Starship Asterisk*

by **dalessio** » Wed May 14, 2008 4:00 pm

I am an astronomer and I hope I can help...

The fact that the resolution of the eye is of the order of arcminutes simply means that a point source (a star is a point source for all practical purposes) will appear to be about 4' in size. The atmosphere only makes it harder to see stars, not better. No matter where we are all of the light of a star will be spread across these 4'. The atmosphere makes this substantially worse, and absorbs some of the light. This spreads the image across a larger area >4' and makes the star fainter to our eyes. If we go to higher altitudes we get a purer image. The issue that stars are tough to see at high altitudes >8000 feet is because your eyes are not getting enough oxygen.
However, in a plane, or in the space station the oxygen supply is fine and the sky should appear very clear. We are then completely limited by only our eyes optics and no degrading atmospheric effects. I have never been able to see stars out of a plane window, but those are usually double paned and very dirty (plus don't forget you are looking through air traveling 500 mph, which is terrible optically). From the space station the sky is probably incredible. However, you will not see this from a picture or video camera. These devices are not meant for recording faint objects. Go ahead and try to take a picture of a star from earth with a normal shutter speed, or take a video with stars in it. You will not see any stars.
Remember Hubble is just a camera in space!

So the summary is... Less atmosphere, stars appear smaller, which means they will be brighter and easier to see, as long as you are getting enough oxygen.

by **Sputnick** » Wed May 07, 2008 8:55 pm

Watch - Your space canoe would have been quite a trip. I had the vertigo experience in a canoe on perfectly calm water at about 2 p.m. on a beautiful summer day .. clouds and blue sky reflecting into the mirror of the river which was about a mile wide, with me in the centre .. talk about sudden and total disorientation thinking I was up instead of down .. I guess that's why airplane pilots sometimes dive into the ground or sea.

The universe's 'planes' are interesting mirrors- example, an exploding star sending out mirror plumes.

by **watch24** » Wed May 07, 2008 7:18 pm

Sputnick wrote:
This is a little late, but I camped at Treeline at Jasper National Park, Alberta, Canada, 30 years ago and the stars looked like a mirror image of Jasper village below. It was quite an experience. So .. to get to Jasper Avenue, turn left at Andromeda.

The first time I really took notice of the night sky was in the BWCAW about 10 years ago (Boundary Waters Canoe Area Wilderness--Canada/Minnesota border). I wanted to see the Andromeda galaxy, so a buddy and I took off accross a big lake at 1 A.M. to get away from the tree line. The lake was like glass and reflected the sky perfectly. I was sitting in the bow, so unless I looked down, I couldn't see the bow wave or the front of the canoe. I couldn't see the other side of the lake either. It was the coolest experience I'd ever had--like drifting through the galaxy without a space suit. It actually gave me a bad case of vertigo, and I had to stabilize myself to keep from falling out. You gotta try it sometime!!!

I've been hooked on astronomy ever since.

by **Sputnick** » Thu May 01, 2008 8:30 pm

This is a little late, but I camped at Treeline at Jasper National Park, Alberta, Canada, 30 years ago and the stars looked like a mirror image of Jasper village below. It was quite an experience. So .. to get to Jasper Avenue, turn left at Andromeda.

by **henk21cm** » Mon Apr 28, 2008 4:08 pm

watch24 wrote: This may explain why I've been more impressed with dark skies near sea level than when camping at 3000m.

There are two items that influence vision itself: the amount of oxygen and the amount of sugar in the blood. With little oxygen you will suffer from "tunnel vision". When the amount of sugar in the blood is raised, the eye will focuss slightly different. There is one main item that influences the interpretation of vision: toxins, like alcohol, LSD or other drugs. "Impression" is a psychological factor, which i count as interpretation. Similarly, the sky in a forrest is less impressive than the sky, as visible on a wide open plain, like the APOD of 2008-04-16.

watch24 wrote:Someone who's used to higher altitudes and in really good shape, like Serge Brunier, probably doesn't need as much oxygen to function well,

What SergeBrunier meant with "plus difficile" is that the feeling of enjoying the stars is more difficult due to the depreviation of oxygen. That was lost in my translation. Nonetheless stars are visible by the naked eye, even at 5000m altitude, according to Serge.

If we are lucky, he will gather some new amazing pictures. When i contacted him, he was due to fly to the Atacama dessert within a few hours and he wrote that he had even better equipment on this trip. I hope we are in for a new treat.

IMHO that has wrapped up the loose ends of this thread.

by **watch24** » Mon Apr 28, 2008 3:28 pm

The sky at about 3000m is magnificent. At 4000 to 5000 m it more difficult, due to the lack of oxygen in your blood.

This may explain why I've been more impressed with dark skies near sea level than when camping at 3000m. Someone who's used to higher altitudes and in really good shape, like Serge Brunier, probably doesn't need as much oxygen to function well, but the average couch potato may start to have trouble at lower than 4000m--maybe even in the reduced-oxygen environment of a commercial jetliner cabin. Anyway, it explains my original question in this thread about why my friend said they had to take the tour group below 9000' on Mauna Kea to view the night sky....whew!

by **watch24** » Mon Apr 28, 2008 3:05 pm

Translated: Indeed, the stars are visible!!!!!!!!!!!! The sky at about 3000m is magnificent. At 4000 to 5000 m it more difficult, due to the lack of oxygen in your blood. But in all these cases the stars are visible at these high altitudes. Atmospheric scintillation /dispersion has nothing to do with vision in that matter, that is absolutely ridiculous.

So that settles it...the first-hand comment I was waiting for. I guess I was right and wrong at the same time: The vision is degraded (remember, my question never was 'are they visible?'), but reportedly has nothing to do with atmospheric dispersion. Actually the notion of oxygen deprivation making vision difficult was also touted by a geologist here in my office, but it's always good to get a second opinion.

My feelings aren't hurt...my ideas have been called 'ridiculous' by frenchmen before.

Thanks for asking![/u]

by **henk21cm** » Sun Apr 27, 2008 1:35 pm

watch24 wrote: Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

G'day Marty,

Eating is proof of the pudding, Serge Brunier, creator of the APOD of 2008-04-23, has been at high altitude and knows from first hand whether stars are visible at high altitude. I wrote him an e-mail and he has an answer. His reply:

e-mail of Serge wrote: Bien sur, les etoiles sont visibles !!!!!!!!!!!!!!!!!!!!!!!!!!!

A 3000 metres environ, le ciel est fantastique.
A 4000 metre, 5000 metre, c est plus difficile, du fait du manque d oxygene
dans le sang.

Mais, dans tous les cas, les etoiles sont visibles en tres haute altitude...

La scintillation n a rien a voir la dedans : c est absolument ridicule...

Translated: Indeed, the stars are visible!!!!!!!!!!!! The sky at about 3000m is magnificent. At 4000 to 5000 m it more difficult, due to the lack of oxygen in your blood. But in all these cases the stars are visible at these high altitudes. Atmospheric scintillation /dispersion has nothing to do with vision in that matter, that is absolutely ridiculous.

by **henk21cm** » Fri Apr 25, 2008 8:43 pm

watch24 wrote: Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

G'day Marty,

If light is spread out over more cells, the intensity per cell will decrease. The same amount of energy is spread out over a larger surface, so the energy flux decreases. When an object is too dim, when it emits too little light, the eye can not trigger on the low amount of photons collected. As a result we do not see any stars less luminous than magnitude 5.5 to 6 with the naked eye. When light is spread out over many cells in our eyes, more than under normal circumstances, the energy flux decreases beyond a point that it is too low to trigger our cells.

A simple experiment you can perform yourself at any altitude: when you wear glasses, or contact lenses, look up the sky and count stars in a particular region of the sky, with these glasses or contact lenses. This we call normal situation. Then remove your glasses or contact lenses and count again. We call that blurred. If you don't wear glasses, look up the sky without glasses. We call that normal. Then borrow those of your parents, sisters, brothers or wive, (non plural!) and repeat the count with those borrowed ones, which we call blurred.

My prediction is that you will count more stars under normal conditions than when under blurred conditions: removing your normal glasses or adding glasses that are not yours.

but now maybe we need a neurologist -

I am not such a person. The only one i know and which comes close to a neurologist is a professor in Eindhoven, "Dept. of Biomedical Imaging". I could e-mail him and hope he has an answer and is willing to give the answer. It may take some time, specially since next week is spring holiday.

by **watch24** » Thu Apr 24, 2008 9:05 pm

The size (length, width, radius) of such a cell is therefore about 2E-6 m, that is the same order of magnitude as the center part of the Airy disk. Thus the limitations of optics (i.e. ideal optics) will cause diffraction of a point source, and spread the light out over at least one cell.

So the projection of a point source on the retina 'always hits a light sensitive cell', it never falls in between of the cells. That would be the obvious reason why you would miss a star.

So assuming you are correct in all your calculations (and given my expertise and time resources, I must do so), then the physics indicates we should be able to see stars even in the vacuum of space--assuming our eyes didn't pop out of their sockets first.

Still...I gotta wonder if that reduction of the number of cells receiving that point of light is interpreted differently in the brain than if that same intensity of light is spread over a much larger number of cells due to atmospheric dispersion. (see July 25, 2000 APOD) That could imply a degraded starfield image 'perception' at high altitudes, don't you think?

My hats of to your expertise in physics, but now maybe we need a neurologist -

Marty

by **henk21cm** » Thu Apr 24, 2008 7:38 pm

watch24 wrote:Unfortunately, being a time exposure, it doesn't answer my question about naked-eye observation at those altitudes.

G'day Marty,

I cannot answer your question from first hand experience. I live 5 m below see level, and the highest point i ever saw a star was when i was just a few hundreds of meters high.

There are reasons which tell me that the height and thus air pressure is of no importance. We could ask Serge Brunier, he has been there.

Each optical element (mirror or lens) has to endure Airy diffraction. The separation angle i told you yesterday is roughly the angle caused by this type of diffraction. See e.g. http://en.wikipedia.org/wiki/Airy_disc. Even the eye has Airy diffraction. The Airy angle in our eye will be roughly 600 nm / 4 mm = 150 micro radians. The radius of our eye is 2.5 cm. Such an angle will lead to a distance on the retina of 1.5E-4 * 2.5E-2 = 4 E-6 m.

Our eye has approximatedly 100 millions light sensitive cells. The surface of the retina is approximately 5 square centimeters. Each cell will, on average, occupy roughly 5E-8 square centimeters, that is 5E-12 square meters. The size (length, width, radius) of such a cell is therefore about 2E-6 m, that is the same order of magnitude as the center part of the Airy disk. Thus the limitations of optics (i.e. ideal optics) will cause diffraction of a point source, and spread the light out over at least one cell.

So the projection of a point source on the retina 'always hits a light sensitive cell', it never falls in between of the cells. That would be the obvious reason why you would miss a star. You can compare it with a bed of nails, as used by some artists/performers. When you drop from a few meters high a marble on the bed of nails, the marble has a chance to fall between the nails. Now we add optics to the equation. The limitation to optics (Airy diffraction) makes the marble larger: they become melons. A melon -when dropped on a bed of nails- will always hit a nail.

Whether the light is diffracted by the atmosphere, is of no importance, since not the atmosphere is the dominant player, the optical limitations in the human eye are.

I read in a publication of the JPL about the Spitzer telescope (Bachus, Velusamy, Thompson and Arballo, 2005). The Airy diffraction, as projected on the CCD, has a size which is comparable with the size of one element of the CCD. This situation is the same as in the human eye. Much smaller CCD elements are useless, since the Airy pattern would spread out the light over many cells, not adding information.

Regards,
Henk

by **bystander** » Thu Apr 24, 2008 5:48 pm

Mac Coak wrote:Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

watch24 wrote:A little disturbing isn't it? "All" the short-exposure shots I've ever seen from space (and the moon) are also black-skied, but I've never seen an explanation. It could be due to the bright foreground subjects (planets or orbital craft), but I suspect it's due to the lack of atmospheric dispersion of light. It would be nice to get a confirmation from someone who knows first-hand.

This discussion was had sometime ago, haven't found the link. I think the explanation was oversaturation from the bright foreground. Same with pictures taken on the moon. But, from what henk21cm says, there should be no problem with ISS astronauts seeing stars.

by **watch24** » Thu Apr 24, 2008 2:32 pm

Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

A little disturbing isn't it? "All" the short-exposure shots I've ever seen from space (and the moon) are also black-skied, but I've never seen an explanation. It could be due to the bright foreground subjects (planets or orbital craft), but I suspect it's due to the lack of atmospheric dispersion of light. It would be nice to get a confirmation from someone who knows first-hand.

by **Mac Coak** » Thu Apr 24, 2008 1:53 pm

SO - by this discussion one is led to believe that if there is life in the universe on some other planet that has a very thin atmosphere then that life would not "see" any stars or other planets other than their own star. APOD 20 November 2007 (picture taken with a camera but not a time exposure) shows a dark background "sky" behind Earth, the sun is behind the camera obviously.

by **Mac Coak** » Thu Apr 24, 2008 1:33 pm

Can astronauts aboard the international space station see stars? All the pictures (obviously taken by a camera) of the space station show a dark sky in the background.

by **Axel** » Thu Apr 24, 2008 3:17 am

But but guys, that helicoid-looking cloud is really interesting! Is this a well-known phenomenon?

by **watch24** » Wed Apr 23, 2008 10:29 pm

A good test is picture #12 of Serge Brunier, just above the 500 hPa level:

http://www.sergebrunier.com/gallerie/pa ... ce/12.html

The Nikon camera he used is comparable -regarding to optics- to the human eye.

I looked at the image. Spectacular, and I have to admire the fortitude to even take such a shot (almost 20,000'!). Unfortunately, being a time exposure, it doesn't answer my question about naked-eye observation at those altitudes.

Marty

by **watch24** » Wed Apr 23, 2008 10:18 pm

Thanks for the detailed explanation. I got the gist of what you were saying, but I had trouble keeping up with the optics. I'll have to dig out a textbook tonight. Obviously, my recollections of human eye limitations are equally rusty.

Therefore there is no reason why one should not see stars at high altitudes, e.g. Mauna Kea level.

I'd assumed some stars could be seen, but would one expect to have their vision noticably degraded by the thin atmosphere at 4 or 5km? Also would stars be visible out a spaceship window or from an astronauts visor?

Foolish questions I suppose, but questions I've never had answered. I've never seen the stars as clearly out a jetliner window as I do from my car, even when they darken the cabin for long international flights and I sealed myself against the window with a thick dark blanket. And the some of the best naked eye observation has been near sea level on the back side of Maui and the Osa Peninsula in Costa Rica--better even than wilderness mountaintops in remote Montana. (maybe they just seemed better because I wasn't shivering) Anyway...burning questions of a child coming from an old man.

Thanks again.

[/b]

by **henk21cm** » Wed Apr 23, 2008 9:43 pm

The minimum-sized image, if I recall correctly, is 2 arc seconds (maybe 0.2??), which isn't very big.

The resolution of the eye is about 4 arc minutes. Mizar/Alcor is a double star, in Ursa Major, just visible by the naked eye. It is told (urban legend?) that the native Americans used Mizar/Alcor as a test for the eye.

Stars are much smaller than the eye's ability to see, but the atmosphere spreads the light to a large enough size for the eye to see.

The amount of air at 5 km high is approximatedly half the amount of air at sea level. That is an awful lot of air molecules to spread the light. In meteorology the 500 hPa 'level' is in use. That is about 5 km high. Light is scattered by air molecules. The amount of scattering depends on the wave length and air pressure. The pressure where pressure becomes important is at a level what we call vacuum. Blue light is scattered 15 times more than red light. Thats why the sky is blue. Above the main part of the atmosphere the sky is black, see e.g. yuri planet, April 12th. That is a good approximation fpr vacuum.

Next to scattering of light, which makes a star 'twinkle', any optical lens produces an additional scattering of light, called Fraunhofer diffraction. The smaller the diameter of the lens, the wider the diffraction pattern. This diffraction pattern hampers the separation of two adjacent stars. The separation is roughly equal to the wave length of the light divided by the diameter of the lens, expressed in radians.

With a 0.06 m lens at 600 nm wavelength the separation is roughly 1E-5 radians. 1 radian is 3600 x 180 / pi = 206264 arc seconds. The separation is therefore 2 arc seconds. A good test for this separation are epsilon1 and epsilon 2 Lyrae, which are both dounle stars separated by 2.2 arc seconds. A star is far less than 2 arc seconds in size. Our sun at 10 lightyears distance would have a size of 5 milli arc seconds. That is far less than the smearing of light due to Fraunhofer diffraction. Therefore there is no reason why one should not see stars at high altitudes, e.g. Mauna Kea level.

A good test is picture #12 of Serge Brunier, just above the 500 hPa level:

http://www.sergebrunier.com/gallerie/pa ... ce/12.html

The Nikon camera he used is comparable -regarding to optics- to the human eye.

Regards,
Henk

by **watch24** » Wed Apr 23, 2008 9:14 pm

The quote button I figured out, now I just need to figure out the 'edit' part--when I'm not on company time.

I've never treked the CO rockies, but have been all over northern Idaho, North Cascades, and the Beartooths in WY and MT. Love the night sky there--abosolutely nothing around. Packing my 10" would be rough though.

Meanwhile, there's gotta be a physicist, mountaintop astronomer, pilot, or someone who can answer my dumb question--they just better make the font bigger than 2 arc-seconds or I won't be able to read it!

by **bystander** » Wed Apr 23, 2008 8:57 pm

watch24 wrote:--BTW, I'm obviously still trying to figure out how to respond with quotes properly.

Easy way is just click the quote button on the box you're answering, then edit the text.

The minimum-sized image, if I recall correctly, is 2 arc seconds (maybe 0.2??), which isn't very big. Stars are much smaller than the eye's ability to see, but the atmosphere spreads the light to a large enough size for the eye to see. My question is, if the point of light is smaller than the eye can resolve, can it resolve it by brightness alone without the aid of atmosphere?

Don't know about this. That's why I appealed to our resident physicists, Art Neuendorffer and Chris Peterson.

by **bystander** » Wed Apr 23, 2008 8:55 pm

watch24 wrote:How high were you?--I doubt 13,000'. I'm an avid backpacker, but I've always retreated to the valleys to camp below 11,000 feet where the stars are bright. The problem with the eye is that it can only resolve a minimum sized image, and a star in real time may not have enough light to for the retina to resolve. A CCD or photo plate, however, is limited only by the technology.

Not sure, probably not, certainly above 8,000' (Rocky Mtn Natl Park). But the scattering of light by the atmosphere should reduce the amount of light available, not increase it.

Used to be an avid camper. Camped all over Colorado back when you didn't have to stay in registered campgrounds (70's). Only time the stars weren't brilliant was because of weather. Light pollution wasn't much of a problem in the back woods.

by **watch24** » Wed Apr 23, 2008 8:35 pm

The minimum-sized image the naked eye, if I recall correctly, is 2 arc seconds (maybe 0.2??), which is much bigger than a star's dia. The atmosphere spreads starlight to a large enough size for the eye to see. My question is, if the point of light is smaller than the eye can resolve, can it resolve it by brightness alone without the aid of atmosphere?

--BTW, I'm obviously still trying to figure out how to respond with quotes properly.

by **watch24** » Wed Apr 23, 2008 8:29 pm

bystander wrote:
watch24 wrote:Of course the human eye and brain gather and interpret light in real time and in a much different manor than a telescope, so what I was told is quite possible. I was unable to see stars from a 12,000-ft peak once, but the reason could've been ambient light.

I don't see how less atmosphere would affect the light gathering power of your eyes. If anything, the stars should be more visible, because of the lack of scattering. (again, my opinion). The night skies while camping in Colorado were amazing.

How high were you?--I doubt 13,000'. I'm an avid backpacker, but I've always retreated to the valleys to camp below 11,000 feet where the stars are bright. The problem with the eye is that it can only resolve a minimum sized image, and a star in real time may not have enough light to for the retina to resolve. A CCD or photo plate, however, is limited only by the technology.

by **bystander** » Wed Apr 23, 2008 8:21 pm

watch24 wrote:Of course the human eye and brain gather and interpret light in real time and in a much different manor than a telescope, so what I was told is quite possible. I was unable to see stars from a 12,000-ft peak once, but the reason could've been ambient light.

I don't see how less atmosphere would affect the light gathering power of your eyes. If anything, the stars should be more visible, because of the lack of scattering. (again, my opinion). The night skies while camping in Colorado were amazing.

Art? Chris? Are you watching? What do the physicists say?

Forgive me for cluttering the forum with a duplicate posting. I just joined an hour ago, so wasn't clear on how all this works.

It's ok. Just kind of a pet peeve of mine. Welcome aboard.

Starship Asterisk*

Full Moon next to Venus???? (APOD 23 Apr 2008)

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Hope this helps

But but