Alaska is a wonderful land, and with the Aurora, is also a wonderful sky.
Greetings
César
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 8:42 am
by nemcw
I live close to the Wrangell-St Elias range/Natl Park. It one is looking for a God's country, I believe this is it. From every vantage point, on the slope, on the road, fishing, exploring, simply here, this is country for all the Gods. nem
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 10:01 am
by Mikkel Lund
Has anyone noticed the concentric circles in the middle of the image? Are they camera artefacts?
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 12:09 pm
by neufer
http://en.wikipedia.org/wiki/Wrangell_Mountains wrote:
<<The Wrangell Mountains are a high mountain range of eastern Alaska in the United States. Much of the range is included in Wrangell-Saint Elias National Park and Preserve. Mountains named after explorer, president of Russian-American Company, admiral Ferdinand von Wrangel. The Wrangell Mountains are almost entirely volcanic in origin, and they include the second and third highest volcanoes in the United States, Mount Blackburn and Mount Sanford. The range takes its name from Mount Wrangell, which is one of the largest andesite shield volcanoes in the world, and also the only presently active volcano in the range. The Wrangell Mountains comprise most of the Wrangell Volcanic Field, which also extends into the neighboring Saint Elias Mountains and the Yukon Territory in Canada. The Wrangell Mountains are just to the northwest of the Saint Elias Mountains and northeast of the Chugach Mountains, which are along the coast of the Gulf of Alaska. These ranges have the combined effect of blocking the inland areas from warmer moist air over the Pacific Ocean. The inland areas to the north of the Wrangell Mountains are therefore among the coldest areas of North America during the winter.>>
Click to play embedded YouTube video.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 12:20 pm
by orin stepanek
The auroras dance is missing only one thing! Music!!
Click to play embedded YouTube video.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 3:07 pm
by biddie67
APOD Robot wrote:
....... If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well.
The text seemed to imply that the X-ray and ultraviolet light could only be seen from space. If you had the right kind of sensing equipment, couldn't you see those wave lengths from Earth also?
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 3:12 pm
by Ann
biddie67 wrote:
APOD Robot wrote:
....... If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well.
The text seemed to imply that the X-ray and ultraviolet light could only be seen from space. If you had the right kind of sensing equipment, couldn't you see those wave lengths from Earth also?
Well, remember that the atmosphere of the Earth is relatively opaque to ultraviolet light. I'm not sure about X-rays, however.
Ann
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 3:46 pm
by Third Rock
Mikkel Lund, I noticed the rings and came to the forum to check if someone else noticed them as well. Maybe someone can explain.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 3:51 pm
by Chris Peterson
Third Rock wrote:Mikkel Lund, I noticed the rings and came to the forum to check if someone else noticed them as well. Maybe someone can explain.
Do you see them when you view the image at its full size? At some reduced sizes, I see aliasing in the image, creating odd circles in the star trails.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 4:08 pm
by bystander
Chris Peterson wrote:Do you see them when you view the image at its full size? At some reduced sizes, I see aliasing in the image, creating odd circles in the star trails.
Full size, center frame.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 4:25 pm
by moikey@att.net
How they got Sasquatch to shake that tree, to make that beautiful painted effect, is the biggest mystery in my mind.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 4:26 pm
by Chris Peterson
bystander wrote:Full size, center frame.
Ah, those. Very nice, you don't often see this in terrestrial images, although it is fairly common in astronomical images. The effect is caused by interference between optical surfaces. In this case, it could be between a filter on front of the lens and the first lens surface, between surfaces inside the lens, or between the first and second surfaces of the sensor cover glass. It is a form of Newton's rings. The reason it doesn't normally show up is because it requires monochromatic light to provide significant contrast. This is common in astronomical images made with narrow band filters, but terrestrial images are always broadband. The rare exception is an image like this, where the dominant light source is monochromatic (in this case, probably the 500.7nm forbidden oxygen emission).
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 5:01 pm
by Third Rock
Chris Peterson wrote:
Third Rock wrote:Mikkel Lund, I noticed the rings and came to the forum to check if someone else noticed them as well. Maybe someone can explain.
Do you see them when you view the image at its full size? At some reduced sizes, I see aliasing in the image, creating odd circles in the star trails.
The rings are present with any size, didn't notice anything odd in the star trails.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Wed Oct 06, 2010 5:02 pm
by HikesWithEyes
Wonderful picture. 1 small nit to pick:
That area is referred to as Southeast Alaska, not Eastern Alaska.
Cheers.
Ah, those. Very nice, you don't often see this in terrestrial images, although it is fairly common in astronomical images. The effect is caused by interference between optical surfaces. In this case, it could be between a filter on front of the lens and the first lens surface, between surfaces inside the lens, or between the first and second surfaces of the sensor cover glass. It is a form of Newton's rings. The reason it doesn't normally show up is because it requires monochromatic light to provide significant contrast. This is common in astronomical images made with narrow band filters, but terrestrial images are always broadband. The rare exception is an image like this, where the dominant light source is monochromatic (in this case, probably the 500.7nm forbidden oxygen emission).
The dominant light source is monochromatic, in this case probably the 500.7nm forbidden oxygen emission?
Well, probably not. In the link below you can see an image by Adam Block of planetary nebula NGC 7662, the Blue Snowball, where the dominant light source is indeed the 500.7 forbidden oxygen emission:
You can see that the color of the nebula is very blue-green, corresponding to a wavelength of 500.7nm. The color of the aurora, on the other hand, is strongly yellow-green, almost pure yellow. I'd say that any dominant light source here must be far closer to 560 than to 501 nm, and to be as yellow as that it will probably have to be mixed with some emission of a wavelength longer than 600 nm.
Ann
The BS nebula
Posted: Thu Oct 07, 2010 1:34 am
by neufer
Ann wrote:
The dominant light source is monochromatic, in this case probably the 500.7nm forbidden oxygen emission? Well, probably not. In the link below you can see an image by Adam Block of planetary nebula NGC 7662, the Blue Snowball, where the dominant light source is indeed the 500.7 forbidden oxygen emission: http://www.caelumobservatory.com/gallery/n7662ab.jpg
You can see that the color of the nebula is very blue-green, corresponding to a wavelength of 500.7nm. The color of the aurora, on the other hand, is strongly yellow-green, almost pure yellow. I'd say that any dominant light source here must be far closer to 560 than to 501 nm, and to be as yellow as that it will probably have to be mixed with some emission of a wavelength longer than 600 nm.
Ann wrote:
The dominant light source is monochromatic, in this case probably the 500.7nm forbidden oxygen emission? Well, probably not. In the link below you can see an image by Adam Block of planetary nebula NGC 7662, the Blue Snowball, where the dominant light source is indeed the 500.7 forbidden oxygen emission: http://www.caelumobservatory.com/gallery/n7662ab.jpg
You can see that the color of the nebula is very blue-green, corresponding to a wavelength of 500.7nm. The color of the aurora, on the other hand, is strongly yellow-green, almost pure yellow. I'd say that any dominant light source here must be far closer to 560 than to 501 nm, and to be as yellow as that it will probably have to be mixed with some emission of a wavelength longer than 600 nm.
First of all, that Hubble picture is a great example of Hubble's terribly colored planetary nebulae images. ::puke:: ::puke::
Second, I would assume that the Hubble image is a narrowband image. Today's APOD clearly is a broadband image. The very idea of comparing the color of a narrowband image with a broadband one is so awful and, well, ignorant that I really thought better of you, Art. Sorry!
Ann
Re: The BS nebula
Posted: Thu Oct 07, 2010 1:57 am
by neufer
Ann wrote:
First of all, that Hubble picture is a great example of Hubble's terribly colored planetary nebulae images. ::puke:: ::puke::
All bright nebulae are light furry Púka and only the WISE can see them as they truly are.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Thu Oct 07, 2010 2:39 am
by Beyond
Yes! The BS Nebula indeed. I'm beginning to see why.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Thu Oct 07, 2010 3:35 am
by Chris Peterson
Ann wrote:The dominant light source is monochromatic, in this case probably the 500.7nm forbidden oxygen emission?
Well, probably not. In the link below you can see an image by Adam Block of planetary nebula NGC 7662, the Blue Snowball, where the dominant light source is indeed the 500.7 forbidden oxygen emission...
You absolutely cannot judge the color of emission lines by comparing different images. The filters used on astronomical cameras are designed to have optimal crossovers between bands to give good data for important emission lines. But they are all different, which is one reason why even well defined spectral lines look different in different images. The situation with color cameras (like the DSLR used for the aurora image) is different: they have filters optimized for imaging of terrestrial, continuum sources, and in-camera processing optimized for terrestrial objects. DSLRs do a terrible job of accurately capturing the color of narrow band, astronomical objects.
You cannot assume that the aurora color as seen in this image comes very close to how the eye would see it, nor that it will come close to the color you see in an astronomical image made through RGB filters.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Thu Oct 07, 2010 6:27 am
by Ann
Chris Peterson wrote:
Ann wrote:The dominant light source is monochromatic, in this case probably the 500.7nm forbidden oxygen emission?
Well, probably not. In the link below you can see an image by Adam Block of planetary nebula NGC 7662, the Blue Snowball, where the dominant light source is indeed the 500.7 forbidden oxygen emission...
You absolutely cannot judge the color of emission lines by comparing different images. The filters used on astronomical cameras are designed to have optimal crossovers between bands to give good data for important emission lines. But they are all different, which is one reason why even well defined spectral lines look different in different images. The situation with color cameras (like the DSLR used for the aurora image) is different: they have filters optimized for imaging of terrestrial, continuum sources, and in-camera processing optimized for terrestrial objects. DSLRs do a terrible job of accurately capturing the color of narrow band, astronomical objects.
You cannot assume that the aurora color as seen in this image comes very close to how the eye would see it, nor that it will come close to the color you see in an astronomical image made through RGB filters.
I still find it very, very improbable that blue-green light would come out as yellow to yellow-green.
I agree with you that color images have to be judged with care. That is why I always try to see as many color images as possible of the same object, or of similar objects. I quite agree that the forbidden oxygen emission at 500.7nm may easily look bright green, blue-green or even bluish in photographs, depending on the color balance of the picture. But can it look yellow-green to almost pure yellow, without messing up the overall color balance of the image in a way that is quite obvious? I strongly doubt it. I have to admit, on the other hand, that the landscape in the image looks remarkably red. So yes, if the overall color balance is much too red, then the aurora may in fact be blue-green, and the dominant emission may in fact be OIII emission.
Ann
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Thu Oct 07, 2010 8:44 am
by Mikkel Lund
Chris Peterson wrote:
bystander wrote:Full size, center frame.
Ah, those. Very nice, you don't often see this in terrestrial images, although it is fairly common in astronomical images. The effect is caused by interference between optical surfaces. In this case, it could be between a filter on front of the lens and the first lens surface, between surfaces inside the lens, or between the first and second surfaces of the sensor cover glass. It is a form of Newton's rings. The reason it doesn't normally show up is because it requires monochromatic light to provide significant contrast. This is common in astronomical images made with narrow band filters, but terrestrial images are always broadband. The rare exception is an image like this, where the dominant light source is monochromatic (in this case, probably the 500.7nm forbidden oxygen emission).
Thank you Chris for the great explanation.
Re: APOD: Aurora Over Alaska (2010 Oct 06)
Posted: Thu Oct 07, 2010 12:28 pm
by zloq
Ann wrote:I still find it very, very improbable that blue-green light would come out as yellow to yellow-green.
Ann
Oxygen has many emission lines, including multiple "forbidden" ones. The ones that appear depend on the temperature and density of the plasma. Nebulae are hot and low density, so they emit a lot of [OIII] at 500.7, which is very green. An aurora is much cooler and denser, and its main O emission is [OI] at 557.7nm - more toward the yellow. So - yes - the colors are different because the plasma is very different.
The [OI] line is also present in nebulae, but it's harder to measure since the same line is emitted by our atmosphere, whereas [OIII] is not. In order to measure [OI] in astronomical objects, they look for fast moving ones that are doppler shifted from the terrestrial emission. See Keenan et al., Forbidden Lines of [OI] in the High-Resolution Optical Spectra of Planetary Nebulae, Pub. Astro. Soc. Pac., 107, 1995.