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Douglas,

In another post, I mentioned that we may be talking at cross-purposes in these 
posts. I still think that's a partial problem here.

First, I have not suggested, nor has anyone else as far as I know, that you 
could look at the full Sun high in the sky through a telescope even for a 
single second. If you do, you'll damage your eye, probably permanently. We 
all agree on that point.

The points of disagreement are as follows (as I see it):
1) Minor point: I claim that you could look at the upper limb of the Sun just 
as it's peeking over the horizon through a small telescope and it is not 
dangerous. Why? Because the extinction at the sea horizon is so great as to 
make the Sun only slightly brighter than the Full Moon (this is emphatically 
NOT the case at an inland hill "horizon" at some degrees above the true 
horizon). And also because, quite simply, I've done it. I've looked at the 
last bit of the Sun as it is setting through binoculars. And my eyes are 
fine.

2) Another minor point: if you catch a glimpse of the mid-day Sun through a 
sextant telescope, you will not be instantly blinded. Here I think the timing 
is critical and the strong reflex we have to look away makes all the 
difference. More than half a second strikes me as dangerous.

3) MAJOR point: The intensity of the sunlight (or the light from any other 
celestial body) on the fovea is not increased when you look through a 
telescope. The total area illuminated is greater and that's what does the 
damage (maybe because conduction cannot cool the larger area). 

This third item is distinctly counter-intuitive, and it's not surprising that 
you find it hard to swallow. I quoted some astrophysics texts which I thought 
might have some impact on you --maybe to get you to question your 
assumptions, but apparently not. So how about trying an experiment? I hope 
you agree that the Sun is not different in any sense from any other extended 
object so we can use the Moon as surrogate. Try this: get a small handheld 
telescope or a pair of binoculars. Now look at the Moon with it holding it up 
to one eye. Then open your other eye and compare the two images. You will 
find that the Moon looks just as bright (in terms of surface brightness) 
whether you're looking through the telescope or observing with your unaided 
eye. The image is larger in proportion to the magnification. Next try it with 
a telescope that has a larger aperture but with the magnification low enough 
that you can still see the whole Moon. There is no change. Incidentally, if 
the Moon is unavailable, try it with some other bright object with comparable 
angular extent that you can see clearly separated from other objects (a 
white, illuminated by bright sunlight, boat some distance from shore should 
work).

So what does this imply on the "viewing screen" of the retina? The fovea is 
basically filled by the image of the Moon at unit magnification (since, as 
you've noted, the fovea is about half a degree in the visual field). If I 
hold a 10x telescope up to my eye, the image on the retina is 10x bigger in 
diameter and 100x bigger by area. Only a small portion of the Moon's image 
now falls on the fovea and so tyo study the appearance of the Moon, we have 
to "scan about" across the face of the Moon. Apart from minor variations in 
brightness (maria vs. highlands), the intensity and total flux of energy 
falling on the fovea doesn't change despite the much larger image size. But 
the total flux of energy falling on the retina as a whole is increased by a 
factor of 100. Are we making any progress? Are those cows moving on to other 
pastures yet?? :-)

-FER




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