I instantly regretted sending my last post, even as I was hitting "send". That last statement sounded too trite, even for me. Look, here's what I am saying: A telescope mirror CAN start a fire. To be fair to Brent, yes it is unlikely, and the longer the focal length, the less concentrated the hot spot (which is just the prime focus solar image). You can play with f-ratio and aperture and find some combinations hotter than others to be sure. This really is overanalyzing a basically intuitive situation. I have purposely started fires with mirrors from 10" to 17.5", f-ratios from f/4.5 to f/6. Sometimes it isn't instantaneous...how long it takes depends also on the nature of the tinder in terms of ignition point, heat retention, etc., probably other conditions that the slide-rule types can easily come up with effortlessly but I'd have to dig out 25 year old textbooks to pull it together. You don't hear much about telescope or optically caused fires, but they do happen. We have friends who lost their house to a fire caused by sunlight being focused on a curtain by glass sculpture! Art causing a fire! So now that this dead horse (sorry Rich, poor choice of words) is thoroughly beaten, there it is. Can happen, but usually only as a fluke occurrence if unintentional. And my apologies to Brent for suggesting a pathological fastidiousness. Sorry, home today with a sick kid, time on my hands. _______________________________ Do you Yahoo!? Declare Yourself - Register online to vote today! http://vote.yahoo.com