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