Larry Breed's Fresnel Parabolic Reflector

What it is, is a fresnel parabolic reflector. I started with a 4ft square sheet of acrylic mirror, and cut it into an 18-strip spiral. As I fastened it to its wooden support I pulled each strip a little too far round for it to be able to lie flat. The angle it lies at matches the angle that a similar section of a true parabolic reflector would have -- thus all the angled strips conspire to concentrate sunlight at the focus.

Figuring precisely how much to distort each strip took some hairy trigonometric calculations, accurate to six figures, which I undermined by hand-measuring and hand-drilling the mounting holes. Consequently, the "focal point" is a focal blob about as big as your hand -- but that turned out to be a safety feature, spreading the solar energy and diminishing its intensity. The wooden support for the mirror pivots around a piece of 3" pvc pipe that is aimed at the pole star in the Little Dipper: shazam! an equatorial telescope mount!

An engineering prof from Georgia Tech thought up the mirror a couple of decades ago; I read about his work and thought, "got to build one sometime." Getting involved with Burning Man provided the motivation to turn "sometime" into "now!" Since 1997, we have used my spiral mirror on Monday afternoon, to set aflame the torch of the Naked Fire Goddess; from that flame, the sacred cauldron in Center Camp; and from its flames (actually in '97, these days only in spirit) we light The Man. One year it was cloudy on Monday afternoon, and though we danced and drummed with the mirror we quietly ignited the torch from a cigarette lighter -- a *solar powered* lighter, if you look at it right.