewalkastronomers.org

I used to use a favorite quote of his in my Usenet .signature file:

"What's the use of somebody who doesn't wonder?"

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

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/ but I don't think very many people grind their own mirrors any more.

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.

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 [2] https://archive.org/details/Z-80_Assembly_Language_Programmi...

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

In a recent conversation with someone at Edmund Optics, I told him that I missed their old surplus catalog. He admitted that he did too.

For surplus and oddball optics, I also recommend Surplus Shed.

Talking to some amateur astronomers, it seems that a lot of the interest has shifted to what can be done with a decent scope, digital camera, and software.

A lot of people who didn't build their own got one of these from Edmund Scientific:

https://en.wikipedia.org/wiki/Astroscan

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?

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).

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.

At 17 you couldn't work?

I am amazed by his work. I followed him on cloudynights forum. But let's not kid ourselves. Of course he has enabling and smart parents.

considering the costs of telescopes, that's not a bad return on a hobby

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

That's some mad building skills. Very impressive.

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?

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...).

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.

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

Interesting scaling problem: A kid should probably start out making a 6 inch mirror which only weighs maybe a pound. But mass goes up by a factor of a cube with diameter... the linked article 15 inch mirror blank is likely 75 to 100 pounds? You can buy special thinner blanks but it gets to the point where in a wet slippy fingers situation it doesn't matter all that much if 60 pounds or 80 pounds lands on your toe when you drop it.

Something the other responder didn't mention about wet grinding is after you finish with a coarser grit, perhaps 200 grit, you have to clean the mirror and the local environment VERY completely or else when you switch to finer 400 grit even one tiny grain of 200 grit will ruin and scratch up the attempt at a 400 grit polish. So its kind of self-fixing in that if you're not screwing up, there will never be aerosolized grit because your working area will be very clean indeed. Its a very clean hobby, compared to most.

It is really easy, keeping glass wet (with water) while grinding & polishing keeps it in a slurry so no airborne particles to give you silicosis.

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!

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

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

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

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.

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

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.

I'm glad he didn't try. As a 15yo my teen friend and I spent half a summer grinding a 6" mirror.

Then we were told to Foucault-test the mirror. We couldn't understand the included instructions. We lived in the middle of nowhere, noone to ask, internet didn't exist, library had bupkis. If we had figured it out, we'd still have had to mail it somewhere to be aluminized. End of story.

Few people make their own ICs anymore either.

Yes. Not sure what about this justifies the oohs and ahs.

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

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.

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.

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.

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

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.

There's nothing odd about this sort of human interest story in a local newspaper about what high school students are doing; I think maybe it just sounds odd in the context of a global audience on the internet.

No one will share how actually it's done?

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

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

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

Search for Stellafane telescope making. There are a lot of other resources on the net as well. CloudyNights forum has a great ATM section.

There is no secret in this and if you follow exact instructions you too can make a mirror and Dobson mount. The beauty of this is that it's easy. Just don't complicate and over-analyze and you'll be fine ;)

OP posted an FAQ:

https://old.reddit.com/r/space/comments/frdd1d/im_17_years_o...

You can find it easily if you change the domain to old.reddit.com (as suggested by jakeogh in a sibling comment).

change the domain to old.reddit.com

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