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.