NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Looking at the Sun through a telescope
From: Frank Reed CT
Date: 2006 Aug 6, 21:52 -0500
George H, you wrote:
"First, the intrinsic brightness of a Sun image focussed on the retina,
as seen through a telescope, in terms of energy per square millimetre,
can not be greater than if the Sun was being observed in the same way
by the naked eye. Not brighter, but certainly bigger."
Right. I agree with that. It's that specific intensity business I mentioned
earlier. When you look at a nebula through a telescope, it doesn't get
brighter per unit area (unit solid angle actually) but it is larger. Hence, the
disappointment many novice telescope users experience when they look at the
Orion Nebula through a telescope and discover that, although they can see more
detail, it is still a dusky, barely visible patch of light. Works just the same
way with the Sun --any small piece of the Sun's disk a minute or arc across
looks, on average, just as bright whether it's magnified 3x or a 100x.
You wrote:
"It is known to be damaging to the eye to look at the Sun directly, and we
automatically avert our view. We have developed a fast blink response
to minimise that damage, and avoid a retinal burn; the iris closes
down as well, but more slowly. Through a telescope, if such a retinal
burn can occur, it will be of a larger patch, rather than a tiny spot.
So, to that extent it can be more damaging. But it seems to me that
the likelihood of damage is no greater than it was without that
telescope."
As a number of people have said in this latest thread (Ken Muldrew in
particular) and a few others said in earlier versions of the discussion, the thing
that has changed is probably heat damage. The human eye has evolved over
millions of years to deal with a Sun that is 30 arcminutes in diameter, just a
small "dot" on the retina. It seems reasonable to assume that the eye can deal
with small hot spots generated by occasional glimpses of the Sun. We've all
looked at the Sun (with the unaided eye, no telescope), probably hundreds of
times, and it does not seem to have caused any permanent damage. But if I look
through a 7x sextant telescope without shades, that means the spot on my
retina will be 49x larger in total area. Considering that the human eye operates
so close to natural limits in so many ways, it would not surprise me to
learn that 49 times as much heating, even for a short period of time, might
overload the retina's natural cooling system. I do think that a glimpse of the Sun
at 7x magnification will probably cause significant impairment of vision
lasting for days or weeks, certainly something to be avoided. But based on
reports of "eclipse blindness", I highly doubt that there would be permanent
damage from a quick glimpse of the Sun through a sextant telescope.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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From: Frank Reed CT
Date: 2006 Aug 6, 21:52 -0500
George H, you wrote:
"First, the intrinsic brightness of a Sun image focussed on the retina,
as seen through a telescope, in terms of energy per square millimetre,
can not be greater than if the Sun was being observed in the same way
by the naked eye. Not brighter, but certainly bigger."
Right. I agree with that. It's that specific intensity business I mentioned
earlier. When you look at a nebula through a telescope, it doesn't get
brighter per unit area (unit solid angle actually) but it is larger. Hence, the
disappointment many novice telescope users experience when they look at the
Orion Nebula through a telescope and discover that, although they can see more
detail, it is still a dusky, barely visible patch of light. Works just the same
way with the Sun --any small piece of the Sun's disk a minute or arc across
looks, on average, just as bright whether it's magnified 3x or a 100x.
You wrote:
"It is known to be damaging to the eye to look at the Sun directly, and we
automatically avert our view. We have developed a fast blink response
to minimise that damage, and avoid a retinal burn; the iris closes
down as well, but more slowly. Through a telescope, if such a retinal
burn can occur, it will be of a larger patch, rather than a tiny spot.
So, to that extent it can be more damaging. But it seems to me that
the likelihood of damage is no greater than it was without that
telescope."
As a number of people have said in this latest thread (Ken Muldrew in
particular) and a few others said in earlier versions of the discussion, the thing
that has changed is probably heat damage. The human eye has evolved over
millions of years to deal with a Sun that is 30 arcminutes in diameter, just a
small "dot" on the retina. It seems reasonable to assume that the eye can deal
with small hot spots generated by occasional glimpses of the Sun. We've all
looked at the Sun (with the unaided eye, no telescope), probably hundreds of
times, and it does not seem to have caused any permanent damage. But if I look
through a 7x sextant telescope without shades, that means the spot on my
retina will be 49x larger in total area. Considering that the human eye operates
so close to natural limits in so many ways, it would not surprise me to
learn that 49 times as much heating, even for a short period of time, might
overload the retina's natural cooling system. I do think that a glimpse of the Sun
at 7x magnification will probably cause significant impairment of vision
lasting for days or weeks, certainly something to be avoided. But based on
reports of "eclipse blindness", I highly doubt that there would be permanent
damage from a quick glimpse of the Sun through a sextant telescope.
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
