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
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It's actually a "solar fire starter" so the light is traveling in the opposite direction, but the optical path is the same.
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. 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.
I won't repeat all the instructions , but you can easily find them on the internet. Look for grinding telescope mirrors. First you get a spherical surface and then make it into a parabola. \ You could probably make a form and then spin disks of metal into parabola's.
Mike, since I've DONE it before, using NO machine tools whatsoever, I'd say, "Yes, there was a way before CNC."
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.
For how, I think metalspinning is the likely best bet. At some point (mostly the mold-making with metalspinning) there's some tedious "match the parbola template" work, though mechanical options arise if an ellipse (or a portion of an ellipse) will do just as well for an application. A pattern-following lathe is another approach.
I know at least one guy (not me) who could probably get good enough results with a sandbag and a rack of hammers (and a template to match.)
In any case there's going to be buffing involved for the finish.
"Lloyd E. Sponenburgh" fired this volley in news:XnsA223681E1A05Dlloydspmindspringcom@216.168.3.70:
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.
Ed , way back in the early 80's Radio Shack 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 .
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.
Every "solar fire starter" Ive owned was oval, not round. And they indeed did work pretty well. They were not made out of stainless steel, but chrome plated sheet steel and over time..everyone of them started to rust until the ability to gather enough sun became compromised.
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If you have a lathe with calibrated dials, you can do this with HNC - that's Human numeric control. You use a program to print out a list of coordinates, then you turn the dials of the crossslide and compound to the right settings, perhaps every .010". After all that is done, then you use a Cratex rubberized abrasive wheel in a die grinder or Dremel tool to remove the ridges and then polish on the lathe with jeweler's rouge. I did an ellipsoidal mirror for a 200 W Hg arc lamp some years ago this way, it worked very well.
Snip For a searchlight you probably don't need anything like the precision that you would need for a telescope. If you use a spherical surface then the focus will be 1/2 the radius of curvature for rays near the axis. The figures you quote would mean that some of the rays are well away from the axis so the approximation is not very good. If you could use a mirror with a longer focal length then a spherical surface would probably be good enough for your application. I wonder if you could use a spherical glass laboratory flask, or even a large light bulb to act as a mould for casting resin and then silver the surface?
So, you make a graph of the shape you want, glue it to a bit of something stiff, and cut the profile. Then, mount a block of wood on a faceplate and (cutting on a wood lathe, or a metal lathe if you are good at cleaning up sawdust) turn it to match your template. Put some soft metal onto the faceplate, and spin (pressure-deform the sheet metal, with a knob-on-stick tool and lubricant) the dish.
mike fired this volley in news:kv3o58$bem$1@dont- 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.
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 fired this volley in news: snipped-for-privacy@4ax.com:
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!
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