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Sunlight and Eyes

Southwind17

Philosopher
Southwind17
Joined
Sep 6, 2007
Messages
5,154
Whilst it's widely known that it's dangerous to look directly at the Sun, is it dangerous to look at it through a deep haze? I live in the Middle East, and in the late afternoon, when the Sun's low, the Sun's brightness is often diminished significantly because of haze. It's difficult to know for sure, but I'd guess that often its luminosity is less than that of a full Moon on a clear night, but maybe the lack of contrast with the surrounding afternoon sky gives that perception. I had absolutely no problem observing it a couple of days back mind, at around 18.00, other than the fact that it disappeared quickly (and not because I went blind!).

Moreover, assuming it's OK to observe a low-luminosity Sun, what about observation through a telescope? Presumably, a UV filter would be a wise idea in any event, but again, is there anything inherently dangerous, even if the Sun isn't "bright"?
 
Nix on the scope.

You eye has a 5mm aperture to let light in. The scope has a 70mm lens to gather the light, and focus it all down to your 5mm aperture. That is, 196 times as much light as the naked eye.

Ya ever burn anything with a magnifying glass? same thing will happen to your eye.
 
casebro said:
Nix on the scope.

You eye has a 5mm aperture to let light in. The scope has a 70mm lens to gather the light, and focus it all down to your 5mm aperture. That is, 196 times as much light as the naked eye.

Ya ever burn anything with a magnifying glass? same thing will happen to your eye.
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Actually, my telescope has a 250mm mirror, so even more caution required then, I guess. But what about filters, including UV, to reduce the already diminished light?
 
Never, ever, look directly in the sun without adequate protective filters.

Remember, there are no pain receptors in the retina, so you could be damaging your eyes even if you are able to gaze into the sun without discomfort.

NASA (among other reputable sources) has advice for observing solar eclipses, which should be followed at any time you want to observe the sun:

http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html
 
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I think it's safe to look directly at the sun at sunrise and sunset. Any other time, it's safer to look at a projected image of the sun, so get yourself a good camera obscura and go wild.
 
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Southwind17 said:
Actually, my telescope has a 250mm mirror, so even more caution required then, I guess. But what about filters, including UV, to reduce the already diminished light?
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Contrary to a prior post: the back of the eye can feel pain, which is why it hurts to look at bright objects. However, the problem is that by the time you feel the pain, a lot of damage has already been done.

UV filters aren't going to be much benefit. The problem isn't UV: it's light intensity. All that sunlight is being focused by your eye's lens and could give your retina a first or second degree burn before the pain makes you look away. That will scar over and could permanently affect your vision.

The point is that considering there are plenty of safe and acceptable alternatives, I can't imagine why anybody would want to look at the sun through a telescope.
 
blutoski said:
UV filters aren't going to be much benefit. The problem isn't UV: it's light intensity. All that sunlight is being focused by your eye's lens and could give your retina a first or second degree burn before the pain makes you look away. That will scar over and could permanently affect your vision.
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So what if the intensity of the light reflected off the Moon is, in fact, greater than that of the evening Sun through thick haze, as I described, is it dangerous to observe the Moon, or OK to observe the Sun?

blutoski said:
The point is that considering there are plenty of safe and acceptable alternatives, I can't imagine why anybody would want to look at the sun through a telescope.
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What alternatives are there for observing sun spots?
 
Southwind17 said:
Actually, my telescope has a 250mm mirror, so even more caution required then, I guess. But what about filters, including UV, to reduce the already diminished light?
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While I wouldn't recommend risking your eyesight based on advice from an Internet message board, there is a lot of good information online, on viewing the sun, especially with telescopes. For example:

http://www.astronomyedinburgh.org/publications/journals/39/safety.html

It is interesting to note that UV is not a danger to the retina, as it does not make it that far.

The main danger is cooking the cells in the back of the eye, which can lead to serious damage, often irreversible. This comes from heat, not UV, and includes visible as well as infrared frequencies.

If the sun is filtered by dust, clouds, haze, so that it is not only as dim as the moon, but also cold, due to the infrared being filtered out, it is quite safe to look at it. As most of us know. We have done it all our lives.

Using a scope or field glasses on that cold dim sun, well, isn't that the tricky question. The scientific method would be to measure the frequency and amplitude of the light, as viewed through the device, then comparing it to safe levels.

Or you could do what lots of people have done, and look at the sun with a telescope, when it is cold and dim. This method is more direct, but you run the risk of becoming one of the millions walking around blind from doing this very experiment.

Sure, you might think that when you first put your eye up to the glass, the brightness and heat would alert you to the danger, just like when you accidentally look at the sun or other intense light source. You know, how you blink, shut the yes, look away, because instinct kicks in, and your body knows good and damn well that the heat is too much, so you don't want to stare at the sun?

But somehow, looking through a scope defeats the bodies natural defenses, and a hot bright light source is just fine, and there you are, all happy observing the sun, seeing sunspots maybe, all the while the back of your eye is cooking, and later you go blind.


All because you didn't practice good science. And you can't trust your eyes.
























Did I mention not depending on Internet advice when it comes to your eyesight. I did? Good, good to know.
 
Southwind17 said:
What alternatives are there for observing sun spots?
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If your aim is to observe sunspots, I'd suggest that sunset through a thick haze is the worse possible moment.

You want a clear sky and a solar filter for your telescope (otherwise you'll burn your eyes and damage your telescope), and it might be advisable to wait for a period of sunspot activity, which probably means in a few years time.
 
A 250mm lens gets 2,497 times as much light as your pupil. RTFM that came with the scope.
 
Southwind17 said:
So what if the intensity of the light reflected off the Moon is, in fact, greater than that of the evening Sun through thick haze, as I described, is it dangerous to observe the Moon, or OK to observe the Sun?
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I have no idea: 'haze' is too vague. Are you saying "Is it safe for me to look through a telescope at the sun if it's darkened enough to be safe?" In which case, uh... yeah. But why did you even ask if you already knew the answer?




Southwind17 said:
What alternatives are there for observing sun spots?
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Depends: on the Dobsonian I use either a glass aperture-spanning [solar filter] (don't use the eyepiece-fitted solar filters, and don't use the mylar ones) or if I'm recording it, I just view via CCD connected to eyepiece. I want to watch the quality of the recording anyway, and if something unexpected happens (like... oh I dunno... a haze suddenly clears...) I'll fry a CCD instead of my macula.

There is also the projection method if you don't want to capture the image. Maintains full colour on a white target surface.

I used to use a DST ("dobson solar telescope"). I dropped and broke the reflector in the '80s and have never replaced it, but now that my nieces are interested I've been motivated to build a new one. It's basically a Newtonian with a 95% absorbing reflector at the aperture. See attachment.
 

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AlbedoWP is the measurement of radiation reflected off a surface. The average albedo of the earth is computed to be about 30%, affected mainly by the large amount of water surface and additionally clouds and snow. The moon's albedo is generally about 7%, but that can peak at 12.5% when the moon is in direct opposition - it is not just an illusion that a full moon looks brilliant. The explanation is unusual enough that I'll copy it here:

It is now known that this departure from a Lambert surface [a model of smooth spherical surface reflecting light] is caused by the very porous first few millimeters of the lunar regolith. Sunlight can penetrate the surface and illuminate subsurface grains, the scattered light from which can make its way back out in any direction. At full phase, all such grains cover their own shadows; the dark shadows being covered by bright grains, the surface is brighter than normal.

The picture is further complicated by the fact that the perfectly full moon is never visible from Earth (at such times, the moon is eclipsed). From the Apollo missions, we know that the exact subsolar point - in effect, the fullest possible moon - is some 30% brighter than the fullest moon seen from earth. It is thought that this is caused by glass beads formed by impact in the lunar regolith, which tend to reflect light in the direction from which it comes. This light is therefore reflected back toward the sun, bypassing earth.
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(http://jeff.medkeff.com/astro/lunar/obs_tech/albedo.htm)

In other words, it would be significantly brighter than normal full moon if it weren't in eclipse. While it is incorrect to assert that the moon is really as dark as fresh asphalt or black construction paper because of these anomalies, It is not far off in the main. It is certainly less than viewing the sun through a camera obscura cast on white paper.

Just don't take the chance. You only have one pair of eyes (perhaps 10 sq cm of retina altogether), and no one is very sanguine yet about routine retinal transplants. Buy a filter and enjoy it in safety.
 
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robinson said:
That sounded far too high. It is 900 times as much. Teh Internets said so.
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Not sure how they calculate that, but assuming the pupil is 5mm, a 250mm lens gets 2,500 times as much light as the pupil, because it has an area 2,500 times larger. Simple pi*r2 calculation.
 
blutoski said:
Are you saying "Is it safe for me to look through a telescope at the sun if it's darkened enough to be safe?" In which case, uh... yeah. But why did you even ask if you already knew the answer?
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Well if that had been my question then, yes, I would, of course, already know the answer, and would have had no need to ask. But that wasn't my question. What I'm mostly getting at is whether there's anything inherently unique about direct sunlight, even low-intensity sunlight, that makes it dangerous compared to, say, reflected sunlight, which we know is OK, hence Moon observations. That's why I mentioned UV, for example.

But thanks for your contribution, and to everybody else. The general advice seems pretty clear!
 
Southwind17 said:
What I'm mostly getting at is whether there's anything inherently unique about direct sunlight, even low-intensity sunlight, that makes it dangerous compared to, say, reflected sunlight, which we know is OK, hence Moon observations.
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The answer to that question is no. Solar photons are identical to any other photons, it's just the sheer number in direct unfiltered sunlight that poses a risk.
 
Magnification can counteract the large aperture of the telescope. e.g. you'd need a magnification of 50 to spread the light out over 2500 times the retinal area.

There exist mylar filters for viewing the sun through a telescope. When there is no dust you'll be able to see sun spots. Much preferable to risking your vision.
 
robinson said:
http://www.bmas.org/how2buytscopebk.pdf

Both set of numbers bring up an interesting issue. Does that mean the heat energy is multiplied by that amount?

Either number means a 10 inch mirror can produce light well over half a million degrees F.

I don't buy that either.
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Okay, that's calculated on the basis that the pupil is about 8.3 mm in diameter. Sounds a bit large to me.
 
I would not deliberately look directly at the sun for any reason. If there is no gain then it is not worth the risk. A filter can be faulty.

If you have a telescope and want to see sun spots then project the light from the telescope onto a piece of paper. Helps if no light goes just on the outside of the telescope and onto the paper.

Looking at the reflection of the sun from water or a mirror is not safe.
 
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