Making a shallow parabolic reflector using hand tools?

This is for a searchlight application where the light source shines backwards into the reflector which then reflects the light forwards producing a narrow
beam. The reflector needs to be about 2 to 3 inches in diameter with a focal point about 2 to 3 inches in front.
Presumably this was possible in the days before CNC machinery, so how did they do it?
The following picture gives you an idea of the size, curvature and surface finish required
http://www.motherearthnews.com/~/media/Images/MEN/Editorial/Blogs/Homesteading%20and%20Livestock/Solar%20Fire%20Starter/sundancesolar_2021_9386858.gif
It's actually a "solar fire starter" so the light is traveling in the opposite direction, but the optical path is the same.
Mike
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Mike, are you sure that you can start a fire with a 2" or 3" reflector? My experience tells me you need at least 6".
When I was in high school I had access to rejects from several phisics-lab projects at Princeton University (including a nice ruby-rod laser, but that's another story). One was aluminized glass parabolic mirrors. They were great toys.
I tried starting fires with most of them. <g> The smallest ones that worked at all were around 6" diameter, and for that I needed a little ball of red cedar inner bark, like you'd use with flint-and-steel fire starters.
BTW, the best solar fire starters I've ever used are the $10, 8" x 10" plastic fresnel magnifiers you can get at any office-supply store. The cut (lenticular)sside goes toward the sun.
Good luck. I don't know how to help you machine them, BTW. I made a parabolic microphone that was 5 feet in diameter when I was 18 or 19, using chicken wire, plywood, Plaster of Paris and a cut piece of tempered Masonite for a sweep. The plaster became a male mold for a fiberglass mat reflector. It worked great but it wasn't accurate enough for optical work.
--
Ed Huntress

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Mike, since I've DONE it before, using NO machine tools whatsoever, I'd say, "Yes, there was a way before CNC." <G>
In fact, about the only 'tool' it requires is a hand saw, but you don't use that to make the reflector, you use it to make one of the polishing tools for the reflector.
Look up "hand-grinding telescope mirrors". What you're doing requires a MUCH deeper curve than most Newtonian telescope mirrors have, but it's completely do-able.
The only 'special' device you'll need is a Focault knife-edge tester to check the curvature of the mirror during final figuring; even that you can make with just 'stuff' lying around the shop, and it, too, requires no tools except ordinary screwdrivers and knives and razor blades to make.
A lot of years ago, I spent three weeks of evenings walking around a barrel, grinding a 12" mirror for my telescope. It's not hard, just a slow process. In fact, I'd say anyone who can stand up is capable of it, and if you can't stand, a rolling chair would probably serve to allow it, too.
You'll need two thick glass blanks and an assortment of carborundum dry grit abrasives. The abrasives are available through lapidary supply houses. You used to be able to buy them and the blanks from Edmund Scientific, but now that finished mirrors are so inexpensive, they may not sell them anymore. Since this won't be a precision mirror for telescopy, you can get away with any-ol' glass, instead of the Pyrex we used for telescopes. Some ordinary flint glass about 1/2" thick should do.
Lloyd
Lloyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> fired this volley in

Oh... I might add that, you don't need fine precision here. I figured my 12" mirror to a precision parabolic curve accurate to within 1/8th wavelength of sodium light, but you don't need that level of accuracy.
It would even be possible to laminate thinner glass using two-part acrylic as the cement. You'd get an aberration ring wherever you penetrate a layer, but that wouldn't matter too much for this application.
Lloyd
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Ed Huntress wrote:

Ed , way back in the early 80's Radio Shack <IIRC> sold a small parabolic mirror cigarette lighter . Had a little prong thingy that held the cig -which folded flat to the mirror for storage - and you aimed it by putting the bright spot in the center of the cig . This unit was only like 2X3" oval , and worked great in those seasons when we had decent sun angles . In N. Utah that was from like March thru September . Also made a small ball of like shreded bark or grass ignite readily . Better have the fire laid and ready though , cuz it'd burn up quickly . -- Snag Yes , I was a Boy Scout .
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wrote:

Huh. Well, that's smaller than I would have expected, to actually cause ignition. I was never able to get a mirror that small to do it.
Now, the big fresnel magnifiers probably could roast a squirrel as he was running by. <g>
--
Ed Huntress


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On 08/21/2013 06:07 AM, David Billington wrote:

Yes they did; mine was oval shaped, maybe a little smaller than 3" x 5", and had a little fold out fork in the middle to put the cigarette. It came in a little vinyl pouch.
Jon
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On 8/21/2013 5:27 AM, Ed Huntress wrote:

You don't say what you're trying to accomplish. For fire starting, I'd second the fresnel recommendation.
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email.me:

A 3" mirror with a proper aluminum coating will collect and focus about 4.8 Watts of energy (including losses in reflection) from noon-day sun.
The sun's disk's substended angle of about 0.5 degrees will focus to a dot of less than 0.1" diameter. That's an area of about 0.008 square inches. Just to be real generous, let's say 0.01 square inches (for poor focus).
That's a heat concentration of 480 Watts per square inch. That's not only enough, but FAR more than enough to ignite any tinder capable of absorbing the heat.
What'd you try to light, aluminum foil?
Lloyd
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On Wed, 21 Aug 2013 20:18:52 -0500, "Lloyd E. Sponenburgh"

Mostly little balls of rubbed red cedar inner bark -- the same stuff you use to start a fire with flint and steel, or with a fire bow. Wild grape vine inner bark works well, too.
Have you actually tried this with such a small reflector, Lloyd? I've been a magnifying-glass fire bug since I was 7 or 8, and here's my experience with it:
You need not only a temperature high enough to make the fuzz ball, or dry leaf, or paper glow and char; you also need to be able to de-focus it enough to heat a large enough area that it will produce enough combustable gas to will sustain a flame. You can take a 3" dime-store magnifying glass and char leaves with it all day long, but only rarely will the leaf actually catch fire. Generally, it glows and chars but won't sustain a flame. Once in a while a serendipitous amount of gentle breeze will fan the spark and a leaf, or bark fuzz ball, will flame. But it may take a lot of tries. The first time someone tries flint and steel they usually wind up chasing a little spark right across your piece of charred rope or whatever, until they get the hang of blowing on it just right to start a flame in the rubbed bark.
OTOH, take an 8" x 10" plastic fresenel lens, de-focus it on paper or a leaf to a half-inch spot, and it will burst into flame in two or three seconds.
--
Ed Huntress

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Of course I have, Ed! I think nobody ever taught you how to properly light a fire with a lens! Burning lens 'pocket' kits for the purpose of starting fires were available even in Colonial times.
Weren't you paying attention in class when we described the little Radio Shack cigarette lighter? It has only a small fraction of the area of a 6" mirror (only about 3" x 4-1/2" oval), and will reliably light a weed in the noonday sun.
FWIW, Ed, I've been making telescopes since I was 13. I don't ever remember a single objective (refractor or reflector) that didn't get at least ONCE tested as a 'burner'... just because I could. I also don't remember a single mirror or lens larger than about 2" that wouldn't serve the purpose. That 12" F-6 I made was quick to make a sheet of plywood burst into flame, but I've lit fires with 25-cent plastic dimestore magnifiers. I have a survival kit fresnel lens firestarter with a rectangular size of only about 2" x 3".
Hell! I'd probably go to jail for doing such today, but when I was about 15, I made a "solar timer" that would ignite something at a particular time of day (albeit only if the sun shone). THAT took some planning! You can't just move the sun to test your aiming point!
Lloyd
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On Thu, 22 Aug 2013 06:18:24 -0500, "Lloyd E. Sponenburgh"

Ok, it you say so. Starting an ember on a cigarette, though, is one thing. Generating enough heat (in addition to a high enough temperature) to start a flame is something else. That's the distinction that I was trying to explain, based on my own experience.
Maybe a little ether would help...<g>

Tricky. Fun. Dangerous. That's my kind of experiment.
--
Ed Huntress

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Once you have that reliable ember glowing, you have the makings of a fire. I can light a flame from a burning cigarette on any day of the week (avoiding rain, if possible).
How many grass fires are started along roadways every year from folks' carelessly flinging weeds out their car windows?
Lloyd
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On Thu, 22 Aug 2013 07:28:49 -0500, "Lloyd E. Sponenburgh"

Well, I can too, and I taught flint and steel firemaking when I was a Senior Patrol Leader in the Boy Scouts.
But it takes some technique and a little experience.

A bigger question is, how many are NOT? The chances of starting a fire with a cigarette that way, to make a wild guess, are probably 1:100, unless you're in an absolute tinderbox.
--
Ed Huntress

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Sure, but that's without the skilled hands of a primitive style firestarter working the goods... it's just lying there!
LLoyd
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On Thu, 22 Aug 2013 06:18:24 -0500, "Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote: >Hell! I'd probably go to jail for doing such today, but when I was about

A bit less planning and you have a one year timer. Plenty of time to make a get away:)
--
William

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On Thu, 22 Aug 2013 08:23:33 -0400, William Bagwell

Solar fired cannon have been around a very long time
http://www.pixgrove.com/2012/12/solar-cannon.html
"
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And the 8x10 fresnel lenses can be had at the dollar store..for a buck.
"
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Ed Huntress wrote:

Ed, The small ones didn't work because of the Jersey smog. :(
John
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wrote:

I was waiting for someone to say that. <g> But I lived in western Maryland and then northeast Pennsylvania during most of my firebug years.
What we have here is not so much smog anymore, but, if you're into anything astronomical, we have terrible light pollution.
--
Ed Huntress

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