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