Starship Asterisk*

Amir wrote: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.

Amir wrote: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 wrote:Do you mean "aperture"?

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

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

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

Fil wrote:...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...

Chris Peterson wrote:

neufer wrote: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.

rany wrote: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 wrote:

neufer wrote: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.

neufer wrote: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).

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

Ethen wrote:Camera would have had to been stationary, right?

Chris Peterson wrote:

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

http://www.webexhibits.org/causesofcolor/4.html wrote:

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

http://en.wikipedia.org/wiki/Aurora_%28astronomy%29 wrote: 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.>>

http://en.wikipedia.org/wiki/Nitrogen wrote:
<<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).>>

Chris Peterson wrote:Ionized nitrogen is very much purple.

neufer wrote: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:

http://www.webexhibits.org/causesofcolor/4.html wrote:

Chris Peterson wrote:

neufer wrote: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.

http://www.webexhibits.org/causesofcolor/4.html wrote:
<<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.>>

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

neufer wrote:Artifact of photography.

DennyMo wrote: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?

http://wvlightning.com/faqw10.shtml wrote:
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. >>