APOD: Ash and Lightning Above an Icelandic... (2010 Apr 19)

[Roundup]

Gravity waves in Ash Cloud!!!
http://rapidfire.sci.gsfc.nasa.gov/real ... 0.250m.jpg

[neufer]

The question everyone I've shown the picture to is "Why is the lightning purple?" Any good answers to that one for me? Artifact of photography, spectral emissions of the lightning affected by the composition of the ash, other?

Artifact of photography.

Lightning photographs: The hue of lightning channels in photographs is usually a function of the type of film, camera, exposure and/or recording media used. The same lightning channel can appear blue, purple, red or orange depending on the type of film, length of exposure, and other factors. Slide film is more likely to produce a more purple/blue image, while print film tends to give lightning a more yellow/orange tint. >>

[Chris Peterson]

Artifact of photography.

Your reference to different emulsions producing different colors isn't very relevant to a digital image like this. In fact, lightning ionizes nitrogen, which results in the purple color seen here. The main lightning path is saturated, making it appear nearly white. Less bright paths show as a combination of the blue-white thermal continuum and the purple output of ionized nitrogen. The color of lightning can be further modified by scattering processes, depending on the amount of dust, rain, or hail in the air. This lightning appears to be in fairly clear air.

[owlice]

More!!



http://www.swisseduc.ch/stromboli/perm/ ... html?id=14

So very wicked cool!!!

[biddie67]

Thanks for the above link - great photos!!!!

[biddie67]

Gravity waves in Ash Cloud!!!
http://rapidfire.sci.gsfc.nasa.gov/real ... 0.250m.jpg

Is this a 3D picture?

[neufer]

Artifact of photography.

Your reference to different emulsions producing different colors isn't very relevant to a digital image like this. In fact, lightning ionizes nitrogen, which results in the purple color seen here. The main lightning path is saturated, making it appear nearly white. Less bright paths show as a combination of the blue-white thermal continuum and the purple output of ionized nitrogen. The color of lightning can be further modified by scattering processes, depending on the amount of dust, rain, or hail in the air. This lightning appears to be in fairly clear air.

It wasn't clear to me that it was originally a digital photo.

In any event, this link claims that the nitrogen factor generates a distinctive blue-white color:

<<Lightning is visible as a flash of light because of both incandescence (due to its high temperature it glows blue-white) and luminescence (excitation of nitrogen gas in the atmosphere). Nitrogen, the dominant gas in the atmosphere, is excited by this strong flow of energy, its electrons moving to higher energy states. The distinctive blue-white color of lightning is caused by light emitted as the electrons drop back to their original energy states.>>

[Chris Peterson]

It wasn't clear to me that it was originally a digital photo.

Well, aside from the fact that this is 2010, nobody uses film anymore, and if they did there are only a couple of color emulsions even available... you could always look at the EXIF header, which identifies the camera as a Fuji FinePix S3 Pro.

In any event, this link claims that the nitrogen factor generates a distinctive blue-white color:

It's wrong. The blue white color is from the blackbody continuum. Ionized nitrogen is very much purple.

[owlice]

: has the urge to make popcorn :

[DennyMo]

Ionized nitrogen is very much purple.

Thanks. I hoped it was somthing being ionized, but I didn't know which what it was.

[Ethen]

Don't know about purple lightning. What amazes me is the lateral red hot ejecta above and below the lightning dendrite nearest the ash cloud. It appears to have a time lapse photo signature that is shorter than the star tracks. How could you photograph star tracks from a moving plane anyway? Camera would have had to been stationary, right?

[neufer]

In any event, this link claims that the nitrogen factor generates a distinctive blue-white color:

It's wrong. The blue white color is from the blackbody continuum. Ionized nitrogen is very much purple.

That is a vial of ultrapure nitrogen with the purple's red component
presumably coming from nitrogen returning to it's ground state.

Lightning excited air at one atmosphere might (or might not) be a different situation\
(; it certainly isn't as clear cut to me as you make it out).

Auroral colors:

<<nitrogen emissions: Blue or red.
Blue if the atom regains an electron after it has been ionized.
Red if returning to ground state from an excited state.

oxygen emissions: Green or brownish-red, depending on the amount of energy absorbed.>>

<<Nitrogen also makes a contribution to visible air glow from the Earth's upper atmosphere, through electron impact excitation followed by emission. This visible blue air glow (seen in the polar aurora and in the re-entry glow of returning spacecraft) typically results not from molecular nitrogen, but rather from free nitrogen atoms combining with oxygen to form nitric oxide (NO).>>

[Amir]

What amazes me is the lateral red hot ejecta above and below the lightning dendrite nearest the ash cloud.

is this what you are referring to?

Ash and Lightning Above an Icelandic Volcano.jpg (34.89 KiB) Viewed 8411 times

if so, i think it's a star trail too; the ash came in front of it over time. it is parallel with other trails.

Camera would have had to been stationary, right?

of course he used a tripod. if he didn't, the lightening wouldn't be that sharp & there wasn't any star trail too.
---------
wow look at that post! it has been edited THREE times!well i just had to clean up some mess.

[wonderboy]

I would imagine that its because the dust and other gumph surrounding the volcano is being superheated by the charge from the lightning. So whatever the lightning is hitting on its way to earth is making the light appear purple.

Just a guess, I hope its right.

[Chris Peterson]

Lightning excited air at one atmosphere might (or might not) be a different situation\
(; it certainly isn't as clear cut to me as you make it out).

Of course, the situation in a gas mixture is more complex. Nevertheless, the purple in the image certainly is coming primarily from ionized nitrogen. This color is very commonly seen in lightning, as well as auroras. As it happens, there are no other atmospheric constituents other than nitrogen that can produce this color under lightning or aurora conditions. It certainly is not a photographic artifact.

[wonderboy]

Lightning excited air at one atmosphere might (or might not) be a different situation\
(; it certainly isn't as clear cut to me as you make it out).

Of course, the situation in a gas mixture is more complex. Nevertheless, the purple in the image certainly is coming primarily from ionized nitrogen. This color is very commonly seen in lightning, as well as auroras. As it happens, there are no other atmospheric constituents other than nitrogen that can produce this color under lightning or aurora conditions. It certainly is not a photographic artifact.

I'm with chris........ He gave me money....

JK, makes sense to me! I accidentally posted my post without knowing there was a page 2, it sounded ok so thats why i left it.



Paul

[rany]

How do you capture such a pic. when it has only mili seconds to perform ? What do you use for camera equipment, timers , sensors ??? How do you get these shots ?

[Chris Peterson]

How do you capture such a pic. when it has only mili seconds to perform ? What do you use for camera equipment, timers , sensors ??? How do you get these shots ?

It's not difficult. You don't need anything special, just a camera capable of fairly long exposures. A DSLR or other digital camera with manual mode control works well. You aim the camera wherever you are hoping for lightning, and open the shutter. How long you leave it open depends on the background brightness. The darker the background, the longer you can open the shutter. And then you just keep doing this, over and over. With a little luck, you eventually catch some nice events.

It doesn't hurt to have an erupting volcano in your backyard, of course!

[Fil]

Artifact of photography.

Your reference to different emulsions producing different colors isn't very relevant to a digital image like this. In fact, lightning ionizes nitrogen, which results in the purple color seen here. The main lightning path is saturated, making it appear nearly white. Less bright paths show as a combination of the blue-white thermal continuum and the purple output of ionized nitrogen. The color of lightning can be further modified by scattering processes, depending on the amount of dust, rain, or hail in the air. This lightning appears to be in fairly clear air.

Another "scattering" process can be the objective lens used on the camera (some chromatic aberration). This is very common, as lenses tend not to be super corrected everywhere on the spectrum. But the rest is true as well.
The idea to take lightning pictures of these subjects during the day is to close the photographic aperture quite enough, to make exposures last longer, cause lightning not to saturate too much (not to appear too thick) and to get as much lightnings as we can in the longer exposure.. Some pictures of this volcano also have star trails in them, that illustrates how long the exposure can get!

[Amir]

Another "scattering" process can be the objective lens used on the camera (some chromatic aberration).

is "chromatic aberration" a problem is such a short focal length (Cameras)?

...The idea to take lightning pictures of these subjects during the day is to close the photographic aperture quite enough, to make exposures last longer, cause lightning not to saturate too much (not to appear too thick) and to get as much lightnings as we can in the longer exposure...

what Diaphragm is usually used to capture a sharp and not so thick Lightening picture?

[WildGuruLarry]

is "chromatic aberration" a problem is such a short focal length (Cameras)?

Chromatic aberration tends to be more pronounced on short focal length (wide angle) lenses, due to the fact that it is difficult to make a lens that can accept light from a wide set of angles without perfectly counteracting refraction problems.

what Diaphragm is usually used to capture a sharp and not so thick Lightening picture?

Do you mean "aperture"? For most lenses you want to try to use an aperture close to the middle of the range. If it is open too much, you can get artifacts from imperfect construction or design of the lens. If it is closed too much, you can get diffraction artifacts. The advice I usually use is "f/8 and be there". Especially for landscape shots where there is not much in the foreground. For this shot, it would probably look identical if shot from f/4 to f/16 (as long as you compensated properly with the shutter speed).

[Amir]

Do you mean "aperture"?

aperture is the diameter of the Lens/Mirror & could not be referred as f/x i guess.
the one you mentioned (f/8) is focal ratio (aperture/focal length). which i've heard the term "Diaphragm" (=a device for varying the effective aperture of the in a camera or other optical system) instead of focal ratio when talking about Cameras.

thanks for the hint, i'm gonna try it some day. would f/8 be still appropriate when you're waiting for lightening with open shutter (like this APOD Picture)?

[WildGuruLarry]

aperture is the diameter of the Lens/Mirror & could not be referred as f/x i guess.
the one you mentioned (f/8) is focal ratio (aperture/focal length). which i've heard the term "Diaphragm" (=a device for varying the effective aperture of the in a camera or other optical system) instead of focal ratio when talking about Cameras.

A "diaphragm" is a device for controlling aperture, but nobody really talks about that directly, unless they are discussing how many blades it has. The aperture is the diameter of the opening in the diaphragm. "f/8" would be the focal ratio like you said, but it is also the aperture. For example, on a 50mm lens, if you set a focal ratio of f/8, then the aperture will be equal to f/8, or 50mm/8, or 6.25mm. So most people would say "shoot with an aperture of f/8", rather than "shoot with a diaphragm of f/8". At least, in my experience in North America.

thanks for the hint, i'm gonna try it some day. would f/8 be still appropriate when you're waiting for lightening with open shutter (like this APOD Picture)?

It really depends on a lot of factors, but that's where I would start (probably with ISO 100 and exposure time of 30s), and then I would adjust accordingly after a few test shots to make sure the background is being exposed properly.
I wish the EXIF data had not been stripped from this photo (and all of his others as well, as far as I can find). It is a very useful instructional tool for us budding photographers.

[Amir]

thanks for the explanation. i guess your sentence (At least, in my experience in North America.) reminded me the fact that i'd never speak English like an American. yeah, you're definitely right. i shouldn't have discussed something i've never used or heard in english. in fact in my language we say Diaphragm. that's why i thought everybody should use the same term!!