Making a shallow parabolic reflector using hand tools?

Mike B · 2013-08-21T11:25:19+00:00

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 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 B 13 years ago

Thanks for all the suggestions. To clarify, it will be a light, but the optics will be similar if not identical to a solar fire starter, but the light source will be at the focal point of the mirror. I used that picture as it provided the best illustration of what is needed as all the picture of searchlights I could find were lacking in detail.

Perhaps I should explain a bit more. If you are not a cyclist then some of the following probably won't make much sense, but bear with me.

Until about 8 years ago all bike lights were essentially rubbish, then a revolution in high powered LED technology resulted in a series of 0.5w (half a watt) and 1w bike lights that were streets ahead in reliability and battery life compared to their incandescent cousins.

Technology marched on and now we have 3w, 5w and even 10w LEDs powered by lithium batteries. Unfortunately the optical design hasn't kept pace and so the minor "spill" (unfocused light directly from the emitter) has become so bright that these lights are blinding to other oncoming cyclists. This is a particular problem on cycle paths where the separation between cyclists travelling in opposite directions can be as little as 6 feet and the combined closing speed can be 40mph+.

Speaking from experience, it's like looking into the sun. All you can see is a blinding dot of light coming towards you. You cannot see the road in front which means that you cannot judge your position or speed. A head on collision is a real possibility.

There are two possible solutions:

1) Join the arms race by buying one of these super bright lights. I'll be able to see OK, but everyone else without an equally bright light will be blinded. Obviously this is not good.

2) Use a bright light that has an ultra-low spill design - like a search light. I'll be able to see the road and nobody will be blinded.

Finding such a light that will fit on a bike has not been easy. Currently I only know of 2 manufactures, one has a poor reputation for quality, the other uses ABS plastic enclosures, which means that prolonged running at 3w+ isn't an option because of the heat dissipation problems.

So, at the moment it looks like I'll need to build my own. I've ordered a solar fire starter from Ebay in the hope that the optics will be good enough. If it doesn't work out then it looks like I have to learn the art of metal spinning.

Mike

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William Bagwell 13 years ago

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

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Lloyd E. Sponenburgh 13 years ago

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

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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Ed Huntress 13 years ago

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.

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Lloyd E. Sponenburgh 13 years ago

Ed Huntress fired this volley in news: snipped-for-privacy@4ax.com:

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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Kristian Ukkonen 13 years ago

I remember someone casting one from resin.. Make a container that SPINS (rotates) and put the resin in the spinning container. RPMs decide the curvature of the resin. I remember they used a LP player to get an even RPM. After it has hardened, coat it with silver or other reflective coating.

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dcaster 13 years ago

On Thursday, August 22, 2013 8:19:04 AM UTC-4, Mike B wrote: Unfortunately the optical design hasn't kept pace and so the

Another approach would be to use the bright LED but direct the light thru a tube with a b]ack inside to reduce the spill. Maybe a piece of electrical conduit, a piece of pvc pipe, a piece of copper pipe. Or to prove concept some black paper.

Dan

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John 13 years ago

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

John

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Ed Huntress 13 years ago

I was waiting for someone to say that. 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.

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Gunner Asch 13 years ago

Solar fired cannon have been around a very long time

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"There are no leftists in mainstream American politics.

Just two right wing parties, one hard right and one softer." Christopher A. Lee, 8/18/2013

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David Harmon 13 years ago

On Wed, 21 Aug 2013 20:49:32 -0500 in rec.crafts.metalworking, "Lloyd E. Sponenburgh" wrote,

And the reflector focuses all those different wavelengths at the same distance.

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Lloyd E. Sponenburgh 13 years ago

David Harmon fired this volley in news:cOSdnSZI0KireYbPnZ2dnUVZ snipped-for-privacy@earthlink.com:

Ayup -- as opposed to a lens which has a different coefficient of refraction for each wavelength (unless you're dealing with fairly expensive achromats, which I would doubt would be employed in this sort of application ).

Lloyd

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User Bp 13 years ago

This does work, it was popular in amateur astronomy in the late '70s. The trick is getting a good surface finish as the resin shrinks during curing. Upper surfaces (which we tried) always had defects which rendered them useless for astronomy, but might be ok for a searchlight. Supposedly the best method was to pour the resin into a dish of mercury and spin the whole works. We never tried it, but the resin/mercury interface was claimed to have defects only at the rim which could be removed. That formed a negative master from which copies could be molded.

In the end it was more work than other methods.....

bob prohaska

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Lloyd E. Sponenburgh 13 years ago

snipped-for-privacy@rahul.net (Edward A. Falk) fired this volley in news:kvirln$2jj$1 @blue-new.rahul.net:

The largest glass mirror in the world was.

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Lloyd

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Lloyd E. Sponenburgh 13 years ago

"Lloyd E. Sponenburgh" fired this volley in news:XnsA22998145191Blloydspmindspringcom@216.168.3.70:

Obut... That doesn't finally shape the mirror. It only gets it to a rough curve, so that not so much glass has to be ground away in the grinding and figuring process.

Lloyd

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Pete Keillor 13 years ago

I read recently in Sky & Telescope about an operational research scope that only looks straight up. Mirror is the mercury pool. They did shape the container so that a minimal amount of mecury is required.

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Pete Keillor

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Lloyd E. Sponenburgh 13 years ago

snipped-for-privacy@rahul.net (Edward A. Falk) fired this volley in news:kvlcgh$31m$1 @blue-new.rahul.net:

Plus, it's not as reflective as a proper aluminum/dichroic coating. Plus mercury slowly oxidizes in air, and the oxides float on the surface. Plus any little seismic variation would screw your curve...

It makes little sense, except perhaps for proving you can do it.

Lloyd

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DoN. Nichols 13 years ago

I suspect that it also has a sealed argon (or other inert gas) above it, to prevent oxidation of the surface of the mercury.

Cost is mentioned as a benefit, and the largest is 6 meters -- so it is bigger than the Palomar 200" mirror, and cost a *lot* less to make. From the site mentioned above:

====================================================================== The greatest advantage of a liquid mirror telescopic is its small cost, about 1% of a conventional mirror. This cuts down the cost of the entire telescope at least 95%. ======================================================================

And -- it goes on to list projects for which the inability to tilt it is not a particular disadvantage.

Granted -- I don't see signs of any kind of enclosure in the photo for an argon atmosphere.

Enjoy, DoN.

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