Starship Asterisk*

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.

APOD Robot wrote:
....... If viewed from space, auroras can be seen to glow in X-ray and ultraviolet light as well.

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?

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.

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.

bystander wrote:Full size, center frame.

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.

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).

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.

neufer 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: 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.

Continuous optical spectrum rendered into the sRGB color space.

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HST image of NGC 7662 planetary nebula 2.5°west-southwest of the star Iota Andromedae. It appears blue-green and has a very faint central star that is variable with a magnitude range of 12 to 16. http://www.daviddarling.info/encyclopedia/B/Blue_Snowball_Nebula.html

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Ann wrote:
First of all, that Hubble picture is a great example of Hubble's terribly colored planetary nebulae images. ::puke:: ::puke::

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...

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.

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).

Ann wrote:I still find it very, very improbable that blue-green light would come out as yellow to yellow-green.

Ann