Homebrew PCBs

On 21/01/12 04:51, Jeff Heiss wrote:
> Russell, where did you find the cylinder reflector?  Did you buy it,
> scrounge it from something you took apart?

I derived the parabola formula then cut out a parabolic arc in two supporting 
sheets of wood then bent some shiny 1mm aluminium sheet into it.

I just re-derived the formula and it seems correct from rough sketches:

x = 2.SinQ/(1 + CosQ)
y = (d - f + (d + f).CosQ)/(1 + CosQ)

x: horizontal distance from center of parabola
y: vertical distance below x-axis
d: distance between x-axis and lowest part of parabola
    (parabola is assumed to be below x-axis and d and f are positive numbers)
f: distance between focal point and lowest part of parabola
Q: angle of ray from vertical (ray comes from focal point)

Q = arTan(x/(d - f))

It was years ago and the formula does not look familiar (i must have had it in a 
different form then).