Aurora in the Distance (APOD 19 Nov 2007)

[Chris Peterson]

Undoubtedly, that lens open as it is on F4 is flaring quite badly from the moonlight, you can see the huge flare halo and its numerous flare spikes radiating out from the moons light source. But they are all WHITE - as they should be for a white source. However, this single spike is green and almost identical in hue and intensity to the aurora.

Only very low quality lenses produce lens flare that is the same color as the source. The flare is caused by internal reflections, which are off of antireflection coatings. Such coatings are highly wavelength dependent, and in fact usually reflect shorter wavelengths. That's why camera lenses normally appear green, blue, or violet. This lens appears to have good quality AR coatings, so only a tiny fraction of the moonlight is internally reflected- nowhere near enough to saturate to white.

The fact that the flare is the same color as the aurora can be explained by the fact that both are showing a nearly monochromatic green. Color cameras generally have problems with monochromatic sources, since they often stimulate only one pixel- in this case the green ones. As a result, two different colors of green can't be distinguished from each other.

I do not think this can be explained away as lens flare, and as its orientation is different in a subsequent shot, yet it is still present, indicates that it is real, but as yet unidentified.

You can't tell how the orientation changes, because the pictures have been cropped. But to a good first approximation, the flare points towards the optical axis of the lens (the center of the image) in both cases, just as would be expected.

[iamlucky13]

I agree with Chris. I've seen plenty of green and magenta flares off of my lenses from the coatings.

Lance, another thank you for sharing your pictures and comments. I really like the reflection off the aurora off the ice to the left, giving way to the moon's reflection on the right.

[DerekSmith]

I agree with Chris. I've seen plenty of green and magenta flares off of my lenses from the coatings.

snip..

Would you like to share an example of one of your flares with us that you think demonstrates this effect. And don't forget, the flare needs to point 'away' from the light source with its brightest point furthest from the moon.

The most chromatic artefacts tend to be iris effects, so none of these please if they just happen to be the right colour, we need a ray/cone that points 'away' from the light source.

Derek

[DerekSmith]

Only very low quality lenses produce lens flare that is the same color as the source. The flare is caused by internal reflections, which are off of antireflection coatings.

Chris, the lens flare around the moon is white and red and large, but I doubt that the white flare is because this is a low quality lens.

Have you ever seen or can you post an example image which demonstrates a flare like this. I have searched a large number of flare images and never come across an effect like this.

So I stand by my original statement, I believe it is unlikely that this is a flare effect, but I am keen that someone shows us an example that matches of a known flare effect

[Chris Peterson]

Would you like to share an example of one of your flares with us that you think demonstrates this effect. And don't forget, the flare needs to point 'away' from the light source with its brightest point furthest from the moon.

Getting the same kind of flare means using the same kind of lens. Why not just look through some of Lance's other images? Here are a few: 1, 2, 3 that show similar flaring. The small differences are explainable by considering that the Moon subtends a larger angle in its images than the artificial light sources in the other images.

Of course, very much to the point, there's no reasonable object that could explain the flare in the aurora images. Lens flare is simply much more likely than any sort of atmospheric phenomenon that was stable over a minute or longer.

[Chris Peterson]

Chris, the lens flare around the moon is white and red and large, but I doubt that the white flare is because this is a low quality lens.

That isn't the kind of flare I'm talking about. I'm referring to internal reflections between elements that produce secondary images of bright sources. They are common image artifacts, and other nighttime shots by Lance also show them.

So I stand by my original statement, I believe it is unlikely that this is a flare effect

If not a flare, then what?

[DerekSmith]

Hi Chris,

I had looked at his other images but not with the detail that you have.

Two of the images at least show the same effect.

I'm sold - you are right and I sign on to this being a camera artefact.

Derek

[Lance McVay]

One thing to keep in mind is that this is an extremely wide lens at 12mm rectangular. It's nothing like a telescope lens in any form. I've owned fisheye lenses that weren't this wide. I'm also using it for what was most likely not its primary designed purpose, which is a lot of night time shooting with bright light sources included.

Here is an image I took last night at 3200 ISO, absolutely untouched out of the camera. I haven't even leveled the horizon. I don't think anybody is going to be arguing over whether this is a celestial body or an artifact.

[/img]

[iampete]

If it weren't for the star being inside the circular shape, I'd swear it had to be a real object rather than an artifact.

However, I'm curious - what kind of artifact is it, i.e., what is the cause? To the best of my understanding, lens flares tend to exhibit "rays" and/or elongations toward/away from major light sources, and aperture-type flares don't produce specific "objects", per se. Is it dirt on the lens, or on the film, or ???

[Lance McVay]

I believe it is a reflection of the moon, colored by the lens coating. If you look at the rocks at the bottom of the scene, you will notice they are lit by a source to the left, which was the moon. The artifact shows the same phase as the moon was that night.

[Mushhushshu13]

You know, the camera that took the shot was left to accumulate light for a while. The thing you're talking about is probly the result of the over exposure. If I leave my camera to absorb light for two secs, the picture gts all blurry and weird.

PS: WHat exactly is that objct anyhow?? I cant find it! FRUSTRATION![/img]

[NoelC]

It's lens flare, plain and simple.

Lenses are comprised of a whole slew of elements, most with curved surfaces, and as such secondary reflections of bright objects can be shaped strangely. They are usually oriented (pointed, angled, whatever you want to call it) toward or away from the light source, as Lance's green flare is.

Anyone who's taken flash photos into a mirror knows this.

It's a beautiful, beautiful photo, Lance. Don't let the nay-sayers get you down.

By the way, I've recently upgraded from a 20D to a 40D and the prowess of the new model in capturing long nighttime exposures is unprecedented. While by no means a beautiful shot, consider this 40D long exposure night image: http://forum.ourdarkskies.com/gallery_i ... _85842.jpg

-Noel

[iamlucky13]

Very nice picture NoelC. I couldn't see any noise at the size of the image you linked to. Looks like the new Canon sensor is quality instrument. The lighting is very attractive, although it's a pity it washes out most of the stars.

[NoelC]

Yes, that's the problem here. The light pollution is horrendous!

In 2005 a hurricane (Wilma) killed most of the electric power infrastructure here for about a week. The views of the sky were to die for. Sigh.

-Noel