NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Looking at the Sun through a telescope
From: Frank Reed CT
Date: 2006 Aug 2, 15:19 -0500
Ken mentioned recently on the list that looking at the Sun through a sextant
telescope is no more dangerous than looking at the Sun with the naked eye. I
remember the underlying logic for this point when it was discussed a couple
of years ago. It depends on the optical principle which says that no optical
system can change "specific intensity" --the astronomers' name for brightness
per square arc minute. In other words, when you look at the Moon, as an
example, through a telescope, the surface is not brighter at any given point
compared to the Moon you see directly with your two eyes. The angular size of the
Moon is magnified when seen through the telescope, but on average any slice
of it that subtends a half-degree wide circle will look almost exactly as
bright as the Moon does in the sky (without a telescope). Similarly when you see
the Andromeda Galaxy through a large telescope, it is a disappointing dusky
band of light --larger than the fuzzy patch we see with the unaided eye but
having the same surface brightness. By that logic, the Sun is not brighter
when seen through a telescope, only bigger. So it shouldn't be more harmful than
looking at the Sun directly with your eyes for a fraction of a second.
Is there more to it? If you hold a piece of paper up in front of the ocular
(eyepiece) of a medium-sized backyard telescope when it is focused on the
Sun, the paper will burst into flames within a second or two. Isn't that a hint
of danger? On another note, a spot of very bright light on the retina
illuminating an area 1 millimeter across may have different effects than light of
the same brightness illuminating an area 7 millimeters across. To what extent
have the human eye's defense mechanisms evolved to deal with the threat of the
"normal" Sun that might not apply to a magnified Sun?
Anyway, I'm just throwing this out there for discussion. It's 98 degrees
Fahrenheit in Chicago today, so talking about the Sun seems topical.
-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 2, 15:19 -0500
Ken mentioned recently on the list that looking at the Sun through a sextant
telescope is no more dangerous than looking at the Sun with the naked eye. I
remember the underlying logic for this point when it was discussed a couple
of years ago. It depends on the optical principle which says that no optical
system can change "specific intensity" --the astronomers' name for brightness
per square arc minute. In other words, when you look at the Moon, as an
example, through a telescope, the surface is not brighter at any given point
compared to the Moon you see directly with your two eyes. The angular size of the
Moon is magnified when seen through the telescope, but on average any slice
of it that subtends a half-degree wide circle will look almost exactly as
bright as the Moon does in the sky (without a telescope). Similarly when you see
the Andromeda Galaxy through a large telescope, it is a disappointing dusky
band of light --larger than the fuzzy patch we see with the unaided eye but
having the same surface brightness. By that logic, the Sun is not brighter
when seen through a telescope, only bigger. So it shouldn't be more harmful than
looking at the Sun directly with your eyes for a fraction of a second.
Is there more to it? If you hold a piece of paper up in front of the ocular
(eyepiece) of a medium-sized backyard telescope when it is focused on the
Sun, the paper will burst into flames within a second or two. Isn't that a hint
of danger? On another note, a spot of very bright light on the retina
illuminating an area 1 millimeter across may have different effects than light of
the same brightness illuminating an area 7 millimeters across. To what extent
have the human eye's defense mechanisms evolved to deal with the threat of the
"normal" Sun that might not apply to a magnified Sun?
Anyway, I'm just throwing this out there for discussion. It's 98 degrees
Fahrenheit in Chicago today, so talking about the Sun seems topical.
-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
-~----------~----~----~----~------~----~------~--~---
