"Grant Erwin" wrote: (clip) The intensity of the light drops off as the
cube of the distance to the arc.
^^^^^^^^^^^^^^^^^^^^^^
Come on, Grant. You don't know the inverse square law? You had to derive
your own, and then get it wrong?
^^^^^^^^^^^^^^^^^^^^^^
Grant wrote: The above statement is rubbish. Perhaps Leo has never looked
at the stars, (clip) [Referring to Leo's statement that the intensity of an
image doesn't drop off with distance.]
^^^^^^^^^^^^^^^^
The inverse square law (not inverse cube) does mean that the amount of light
per unit area decreases with distance, but that is not the end of the story.
The light reaching a lens is collected and focused on the image plane. If
you do the math, you find out that the brightness of the image is
proportional to the brightness of the source. That is why exposure control
on a camera works. If you take a picture of an outdoor scene, for example,
the foreground and the background receive the same exposure. In fact, if
you take a picture of a full moon, it needs the same exposure as a picture
taken on a sunny day.

I do not know off the top of my head why stars don't burn your eyes. I
think it has to do with the fact that the images are so small that there is
not enough energy to heat up the retinal cells. Are there any astronomical
photographers in the group?