Thanks - I figured this was the case but I figured, if nothing else, you guys would have a good laugh over it.

I should have remembered that "6 orders of magnitude" from my high school infatuation with astronomy. I've heard that in the tropics, it's actually possible to start a fire with a raindrop. I'm not sure what size of raindrop, but I decided to take your technique and work backwards, starting with the Mount Palomar 200-inch reflector. 200 inch diameter equals 100 inch radius equals area of 10000 * pi square inches. Divide this by 10^6 to get .01 * pi. Divide by pi and take the square root of .01 to get radius of .1 inch or diameter of .2 inches.

So - if it's possible to start a fire in sunlight with a lens 1/5 of an inch across, then theoretically it *might* be possible to start a fire in moonlight using the largest optical telescope* in the world. But as you say, a bit big to fit in the back pocket <img src="images/graemlins/wink.gif" alt="" />

*I know the Russians built one that's even bigger, but I understand it never did work properly.