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: Ken Muldrew
Date: 2006 Aug 3, 22:59 -0500
From: Ken Gebhart <gebhart@celestaire.com>
> Well, I didn't mean to make light of the dangers of looking at the sun
> through a telescope on a sextant. However, as someone involved with
> the manufacture of sextants, I am aware of the possibility on product
> liability litigation in this area. So, I have collected any and all
> anecdotes and opinions on the subject in order to arm ourselves for
> this possibility.
Perhaps the data for people looking at the sun with binoculars (of a similar
power to a sextant scope) will provide more information. Or maybe
birdwatchers with spotting scopes catch an odd glimpse of the sun (though
these are likely to be more powerful than sextant scopes).
> Apart from the scientific discussion of light, lenses and retinas, I
> am not aware of any instances of actual eye damage occurring from use
> of a modern day sextant (or for that matter, an astronomical
> telescope). If any list members know of such things, I would
> appreciate any leads they may have.
Timothy Ferris wrote of his experience looking through a telescope during a
total eclipse where he kept looking just a little too long (and I think suffered a
permanent retinopathy). This was in Seeing_in_the_Dark, I think.
I don't know of any specific anecdotes with sextant scopes or binoculars, but
like I said earlier, the damage caused through a scope will likely be due to
heat rather than the photochemical injury that occurs with the naked eye.
Therefore it isn't due to the brightness of the light, so the argument that the
scope won't increase the brightness doesn't apply here. The heating power
of the sun through a lens goes up approximately as the square of
magnification. So if the temperature of the retina is increased by 4°C by
naked eye exposure (my earlier claim of 2°C was wrong) then we might
naively guess that a 6x scope would increase the retinal temperature by
24°C. Retinal burning (which is a permanent injury) occurs when the
temperature of the retina rises by 10-12°C (there's a time dependence, but
I'm not sure what it is). So you might get away with several, or even tens, of
seconds with a 3x or 4x scope but even a quick flash with a 6x could be
enough to cause permanent damage (again, these are just guesses). But
this is for the full sun to be focussed on the retina. For the case of a sliver of
sun getting past the shades, the heating may be less.
Also, having experienced snow blindness a couple of times (a
photochemical injury similar to naked eye sungazing) I wouldn't recommend
experimenting with this. It would be less painful to just pour a bunch of sand
underneath your eyelids and leave it there for a day or two.
Ken Muldrew.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
From: Ken Muldrew
Date: 2006 Aug 3, 22:59 -0500
From: Ken Gebhart <gebhart@celestaire.com>
> Well, I didn't mean to make light of the dangers of looking at the sun
> through a telescope on a sextant. However, as someone involved with
> the manufacture of sextants, I am aware of the possibility on product
> liability litigation in this area. So, I have collected any and all
> anecdotes and opinions on the subject in order to arm ourselves for
> this possibility.
Perhaps the data for people looking at the sun with binoculars (of a similar
power to a sextant scope) will provide more information. Or maybe
birdwatchers with spotting scopes catch an odd glimpse of the sun (though
these are likely to be more powerful than sextant scopes).
> Apart from the scientific discussion of light, lenses and retinas, I
> am not aware of any instances of actual eye damage occurring from use
> of a modern day sextant (or for that matter, an astronomical
> telescope). If any list members know of such things, I would
> appreciate any leads they may have.
Timothy Ferris wrote of his experience looking through a telescope during a
total eclipse where he kept looking just a little too long (and I think suffered a
permanent retinopathy). This was in Seeing_in_the_Dark, I think.
I don't know of any specific anecdotes with sextant scopes or binoculars, but
like I said earlier, the damage caused through a scope will likely be due to
heat rather than the photochemical injury that occurs with the naked eye.
Therefore it isn't due to the brightness of the light, so the argument that the
scope won't increase the brightness doesn't apply here. The heating power
of the sun through a lens goes up approximately as the square of
magnification. So if the temperature of the retina is increased by 4°C by
naked eye exposure (my earlier claim of 2°C was wrong) then we might
naively guess that a 6x scope would increase the retinal temperature by
24°C. Retinal burning (which is a permanent injury) occurs when the
temperature of the retina rises by 10-12°C (there's a time dependence, but
I'm not sure what it is). So you might get away with several, or even tens, of
seconds with a 3x or 4x scope but even a quick flash with a 6x could be
enough to cause permanent damage (again, these are just guesses). But
this is for the full sun to be focussed on the retina. For the case of a sliver of
sun getting past the shades, the heating may be less.
Also, having experienced snow blindness a couple of times (a
photochemical injury similar to naked eye sungazing) I wouldn't recommend
experimenting with this. It would be less painful to just pour a bunch of sand
underneath your eyelids and leave it there for a day or two.
Ken Muldrew.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
