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It is easily experimentally demonstrated that a focussed image of the Sun can be used to start a fire. Furthermore, by thermodynamic considerations or the conservation of étendue, it can be shown that moonlight cannot be focussed such that a fire can be created. However, it is theoretically possible to magnify starlight enough to create a fire, as the surface temperature of a distant star is still above the ignition temperature of tinder.

According to this article, there was a paper published in 2002 that included a design for a telescope capable of this. Does anyone know what the paper was? I've tried searching on Google Scholar and have found nothing relevant so far. My back of the envelope calculations suggest that in order to magnify Betelguese such that it had the apparent diameter of the Sun would require a magnification of at least 36,000 times. Assuming we were then going to focus this image to a small disk using a microscope objective, this would suggest we need an aperture of the order of 100s of metres in diameter... I'd be interested to see if there's a trick around this.

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    $\begingroup$ See Is it possible to start fire using moonlight? Unfortunately it's hard to glean from that thread, but the underlying assumption that goes into the claim that moonlight can't start a fire is simply false. The moon is not merely a blackbody radiating at whatever temperature a thermometer buried in the dust would read. $\endgroup$ – user10851 Jul 12 '16 at 0:20
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – David Z Jul 12 '16 at 13:15
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Listening to the recording that the article was derived from, it appears that the article is:

Donald Lynden-Bell. Exact Optics : A unification of optical telescope design. Mon. Not. R. Astron. Soc. (2002)

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    $\begingroup$ The problem is going to be the diffraction pattern even though you have a perfect focus. If you picked the brightest star Sirius for example, it is effectively a point source of light at 0.006" angular size, so you would have to focus to an exact point to concentrate enough energy, which is impossible because of the diffraction pattern of any size mirror or lens. $\endgroup$ – Peter R Jul 12 '16 at 20:36
  • $\begingroup$ @PeterR If you could rewrite this to an answer, I would definitely upvote it. $\endgroup$ – Thriveth Aug 15 '16 at 14:04

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