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/~/med...21_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

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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

On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.

Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...e_lighter.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On 21/08/13 14:02, Spehro Pefhany wrote:
On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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
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.
Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...e_lighter.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.



Best regards,
Spehro Pefhany
I can remember RadioShack had ones like those back in the mid 1970s, at
least the Huntington LI, NY branch did. Definitely smaller than 6".

On Wed, 21 Aug 2013 09:02:19 -0400, Spehro Pefhany
wrote:

On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.

Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...e_lighter.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.

Those things were advertised in magazines when I was about 10 years
old, but lighting a cigarette doesn't require as high a temperature as
igniting, say, some red cedar inner bark.

--
Ed Huntress





Best regards,
Spehro Pefhany

On Wednesday, August 21, 2013 7:25:19 AM UTC-4, Mike B wrote:


The reflector needs to be about 2 to 3 inches in diameter with a focal point

about 2 to 3 inches in front.




Mike

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.

Dan

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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?


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

In article ,
Mike B someone@noplace wrote:

Presumably this was possible in the days before CNC machinery, so how did
they
do it?

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.

--
Cats, coffee, chocolate...vices to live by
Please don't feed the trolls. Killfile and ignore them so they will go away.

"Lloyd E. Sponenburgh" lloydspinsidemindspring.com fired this volley in
. 3.70:

Some ordinary flint glass about 1/2" thick should
do.


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

Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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".


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 .

And this might come in handy for doing the math...

http://mscir.tripod.com/parabola/



"Mike B" someone@noplace wrote in message
...
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/~/med...21_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

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

I can remember RadioShack had ones like those back in the mid 1970s, at
least the Huntington LI, NY branch did. Definitely smaller than 6".

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

On Wed, 21 Aug 2013 10:28:57 -0500, "Snag's Shop"
wrote:



Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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".


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 .

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


--
Snag
Yes , I was
a Boy Scout .

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.


"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

On Wed, 21 Aug 2013 09:02:19 -0400, Spehro Pefhany
wrote:

On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.

Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...e_lighter.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.

http://www.amazon.com/Solar-Spark-Li...arette+lighter

The Greens use it to light up a joint. Pot is Green. Tobacco is not.

http://www.wildwoodsurvival.com/surv...tor/index.html

http://www.ebay.com/itm/Windproof-Wa...-/350857024511

Radio Shack had the oval ones as I recall.





Best regards,
Spehro Pefhany

"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

In article ,
David Billington wrote:

On 21/08/13 14:02, Spehro Pefhany wrote:
On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...Blogs/Homestea
ding%20and%20Livestock/Solar%20Fire%20Starter/sundancesolar_2021_9386858.g
if
It's actually a "solar fire starter" so the light is traveling in the
opposite
direction, but the optical path is the same.


Mike
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.
Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...garette_lighte
r.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.



Best regards,
Spehro Pefhany
I can remember RadioShack had ones like those back in the mid 1970s, at
least the Huntington LI, NY branch did. Definitely smaller than 6".

I had one of the Radio Shack ones as well... bought in w Los Angeles. As
I recall, didn't work well if there was even the slightest overcast or
haze.

Erik

Mike B wrote:

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?
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.

Jon

In article , Mike B
someone@noplace.? writes
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?
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?

--
Chris Holford

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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.

Not near as much fun as making one from scratch,
http://www.4bobandbob.com/pages/01.html

All the parabolics are sold out and were 4" the non parabolics range from 2
1/2" to 4". Seem to recall they once sold a 6" many years ago... Have not
bought from them in ~25 years, nice to see they are still in business.
--
William

On Wednesday, August 21, 2013 4:25:19 AM UTC-7, Mike B wrote:
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.


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.

On 8/21/2013 5:27 AM, Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.

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

mike fired this volley in news:kv3o58$bem$1@dont-
email.me:

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.

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

On Wed, 21 Aug 2013 14:07:53 +0100, David Billington
wrote:

On 21/08/13 14:02, Spehro Pefhany wrote:
On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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
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.
Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last time
I was on Venice beach so I guess it will have to be a tobacco product
or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...e_lighter.html

I guess it's aimed at the green consitutuent of the smoking public who
mostly lights up a fag at midday- likely an exceedingly narrow sliver
of the Venn diagram.



Best regards,
Spehro Pefhany
I can remember RadioShack had ones like those back in the mid 1970s, at
least the Huntington LI, NY branch did. Definitely smaller than 6".

I had one of those. IIRC, it was an elliptical shape about 3x5".
The diabolical elliptical parabolical lighter.

--
Truth loves to go naked.
--Dr. Thomas Fuller, Gnomologia, 1732

Larry Jaques fired this volley in
:

On Wed, 21 Aug 2013 14:07:53 +0100, David Billington
wrote:

On 21/08/13 14:02, Spehro Pefhany wrote:
On Wed, 21 Aug 2013 08:27:43 -0400, the renowned Ed Huntress
wrote:

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...torial/Blogs/H
omesteading%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
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.
Years ago I had some premiums that were folding aluminized parabolic
mirrors that could allegedly be used to light a cigarette. They were
at most 3" diameter when opened. Never tried it myself that I can
remember. I've probably still got one around somewhere, I'll have to
try it. I didn't spend the $40 to get medically "evaluated" last
time I was on Venice beach so I guess it will have to be a tobacco
product or paper.

This is a similar idea without the folding part:
http://www.alibaba.com/product-gs/11...olar_cigarette
_lighter.html

I guess it's aimed at the green consitutuent of the smoking public
who mostly lights up a fag at midday- likely an exceedingly narrow
sliver of the Venn diagram.



Best regards,
Spehro Pefhany
I can remember RadioShack had ones like those back in the mid 1970s,
at least the Huntington LI, NY branch did. Definitely smaller than 6".

I had one of those. IIRC, it was an elliptical shape about 3x5".
The diabolical elliptical parabolical lighter.

--
Truth loves to go naked.
--Dr. Thomas Fuller, Gnomologia, 1732


Yeah... way smaller than 6". Consider -- you can light a fire with a 2"
magnifying glass. They pass less than 80% of the heat-producing
wavelengths.

A properly aluminized mirror reflects over 97% of all visible
wavelengths, and most IR.

That means it's more efficient at lighting fires than a magnifying glass
of the same size.

Those dumb Radio Shack lighters were just polished stainless steel -- not
hardly reflective at all, in general optical terms.

LLoyd

On Wed, 21 Aug 2013 20:18:52 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

mike fired this volley in news:kv3o58$bem$1@dont-
email.me:

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.

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?

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


Lloyd

On Wed, 21 Aug 2013 18:05:35 -0700, mike wrote:

On 8/21/2013 5:27 AM, Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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.

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

And the 8x10 fresnel lenses can be had at the dollar store..for a
buck.


"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

Ed Huntress fired this volley in
:

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:

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

On Thu, 22 Aug 2013 06:18:24 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Ed Huntress fired this volley in
:

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:

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".

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



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!


Tricky. Fun. Dangerous. That's my kind of experiment.

--
Ed Huntress


Lloyd

On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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.


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

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
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!

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

Ed Huntress fired this volley in
:

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.



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

On Thu, 22 Aug 2013 07:28:49 -0500, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Ed Huntress fired this volley in
:

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.



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).

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.


How many grass fires are started along roadways every year from folks'
carelessly flinging weeds out their car windows?

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



Lloyd

Ed Huntress fired this volley in
:

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.


Sure, but that's without the skilled hands of a primitive style
firestarter working the goods... it's just lying there!

LLoyd

On 8/21/2013 14:25, Mike B wrote:
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?

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.

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

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+.





There are two possible solutions:


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.




Mike


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

Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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

John

On Thu, 22 Aug 2013 19:38:07 -0400, John
wrote:

Ed Huntress wrote:
On Wed, 21 Aug 2013 19:25:19 +0800, Mike B someone@noplace wrote:

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/~/med...21_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

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

John

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

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

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
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!

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

Solar fired cannon have been around a very long time

http://www.pixgrove.com/2012/12/solar-cannon.html


"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

On Wed, 21 Aug 2013 20:49:32 -0500 in rec.crafts.metalworking,
"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote,
Yeah... way smaller than 6". Consider -- you can light a fire with a 2"
magnifying glass. They pass less than 80% of the heat-producing
wavelengths.

A properly aluminized mirror reflects over 97% of all visible
wavelengths, and most IR.

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

David Harmon fired this volley in
m:

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

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 G).

Lloyd