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chornedsnorkack
2009-Mar-13, 08:48 AM
The recommendations about looking at Sun at eclipses suggest that the filter must decrease the brightness by at least 4,5 orders of 10.

This means 11,25 magnitudes.

Now, the Sun is an extended body, about 30 arc minutes across.

The resolution of eye is about 1 arc minute, so a point source is one which is smaller than 1 arc minute. If the disc like Sun but 30 000 times dimmer will burn the eye, then an object 900 times dimmer but also 900 times smaller at 1 arc minute across will be focused to a sharp spot in the eye and burn it. So, it would mean 27 million times less than the light of the Sun. Which means something like 18,6 magnitudes!

The Sun is magnitude -26,7. Thus, a point source of light at -8 should burn the eyes.

The brightest star, Venus, is about -4,4, and the brightest fixed star, Sirius, is -1,4. The Moon is -12,7, and it is an extended body. But a spot source of light as bright as Moon should easily burn eyes.

What about supernovas? How far must a supernova be to be safe to look at?

astromark
2009-Mar-13, 10:09 AM
Quote;
"What about supernovas? How far must a supernova be to be safe to look at?"

If a Nova event is so bright as to be damaging to sight... We are in trouble. Other energies are more harmful that a little bright light sorce... you could 'NOT LOOK.' or a deep user friendly cave might be best...

This thread seems to have sprung from the close relative. re, magnitudes.
Very good to see all those numbers... the full moon at -12.7 is bright but, your sight is in no danger from it. The sun has the same angular size, but can only be viewed through filtering or at night.... sorry:) As for your question; If the brightness makes you squint... look away. Its not good for your eyes. The numbers do not matter if you can no longer see.

chornedsnorkack
2009-Mar-13, 04:31 PM
"What about supernovas? How far must a supernova be to be safe to look at?"

If a Nova event is so bright as to be damaging to sight... We are in trouble. Other energies are more harmful that a little bright light sorce... you could 'NOT LOOK.' or a deep user friendly cave might be best...

This thread seems to have sprung from the close relative. re, magnitudes.
Very good to see all those numbers... the full moon at -12.7 is bright but, your sight is in no danger from it. The sun has the same angular size, but can only be viewed through filtering or at night.... sorry:) As for your question; If the brightness makes you squint... look away. Its not good for your eyes. The numbers do not matter if you can no longer see.

Well, consider the early historical supernovae. No fixed star has been brighter than Venus for over nine centuries (because Tycho Brahe's supernova in 1572 was slightly dimmer). But there were big and bright supernovae in Taurus in 1054 and Lupus in 1006. How bright were they? And did they cause harm on Earth at other energies?

What would a supernova like the Lupus supernova of 1006, or a slightly brighter one, look like when stared at? How close and bright would such a supernova have to be to do other harm?

And how big binoculars and telescopes are safe for observing night sky?

grant hutchison
2009-Mar-13, 04:55 PM
This is another surface brightness question. The solar disc has a high enough surface brightness to generate a damaging level of retinal illumination. It will continue to have that same surface brightness as we move farther from the sun: the disc just subtends a smaller angle. So the spot of damage to the retina will decrease in size.
What saves us from incurring tiny points of retinal damage from all the stars in the sky is diffraction limitation: once the solar disc gets down to about one minute of arc in diameter, it stops getting smaller. At that point, the apparent surface brightness starts to decline with increasing distance (as less light is diffracted to cover the same size of spot), and so the level of retinal illumination goes down with increasing distance, quickly reaching safe levels.

I have some similar calculations somewhere (I was working out what apparent magnitude would be necessary to "show through" the blue sky). I'll dig them out and see what sort of answers I get with new parameters plugged in.

Grant Hutchison

chornedsnorkack
2009-Mar-13, 08:42 PM
This is another surface brightness question. The solar disc has a high enough surface brightness to generate a damaging level of retinal illumination. It will continue to have that same surface brightness as we move farther from the sun: the disc just subtends a smaller angle. So the spot of damage to the retina will decrease in size.
What saves us from incurring tiny points of retinal damage from all the stars in the sky is diffraction limitation: once the solar disc gets down to about one minute of arc in diameter, it stops getting smaller.
Yes. At this point, we are at magnitude of about -19.


At that point, the apparent surface brightness starts to decline with increasing distance (as less light is diffracted to cover the same size of spot), and so the level of retinal illumination goes down with increasing distance, quickly reaching safe levels.

Not so quickly. Seeing how a filter must decrease the surface brightness of the solar disc by 11 magnitudes to make the disc safe to see, the brightness of a spot would also have to be decreased by 11 orders of magnitude.

Coming from the other direction, increasing the brightness of Sirius by just 7 orders of magnitude would also reach magnitude -8,4. If Sirius were magnified 25 times, something which could be done with perhaps 10 or 15 cm diametre scope, Sirius A would be magnitude -8, and Pup would be +1,7. Bright enough to see by naked eye, but will staring intently at Sirius with magnitude -8,4 be safe for the eye, and will the squint reaction hamper noticing the Pup?

grant hutchison
2009-Mar-14, 12:34 AM
Not so quickly. Seeing how a filter must decrease the surface brightness of the solar disc by 11 magnitudes to make the disc safe to see ...That's making it really, really, really safe to see. An incandescent lamp filament will leave you with an afterimage, but doesn't produce permanent damage. That's about a hundredth of the surface brightness of the solar disc: five magnitudes, or a tenfold increase in distance from the diffraction limit. You'll get that about 300AU from the Sun, which is just a hop and a skip in interstellar terms.

Grant Hutchison

chornedsnorkack
2009-Mar-14, 10:04 AM
That's making it really, really, really safe to see. An incandescent lamp filament will leave you with an afterimage, but doesn't produce permanent damage. That's about a hundredth of the surface brightness of the solar disc

Visual or bolometric?

Hornblower
2009-Mar-14, 12:45 PM
Visual or bolometric?
That looks like visual to me. Assuming a filament temperature of 3000K and similar emissivity, the bolometric intensity, including all infrared, should be about 1/16 that of the Sun, still a large reduction.

grant hutchison
2009-Mar-14, 02:09 PM
Visual or bolometric?Visual.
The visual wavelengths are the only ones that are actually focused on our retinas. The out-of-focus retinal spot generated by IR is therefore larger, as would be the spot produced by UV if it weren't filtered before focusing by the structures of the anterior eye. So point sources will deliver a more dilute retinal hit at wavelengths outside the visual.
I presume that's why you were asking about the visual brightness that is safe to see.

Grant Hutchison

undidly
2009-Mar-19, 01:43 AM
The recommendations about looking at Sun at eclipses suggest that the filter must decrease the brightness by at least 4,5 orders of 10.


The Sun is magnitude -26,7. Thus, a point source of light at -8 should burn the eyes.



OH dear.

Heavens Above tells me that tonight I can observe an iridium satellite flare with a magnitude of -8.

I have seen bright flares before and could feel them in my eyes.
Don't know the magnitude of these flares.
Heavens Above does not give a warning.
I will be watching anyway but be ready to look away.
The flare will be visible for about 5 seconds.

a1call
2009-Mar-19, 02:12 AM
Visual.
The visual wavelengths are the only ones that are actually focused on our retinas. The out-of-focus retinal spot generated by IR is therefore larger, as would be the spot produced by UV if it weren't filtered before focusing by the structures of the anterior eye. So point sources will deliver a more dilute retinal hit at wavelengths outside the visual.


Grant Hutchison

*- Does that mean that very near-sighted people have some level of protection against bright lights that could be damaging to healthy sighted people?

*- Could/has near-sightedness be/en an evolutionary trait for Eskimo races against snow blindness? (Not sure-if/implying the trait is present in Eskimo more than average or to a higher degree)

ETA: One supportive quote (though attributed to indoors work):


Some populations (the Inuit eskimos, for example) have shown a statistical shift toward myopia when, over many years, some members of their society changed from outdoor activity to working inside, doing much closer work. This fact, however, does not mean that if an individual does a lot of close work, he or she will become myopic.

Source (http://www.triadpublishing.com/eyecarereports/myopia-book.shtml)

grant hutchison
2009-Mar-19, 02:43 AM
*- Could/has near-sightedness be/en an evolutionary trait for Eskimo races against snow blindness?Snow-blindness is a corneal burn from ultraviolet: sunburn of the eyeball. So focus isn't an issue.

Grant Hutchison

cjl
2009-Mar-19, 02:51 AM
OH dear.

Heavens Above tells me that tonight I can observe an iridium satellite flare with a magnitude of -8.

I have seen bright flares before and could feel them in my eyes.
Don't know the magnitude of these flares.
Heavens Above does not give a warning.
I will be watching anyway but be ready to look away.
The flare will be visible for about 5 seconds.

-8 isn't dangerous. See Grant Hutchison's point for details.

a1call
2009-Mar-19, 03:00 AM
Snow-blindness is a corneal burn from ultraviolet: sunburn of the eyeball. So focus isn't an issue.

Grant Hutchison

Directional UV reflection of the sun does not normally (in a 20/20 vision) focus on the retina but wouldn't it be even more diffused in a near-sighted individual?

undidly
2009-Mar-22, 06:54 AM
I am north of Sydney,Australia.
In 2 hours there is a -9 iridium visible 3.6 km E of home.
I will be there.
Can hardly wait.
Missed the -8 that I mentioned in an earlier post.

chornedsnorkack
2010-May-23, 09:31 AM
That's making it really, really, really safe to see. An incandescent lamp filament will leave you with an afterimage, but doesn't produce permanent damage. That's about a hundredth of the surface brightness of the solar disc: five magnitudes, or a tenfold increase in distance from the diffraction limit.

For a point source, this means about -14 magnitude. Which means observing (vicinity of) first magnitude stars with telescopes at least a couple of metres across.

How do astronomers using big telescopes at night deal with dazzle and afterimages?

Hornblower
2010-May-23, 10:27 AM
For a point source, this means about -14 magnitude. Which means observing (vicinity of) first magnitude stars with telescopes at least a couple of metres across.

How do astronomers using big telescopes at night deal with dazzle and afterimages?They use filters as needed to dim a bright star to comfortable levels.

ngc3314
2010-May-23, 12:21 PM
Yes. At this point, we are at magnitude of about -19.
Coming from the other direction, increasing the brightness of Sirius by just 7 orders of magnitude would also reach magnitude -8,4. If Sirius were magnified 25 times, something which could be done with perhaps 10 or 15 cm diametre scope, Sirius A would be magnitude -8, and Pup would be +1,7. Bright enough to see by naked eye, but will staring intently at Sirius with magnitude -8,4 be safe for the eye, and will the squint reaction hamper noticing the Pup?

Imprecision alert - magnitudes are not the same as orders of magnitude. Two orders of magnitude in intensity is 5 stellar magnitudes. 25x in intensity would be 3.5 magnitudes, putting Sirius A at visual magnitude -5.1 and Sirius B at about 4.6. To have the listed equivalent visual magnitudes, they would need to be brightened by 360 times.

EDG
2010-May-23, 04:51 PM
What saves us from incurring tiny points of retinal damage from all the stars in the sky is diffraction limitation: once the solar disc gets down to about one minute of arc in diameter, it stops getting smaller. At that point, the apparent surface brightness starts to decline with increasing distance (as less light is diffracted to cover the same size of spot), and so the level of retinal illumination goes down with increasing distance, quickly reaching safe levels.

Would the light be getting diffracted by the atmosphere?

Hornblower
2010-May-23, 06:02 PM
Would the light be getting diffracted by the atmosphere?No, it is diffracted by the edge of the pupil of your eye. Atmospheric distortion smears the image out slightly, but not that much.

George
2010-May-23, 06:16 PM
The visual wavelengths are the only ones that are actually focused on our retinas. The out-of-focus retinal spot generated by IR is therefore larger, as would be the spot produced by UV if it weren't filtered before focusing by the structures of the anterior eye. So point sources will deliver a more dilute retinal hit at wavelengths outside the visual.
Due to the apparent size of the Sun, I assume the lack of focus for IR is less important and that IR damage is possible. I am curious about this since, using binoculars, I once enjoyed observing a setting yellowish-orange Sun that was at a very comfortable low level of brightness. Then I soon realized that the our atmosphere is transparent for a number of IR bands. Perhaps, though, the weaker degree of refraction by the lenses within the binoculars diffuse much of this IR. Any idea?

chornedsnorkack
2010-May-23, 06:19 PM
Imprecision alert - magnitudes are not the same as orders of magnitude. Two orders of magnitude in intensity is 5 stellar magnitudes. 25x in intensity would be 3.5 magnitudes, putting Sirius A at visual magnitude -5.1 and Sirius B at about 4.6. To have the listed equivalent visual magnitudes, they would need to be brightened by 360 times.

What I meant is 25 times in linear magnification - 625 times the light collecting area. Something like 15 cm aperture then.

Filters would equally dim the light of Sirius A and of B. So what is done to see Sirius B?

Shaula
2010-May-23, 06:45 PM
Blocking plates (an Occultating disk) are used to block out the light from the primary IIRC.

Nereid
2010-May-23, 11:19 PM
Well, consider the early historical supernovae. No fixed star has been brighter than Venus for over nine centuries (because Tycho Brahe's supernova in 1572 was slightly dimmer). But there were big and bright supernovae in Taurus in 1054 and Lupus in 1006. How bright were they? And did they cause harm on Earth at other energies?

What would a supernova like the Lupus supernova of 1006, or a slightly brighter one, look like when stared at? How close and bright would such a supernova have to be to do other harm?

And how big binoculars and telescopes are safe for observing night sky?
Making robust estimates of the peak magnitude of historical supernova is extraordinarily difficult.

However, IIRC, the brightest such reached peaks of ~-8 (or was it -10?).

How big a telescope would you need for Sirius to be as bright as the Sun (assume seeing of 1", and no losses)?

chornedsnorkack
2013-Mar-13, 05:50 PM
How big a telescope would you need for Sirius to be as bright as the Sun (assume seeing of 1", and no losses)?

Do you mean integrated brightness, or surface brightness?