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 .