Aurora Question?

[lynngrubbs]

I just checked out the pictures from the Aurora Gallery for the Nov 7 Aurora displays caught by other's with their camera's. The very last photo in the group shot by<br />Abe Megahed of the sky in Wisconsin has a very strange thing right in the middle of the photo, it like a bunch of circles one after another, sort of like a target effect, in the sky? Does anyone know what cause's this effect, its pretty cool looking and looks to be part of the sky/aurora...but what is it? I'm going to post the link to the picture but its a long one.....<br /><br />Here goes:<br /><br />http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=ig228_14_02.jpg&cap=Abe%20Megahed%20took%20this%20vivid%20photograph%20of%20an%20aurora%20over%20the%20city%20of%20Madison%2C%20Wisconsin%2C%20on%20Nov%208.%20This%20amazing%20aurora%20display%20was%20so%20bright%20that%20it%20was%20beautifully%20visible%20a%20mile%20from%20downtown%20Madison.%20Colors%20visible%20in%20the%20display%20were%20green%2C%20yellow%2C%20red%2C%20orange%2C%20and%20even%20violet%21%20Click%20to%20enlarge.

 

[spacechump]

It looks like a diffraction pattern formed from the lens.

 

[igorsboss]

The shape appeared to me to have 9 sides to it, not a precise circle. Since a camera's iris approximates a perfect circle with several overlapping black metal leaves, I suspect the iris was involved in this effect.<br /><br />I'd say the effect was from diffraction from the camera's iris. The lens was probably stopped down significantly.<br /><br />Also note that the image has a large depth-of-field. That is, both close and distant things (trees and city) are both in focus. This effect is most commonly accomplished by "stopping down" (making the iris small), and decreasing the shutter speed.<br /><br />The iris diffraction pattern is results from the small iris blocking out some of the light, causing the near-circular pattern.

 

[alkalin]

I agree. When working with coherent light, I have frequently observed rings such as this being produced between the lens surfaces. There is probably enough bright green coherence in this shot to do it.

 

[lynngrubbs]

Oh, ok...I'm not familar with effects of the Aurora, but I thought this might be some kind of effect that was created by the Aurora lights instead of a camera lens/iris. Are any of you familar with Aurora, or is this just shooting in the dark for an answer?

 

[alkalin]

Hmm.........I have witnessed a few ‘northern lights’ as we called them many years ago in the northern mid-west when I used to live there. I remember how beautiful they could be, and sometimes how rapidly they could change and appear to move and writhe like a serpent. <br /> <br />But I had not seen anything even close to what appears to me to be a lens artifact in that image.<br /><br />If you are not familiar with lens testing...it is by interferometry that utilizes ‘fringing’ effects that are very typical of what you see in the image. The fringes have what we would call power because the two lens surfaces making them are slightly different in curvature. <br /><br />The necessary ingredients for an interferometer: A certain amount of optical light path, a coherent light source, and two fairly closely matching optical surfaces.<br />