Hmm, I wonder why astronomers use those huge telescopes. Or why a huge
array of mirrors is used for attempts to harness the sun's energy.

THATS about signal to noise ratios and image scale and total energy collected
and detecting FAINTER stuff....a totally seperate issue...trust me, ive been
playing with optics and astronomy for 20 years....

power density (how hot the hot spot gets) is pretty much a function the f
ratio, how bright the source is...in this case a constant....the sun...and the
target is black, white or in between....

here's the scoop....

two lenses.....same f ratio....one 3 times the size of the other...so larger
one has 3 times as long a focal length...

Both form an IMAGE of the sun (it isnt a point, its a disk....)

The longer/bigger lens has an image 3 times as large as the small one, so its
image has 3^2 or 9 times the area.....but the larger lens ALSO collects 3^2 or
nine times the total energy....so the power density (or how bright the focused
sun is) is the SAME for both lenses...

And as an aside....thats why photography lens are so concerned with F
ratios...because thats what determines what the exposure time will be.....the
focal length of the lens ONLY determines the image scale/effective
magnification....

If you half your f ratio..ie go from f2 to f1 youve increased the power density
a factor of 4 times...ie same amount of energy in an image half the size AND
1/4th the area....

So, if you want to compare 5 percent reflection to 80 percent say.....80/5 =
16.....sqrt 16 equals 4....so an F4 system at 80 percent equals an F1 system at
5 percent "burn/melt hazard wise"....

take care

Blll