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Aurora photography and (not) Newtons Rings

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#1 sharkmelley

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Posted 19 May 2024 - 02:24 AM

For many years I've been intrigued by the phenomenon of rings appearing in the centre of images of the Aurora. 

Here's an early example I came across:

https://www.dpreview...s/post/61829327

 

It's caused by having a filter in front of the lens so remove your filter for aurora photography.

 

After the recent powerful aurora, there were many people on forums asking about the rings in their image.  Although the answers generally recognised that the filter triggers the problem, they also described the cause as "Newtons Rings".  But is this right?

 

An example of the argument for Newtons Rings is this webpage:

https://capturenorth...e-newtons-rings

 

This explanation makes no sense to me because if the interference fringes are generated between the plane face of the filter and the curved face of the front lens element then there is no possible way they can appear in the image because they will be destroyed as they pass through the optics.

 

Instead, I wondered if they could be generated within the filter by light bouncing between the plane faces of the filter and interfering either constructively or destructively, which is Fabry-Perot interference:

http://hyperphysics....yopt/fabry.html

 

I found support for this idea here:

 

https://www.alaskaph...urora-borealis/

 

What causes the rings? Charles Deehr, a professor emeritus in physics at the University of Alaska Geophysical Institute, says:

 

“These are interference fringes due to the parallel faces of the filter and to the narrow spectral emission at 5577 Angstroms in the aurora. That green, atomic oxygen emission line is the strongest emission in the aurora near our film and eye peak sensitivity, so it shows up first when there is any device in the optical path which sorts out the spectral emissions.

 

I also found these exact comments in an archived webpage from 2004.  

 

To investigate further, I decided to generate a "fake aurora" with rings at home. Putting a lens and Hoya skylight filter on my Canon 600D, I photographed a tiled fireplace wall illuminated only with the scattered light from light-painting the ceiling with a laser pointer.  This was sufficient to trigger the rings:

 

AuroraExperiment.jpg

 

Here's an enhanced version to make the rings more obvious:

 

AuroraExperimentEnhanced.jpg

 

If the Newton's Rings explanation is correct then the ring spacing will depend on the curvature of the front element of the lens.  However if the Fabry-Perot explanation is correct then the ring formation depends only on the angle of incidence of light at the filter, so the spacing will remain constant relative to other features in the image.  Changing the lens will simply change the size of the image on the sensor. 

 

So I put the same Hoya skylight filter on 4 different lenses, imaging the same fireplace from the the same position, illuminated by scattered light from a laser pointer. Each shot was 30sec at ISO1600 using my Canon 600D with each lens at f/5.6. I've cropped a similar region from each image (slightly bigger than the width of one fireplace tile), rescaled them and placed them side by side:

 

AuroraExperimentResults.jpg

 

The ring spacing relative to the background tile is the same in each case, even though the curvatures of the front element of the lens are very different.  So clearly the Newton's Ring explanation is incorrect and Professor Charles Deehr had it right!  Since the correct explanation has been around for a long time, I wonder where the myth of Newton's Rings came from?

 

Mark


Edited by sharkmelley, 19 May 2024 - 03:17 AM.

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#2 piaras

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Posted 19 May 2024 - 05:45 AM

Looking at the photos without reading the article, I was thinking of when I use an optical flat under monochromatic green light. 
Pierre



#3 BQ Octantis

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Posted 19 May 2024 - 05:57 AM

Hi Mark,

 

Fun analysis! It seems pretty conclusive that they're not Newton's Rings.

 

As to where the idea came from, just looking at the examples of Newton's rings on the wiki, the similarity is striking. And we humans are lazy time-efficient creatures, so we take shortcuts using similarity. We even have the legal premise of prima facie to save time: "prima facie evidence presented at the beginning of [a] trial does not have to be conclusive or irrefutable." In a layperson's experience, hypothesis is transmuted into general acceptance by Occam's razor—and then cemented into irrefutable fact through culture (or even religion). Apparently, this has some sort of evolutionary advantage…

 

BQ

 

P.S. We had an analogous discussion over in the Beginner's forum on Fresnel vs. Fraunhofer diffraction patterns and the Airy disk. What people think is the Airy pattern is something different entirely…

 

https://www.cloudyni...many-questions/


Edited by BQ Octantis, 19 May 2024 - 06:01 AM.


#4 KLWalsh

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Posted 19 May 2024 - 08:13 AM

“This explanation makes no sense to me because if the interference fringes are generated between the plane face of the filter and the curved face of the front lens element then there is no possible way they can appear in the image because they will be destroyed as they pass through the optics.”

While overall I think you may be correct in analysing the cause of the rings, I don’t see why you think the rings would be destroyed in passing through the optics.
I have seen Newton’s Rings with my own eyes, and I have photographed them. If Newton’s Rings can survive the imperfect optics of my eyes and the optics of a camera lens in one instance, why would the rings not survive the optics of a camera lens in this instance?

#5 sharkmelley

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Posted 19 May 2024 - 08:56 AM

While overall I think you may be correct in analysing the cause of the rings, I don’t see why you think the rings would be destroyed in passing through the optics.
I have seen Newton’s Rings with my own eyes, and I have photographed them. If Newton’s Rings can survive the imperfect optics of my eyes and the optics of a camera lens in one instance, why would the rings not survive the optics of a camera lens in this instance?

If you can see Newton's Rings in an object then of course they can be photographed.  No-one is claiming otherwise. But that's not what we are discussing here.  Instead, we are talking about the claim that rings are formed between the filter and the front of the lens.  Clearly these rings are far too close to the lens to be photographed.

 

Another way of thinking about it is to consider the bundle of rays that come to focus at a single point in the image.  Each ray in this bundle will suffer a different amount of constructive/destructive interference because they have passed through different size gaps between the filter and the curved surface of the lens.  This destroys the ring pattern.  On the other hand, if the interference happens within the filter then every ray in the bundle will have the same amount of constructive/destructive interference because they have the same angle of incidence at the filter.  This is what causes the rings in the image.

 

Mark



#6 timelapser

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Posted 19 May 2024 - 12:30 PM

Restating your argument, Mark, for Newton's rings between filter and frontmost lens surface, rays of light from the same incident direction in the sky would undergo different interference depending on the point of incidence of the ray on the filter, and hence on the gap.  But each of those rays should be focused to the same point on the sensor - that's the job of the lens, when focused at infinity.  So the interference is completely smeared - one pixel sees the sum of all bright and dark fringes.

 

But it's not entirely clear what happens with rays with different incident directions, which focus on different pixels.  They're also smeared due to different incident positions/gaps, but there may be some residual phase shift between the two incident directions due to different phase shifts near the centre.

 

Regardless, Newton's rings won't be visible, because they're seen when the gap between the curved and flat surface is small, ie, comparable to the wavelength of the light.  Normally the gap between filter and lens would be huge compared to the wavelength.  This means the interference pattern would be extremely sensitive to the incidence direction, since the phase shift between the two paths would be similarly sensitive.  In practice this means you could never see the interference even without the effect of the imaging lens smearing the pattern, since you're always summing rays over some (small) range of incident directions.

 

I guess the "myth" is just sloppy conclusions based on vague resemblance of the pattern without actually following the rays.

 

Nevertheless it is an interesting effect that I wasn't aware of (I don't use filters normally).  The first example you posted is high contrast/visibility.  That makes me wonder whether the FP effect can be seen with white light, at reduced contrast of course.  Interference can be seen with white light via Newton's rings or oil on water.



#7 KLWalsh

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Posted 19 May 2024 - 07:19 PM

Uhhh… Still not entirely convinced.
The auroral light is essentially at infinity, not ‘up close’ as the light source in an optics lab. So the separation between the filter and the lens is irrelevant wrt light undergoing constructive and destructive interference. Plus, interference is afocal. Eg., one sees a speckle pattern in a laser beam regardless of whether the speckle goes through a lens or not.
I’m still not arguing for or against it being Newton’s Rings. I’m just saying your explanation is unconvincing. The fact that the interference pattern is created at point A doesn’t mean it won’t show up at point B. In the famous ‘Double Slit’ experiment, the interference pattern exists at any distance behind the slits.

#8 timelapser

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Posted 19 May 2024 - 07:56 PM

So the separation between the filter and the lens is irrelevant wrt light undergoing constructive and destructive interference

I don't follow you there.

 

If anyone's unsure I challenge you to see Newton's rings by any means when the flat and curved surface are separated by centimetres or even millimetres instead of just touching.



#9 sharkmelley

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Posted 22 May 2024 - 12:36 AM

I did some further work on this, measuring the diameters in pixels of successive rings in the Canon 18-55mm image (shot at 40mm):

430, 562,686,789,870,960,1038,1101

The Canon 600D has a pixel pitch of 4.3 microns.

 

Assuming the refractive index of the filter glass is 1.5 then a filter thickness of 1.9mm is consistent with the observed set of rings.  Unfortunately, I don't have an accurate way of measuring the filter thickness but I reached an approximate measurement of 1.5-2.0mm.



#10 sharkmelley

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Posted 22 May 2024 - 05:01 AM

Here's a couple of interesting threads regarding Newton's Rings:

Newton's Rings are actually visible when looking at the optics of some refactors because two glass surfaces with slightly different curvatures are in close proximity.  In fact, the effect is sometimes deliberately used to aid collimation. But there's no suggestion that the resulting astro-images suffer from rings.  This supports the argument that Newton's Rings do not "pass through" the optics to degrade images.



#11 Steve OK

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Posted 27 May 2024 - 10:27 AM

I had the rings show up on lots of my aurora photos.  Quite a surprise and a disappointment.  I did leave the skylight filter on the lens I used.  I assumed the rings were so prominent because the light from the aurora of a given color (pink or green) is essentially monochromatic, being generated by a single line of an emission spectrum.  It was beyond me to figure out which set of optical surfaces were responsible, so I appreciate this discussion.  Here's a close crop of the center of one of my images: 

Rings-CN.jpg

 

Steve



#12 otoien

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Posted 27 May 2024 - 03:58 PM

I hit upon this phenomenon back in 2006 when using my D200 with a Nikon 28mm f/2.8 AIS and a Nikon NC filter on the front. At the time I found Charles Deehr's explanation on the matter (whole frame and 100% crop attached). Living in Fairbanks I have since then captured a lot of  aurora, but consequently stopped using filters for aurora captures and have not seen it since then.

 

2006-11-05-0110D-6312-md.jpg

 

2006-11-05-0110D-6312-crop.jpg


Edited by otoien, 27 May 2024 - 03:59 PM.

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