Silly question

And I meant it. How big would the objective have to be for full moonlight (say at nearest distance) to be able to spontaneously light paper on fire. Or Sirius?

These really are for me burning questions...

Jess Tauber

I suspect you can't. A 10" f/4 has a lunar image 0.35" in diameter in the focal plane. If you double the aperture, you quadruple the light grasp, but you also double the size of the lunar image to 0.70", so it is spread out over four times the area. The brightness of the image per unit area is unchanged. You could concentrate the image more by using a faster system, but I don't think it will come close to getting you there.

The Sun is magnitude -26.75 and the Moon is -12.7, so the Sun is 14.05 magnitudes or about 417,000 times brighter than the Moon.

There may be issues with the energy outside the visible spectrum too. Does some we get directly from the Sun get lost when light is reflected off the Moon?

I think you'll still need matches.

Clear skies, Alan