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Douglas, you wrote:
"NEVER never NEVER look at the sun at any stage of its being above the horizon 
with any from of magnification:  binoculars, telescopes - anything."

A brief glimpse will not cause any harm especially when the Sun is just 
peeking a part of its disk above the horizon. The extinction at the horizon 
is around 12 magnitudes on average, so the Sun is reduced in brightness to 
such an extent that it is only about 2 magnitudes brighter than the Full Moon 
(high in the sky). I've looked at the setting Sun's last sliver through a 
small sextant telescope on a number of occasions. It's bright, but nothing 
like the Sun when it's well above the horizon. I should add that the "setting 
sun" for land observers is usually a couple of degrees above the horizon 
because of surrounding topography and then the Sun is much, MUCH brighter. 
Those twelve magnitudes of extinction apply when the Sun is at the normal sea 
horizon.

Also, many of us have caught a bit of sunlight looking through a sextant when 
the filters are not quite positioned correctly. And we're not blind. 
 
You also wrote:
"I have seen 'holes' in the retina with consequent poor central vision due to this in practice."

A few years ago, some of us attempted to find examples of this in the 
literature. The best we could come up with were people who were not fully 
conscious, or otherwise incapacitated (e.g. by drugs), staring at the Sun. 
Other than that, the tales of eye damage seem to be hard to substantiate 
(except for the final stage just before totality in a total solar eclipse).
 
You wrote:
"Outside of visible spectrum:  especially  infra-red emissions are still there 
and can damage the fovea - permanently in seconds if there is any intensity 
in the sun's light at all."

How many seconds? Most people when exposed to dazzling sunlight blink away in 
a fraction of a second (with the single exception of the very last moments 
before totality in a total solar eclipse). But if you're looking at the 
rising or setting Sun, this doesn't happen for the simple reason that the Sun 
isn't anywhere near as bright. Those twelve magnitudes of extinction 
correspond to a factor of about 63,000 reduction in the intensity of the 
light from the Sun. You say also "if there is 'any' intensity in the sun's 
light". Well, ok, but how much is "any"?
 
And you wrote:
"Even some so-called "dark glass" or "smoked glass" filters, especially on 
early sextants are suspect for not filtering out the infra-red sufficiently. 
They were not aware of the poor quality of filtering with some 'coloured 
glass filters' outside of visible spectrum in the ninteenth century for 
example"

Is this true by evidence or just something that's worth considering? Do you 
have any hard data on nineteenth century sextant filters? Again, I own some 
19th century instruments. I have used them. And my eyes are fine. How common 
were bad shades?
 
And you concluded:
"Make sure you use correct filters whenever using a telescope for viewing the sun."

And of course, this is absolutely true when you're looking directly at the 
full disk of the Sun for more than a fraction of a second. It's also 
important when the Sun is almost completely eclipsed, even with no optical 
aid, since that last little bit of the visible disk of the Sun doesn't 
trigger our normal instinct to blink and look away, but the light is still 
full intensity from that small brilliant point of light.

-FER




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