Okay!

The experimental results are in!

Drum roll please.......

I happened to have an F4 mirror handy.....4 inch diameter, 16 inch focal
length, reflectivity 90 percent or so....

Remember I said an F4 80 percenter was equal to an F1 5 percenter? And someone
here noted that their canopy was roughly an F1 system?

Well, the sun is out bright and clear today....

I had 4 targets, some thick black garden plastic (like a very heavy duty
garbage bag material), some black plastic about a millimeter thick, an old
black neoprene mouse pad between and 1/8 and a 1/4 inch thick, and a BONE dry
paper thin piece of wood loaded with sap....

The mirror had no trouble melting the 2 plastics...but a fire seemed
impossible/improbable, though there as plenty of smoke.....and note that
thicker plastics probably wouldnt even smoke....but would probably
melt/distort some....

with a little care I got the wood to ignite, but a similiar piece a 1/4 inch
thick was a no go no matter what I tried, but again a nice amount of smoke....

Now, to me the mousepad seems like the most realisitic substitute target...

When I focused the mirror on that it smoked quite nicely...but trying my best
with all kinds of variations an actual fire just didnt seem possible....

Now, if you increased the power density a factor of 16 I have little doubt it
woulda burst into flames...which is what would happen if an equivalent F1
system at 5 percent suddenly was to operate at a grazing angle and reflect 80
percent or so.....

So, I guess the big question would be HOW much above 5 percent reflection or
how much faster than F1 would be required to start a fire?

Note that this mirror was DESIGNED to focus properly, while I doubt a canopy is
anywhere near as accurate an optical surface (regardless of its percent
reflectivitiy), which would bring down the power density significantly...and to
just accidently get a canopy faster than F1 (ie F ratio less than 1)..that is
ALSO the right shape optically would be pretty improbable....

So, I'll retract my statement some....

If your worried about a FIRE (which was what I was thinking about when I first
posted).....AND you have DO NOT have grazing reflections...your probably
okay....

If you dont want stuff melting and smoking...your gonna have to be a bit more
careful!

Best bet? Be prudent and FACE away (perpendicular?) from the sun (or whatever
the owners manual says to do)....

Again, I guess my main point that got me into this discussion in the first
place was that the grazing angle and its resulting very high reflectivities
could be a MAJOR factor that some folks might NOT be aware of...and hence since
they never had problems at non grazing angles they might not realize the
dangers (fires OR just melting stuff) of what happens when they DO happen to be
in grazing angle conditions....

Anybody want to buy a slightly melted mousepad?

take care

Blll