Hi people, bit of fun... Somebody mentioned using a mirror instead. So the thought occurred to me earlier, just for fun, when reading about the National Ignition Facility project, that maybe the 4.2 billion dollars spent on this could be better used simply by finding a big mountain somewhere, just for fun, and building a humungous 2-kilometer wide parabolic mirror into the side of it and having a moveable target suspended by a crane, following the sun, and generating fusion on the target. Just for fun, I wondered what the upper limit of temperature was that could be reached, in theory, with such a device ? Anyone know how many joules per meter squared the sun can normally deliver to the ground on a nice sunny day? No one has offered any figures, so let's take a guess. My 10 cm-diameter magnifying glass can easily burn paper with a dot that is, at a guess, about 100 times smaller than the glass itself. If the temperature generated is say 250 degrees, then we can start having some fun. Let's assume for the sake of argument that a 10cm parabolic mirror would generate the same amount of heat; I suspect it would be more, but let's assume this to make things easier. My glass / mirror is 5 squared times PI cm squared, which is 78.5 . So an area of 78.5 cm squared gives you a manageable dot of light at 250 degrees celcius. So, a mirror of two kilometers wide = 2000 meters = 200000 centimeters. Square this and times by PI, and you get and area of 125663704000 cm squared. Now, divide this by the area of my magnifying glass, 78.5 cm squared, and you learn that the big mirror is 1600811515.9 times bigger in area. Times this by 250 degrees and you get a temperature of 400,202,878,980 or about 400 thousand million degrees, easily enough to start fusion! Now, obviously, our great scientists would have tried this, I guess, unless they spotted what would be to them an obvious flaw in the scheme. Anybody care to put me right? :-) thanks :-)