[DagT]

Pentax K5IIS - Sigma 180mm f/3.5 APO EX ~ ISO 80 F/32 + 82mm extension tubes, Godox AD180 flash used at 1/2 power. Image contrast has been enhanced.

yes they do require physical contact. I sandwiched a multi-coated Hoya +3 diopter macro lens with a a Hoya UV[0] filter to create the Newton rings in the above image. And I have to say I'm wrong about it being contact between the front element and the filter, but having said that: I don't think it is Fabry-Perot interference either. Because it requires so many environmental conditions to comply for it to occur.

The principal reason that the interference would have to propagate throughout the entire optical path for it to be visible at the sensor plane - if this were the case it would be observable in live view or by looking through the viewfinder. The interference between the first two elements it wouldn't even be visible. It would have to occur near the rear element of the lens. And as it transpires there are two elements that are made of different materials that I suspect that could also be a source of the interference pattern


The point here is how to prevent the interference - and almost universally the advice is to remove the filter.

And, as yoou can see, it is visible in white light. The F-P occures only under certain environmental conditions, such as focussing at infinity, flat filter and monochromatic light.

---------- Post added 02-12-16 at 08:40 AM ----------

Furthermore I have attempted to generate the Fabry-Perot interference using flash with a green gel that had transmission resembling that of the aurora. I stacked a pair of thin framed Hoya multi-coated skylight filters* on my Sigma 18-35mm f/1.8 ART the filters were perfectly parallel combined with a narrow spectrum of light, I had Ideal conditions for Fabry-Perot interference to appear - However, I was unable to provoke the effect:



Which brings to mind green screen photography which uses monochromatic light to aid in the digital removal of backgrounds. Personally I use 550nm Dichroic filters for this as the purity of light they produce makes removal of the background very easy. Curiously, even under these conditions the Fabry-Perot interference patterns do not appear.

Sorry, but you need a clean emmision spectrum to obtain sufficiently monochromatic light. A colour filter is very far away from that.

[GUB]

Sorry, but you need a clean emmision spectrum to obtain sufficiently monochromatic light. A colour filter is very far away from that.

Would the green laser do it?

[Digitalis]

Would the green laser do it?

Possibly, but I would be wary of aiming a laser at a camera sensor, lasers have a history of wrecking them. Also DagT stipulates that the frequency of light has to be exactly 557.7nm for this effect to occur.

Sorry, but you need a clean emmision(sic) spectrum to obtain sufficiently monochromatic light.

And you're suggesting the aurora that has to travel through several hundred miles of mixed gasses and atmospheric scattering, deflection, reflection and refraction arrives perfectly collimated and monochromatic at sea level? There is also the issue that there are multiple gasses that are excited during the aurora, in one single display the frequencies of light can shift quite dramatically.

And, as yoou(sic) can see, it is visible in white light. The F-P occures(sic) only under certain environmental conditions, such as focussing(sic) at infinity, flat filter and monochromatic light.

I never said newton rings aren't visible in normal light. And at 28mm f/2.8 Focus distance would have a minimal effect on the pattern if it is being generated through the mechanism you are suggesting. However the phase, specific spectrum and degree of collimation and specific spacing of the filter required for F-P interference occur in a natural setting are a bit far fetched.

[GUB]

I never said newton rings aren't visible in normal light.

But the effect is only showing up in a monochromatic situation. Your newton ring setup would show in a blue sky.

---------- Post added 02-12-16 at 09:11 PM ----------

And at 28mm f/2.8 Focus distance would have a minimal effect on the pattern if it is being generated through the mechanism you are suggesting

I think it would go in and out of focus just like an object at the Aurora

---------- Post added 02-12-16 at 09:26 PM ----------

Also DagT stipulates that the frequency of light has to be exactly 557.7nm for this effect to occur.

Where did he say that?

---------- Post added 02-12-16 at 09:36 PM ----------

The way I see it the prerequisites for this effect is ;
An area (not just a point) of monochromatic light far enough away for the emanating rays to be in practice parallel.
A filter of high enough precision to act as a interferometer.
(The area has to be large enough to project the rings on.)

[Digitalis]

A filter of high enough precision to act as a interferometer.

You need two planar filters for the F-B lines to appear:



The problem is that the front element of the Tamron SP AF 17-50mm f/2.8 (IF) is distinctly curved, this would alter the appearance of the interference: the lines would get broader as the distance between the filter and the lens increases. And none of the elements in the lens are the same diameter.

Where did he say that?

you need a clean emmision(sic) spectrum to obtain sufficiently monochromatic light

To conclusively prove that the aurora ( which is what DagT is saying) is what is causing the effect - we have to use a light source that is as close to it as possible. So 557.7nm light is what is needed.

i'm going to have to book some time at the Canberra synchrotron.

[DagT]

You need two planar filters for the F-B lines to appear:



The problem is that the front element of the Tamron SP AF 17-50mm f/2.8 (IF) is distinctly curved, this would alter the appearance of the interference: the lines would get broader as the distance between the filter and the lens increases.

But the two plane surfaces are not between the filter and the front lens, it is inside the filter, between the front and back surface of the filter. They are both plane.

Just to add: The reason why you usually use two glass objects with a spacer when you make an F-P is that you want to adjust it, e.g. for colour separation in a certain direction. You will get the same effect from one glass element with reflections on both sides (which you will always have to some degree) but you will loose the possibility for adjusting the filtering.

---------- Post added 02-12-16 at 10:08 AM ----------

Would the green laser do it?

Yes, I used diode lasers of different types, and some superluminence diodes. Actually, when we encountered it the first time I was surprised that the Aurora was sufficiently monochromatic.

---------- Post added 02-12-16 at 10:12 AM ----------

Possibly, but I would be wary of aiming a laser at a camera sensor, lasers have a history of wrecking them. Also DagT stipulates that the frequency of light has to be exactly 557.7nm for this effect to occur.



And you're suggesting the aurora that has to travel through several hundred miles of mixed gasses and atmospheric scattering, deflection, reflection and refraction arrives perfectly collimated and monochromatic at sea level? There is also the issue that there are multiple gasses that are excited during the aurora, in one single display the frequencies of light can shift quite dramatically.



I never said newton rings aren't visible in normal light. And at 28mm f/2.8 Focus distance would have a minimal effect on the pattern if it is being generated through the mechanism you are suggesting. However the phase, specific spectrum and degree of collimation and specific spacing of the filter required for F-P interference occur in a natural setting are a bit fetched.

Think about it: mixed gases and scattering will NOT alter the wavelength. It will selectively absorb or scatter the existing light.

The spacing in the F-P will only affect the size of the rings at a certain wavelength. It will not remove the effect. Had the Aurora been red the rings would still be there as long as the light was sufficiently monochromatic.

[Digitalis]

But the two plane surfaces are not between the filter and the front lens, it is inside the filter, between the front and back surface of the filter. They are both plane.

If that is the case : then why isn't the pattern consistent across the frame? Also, theoretically the filter isn't even needed to make the patterns, there are plenty of lens elements in the Tamron SP AF 17-50mm f/2.8 (IF) that would have sufficient thickness to produce the interference pattern.

Think about it: mixed gases and scattering will NOT alter the wavelength. It will selectively absorb or scatter the existing light.

Filtering it out - I get it.

I was surprised that the Aurora was sufficiently monochromatic.

I would be surprised as well.

---

Last edited by Digitalis; 02-12-2016 at 02:26 AM.

[DagT]

Then theoretically the filter isn't even needed to make the patterns, there are plenty of lens elements in the Tamron SP AF 17-50mm f/2.8 (IF) that would have sufficient thickness to produce the interference pattern.

No, as I said before, after being refracted at the first lens surface they rays from infinity will no longer be parallel. It could happen, in some very rare occations as there are a number of things happening between the elements, but then I would agree with you in that it was unlikely.

---------- Post added 02-12-16 at 10:29 AM ----------

If that is the case : then why isn't the pattern consistent across the frame?

This is explained in the first links I referred to. It occures aliong the optical axis and is then focussed by the lens toward the sensor.
http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/fabry.html

If you look at a laser through the F-P and turn the filter (around an axis perpendicular to the optical axis) you will see that the transmitted intensity varies with the angle. What's happening here is that the lens will focus this variation onto the sensor.

[Digitalis]

It occures(sic) aliong(sic) the optical axis and is then focussed(sic) by the lens toward the sensor.

So if you placed the aurora off center the interference would be diminished?

[DagT]

So if you placed the aurora off center the interference would be diminished?

Yes, allowing for some inaccuracies in the optics.

Edit: it would also depend on the filter thickness. A thin filter would make larger rings covering as larger part of the sensor. Also, of course, depend on the focal length of the lens how far from the center the pattern can be seen.

[BigMackCam]

Whilst this is way out of my league scientifically, it has been a fascinating thread!

[GUB]

Whilst this is way out of my league scientifically, it has been a fascinating thread!

I am relegated back to being a farmer now out picking up hay in the dark.(smoko break) But hey it has been fun.

I think we can label this thread as "solved now" which is more than I can say about the other forums on this fascinating phenomenon.

[DagT]

I am relegated back to being a farmer now out picking up hay in the dark.(smoko break) But hey it has been fun.

I think we can label this thread as "solved now" which is more than I can say about the other forums on this fascinating phenomenon.

I hope so, but I also have to say that without the opposition the explanation probably would not have so good. I don't know if Digitalis agrees (yet ) , but I appreciate his contribution.

[BrianR]

Thanks for your contribution here Dag!

The commonly given Newtons Rings explanation never made sense to me. This is a widespread enough phenomena with aurora photos that I could never quite imagine how so many lens and filter combinations put the glass so close that the teeny amount of contraction you'd see in a bit of cold would be enough to make them touch*, or that it would end up in focus on the sensor/film.

There's a nice example here with the rings going to the edge of the frame: Strange Concentric Circles in Northern Lights Photo - Photo.net photo.net Forum

Is it wrong that I plan to try to cause this Fabry-Perot interference with the next aurora here?


*It can happen at room temperature too, but according to LensRentals.com - Bad Times with Bad Filters it takes just the wrong combination.

[DagT]

Thanks for your contribution here Dag!

The commonly given Newtons Rings explanation never made sense to me. This is a widespread enough phenomena with aurora photos that I could never quite imagine how so many lens and filter combinations put the glass so close that the teeny amount of contraction you'd see in a bit of cold would be enough to make them touch*, or that it would end up in focus on the sensor/film.

There's a nice example here with the rings going to the edge of the frame: Strange Concentric Circles in Northern Lights Photo - Photo.net photo.net Forum

Is it wrong that I plan to try to cause this Fabry-Perot interference with the next aurora here?


*It can happen at room temperature too, but according to LensRentals.com - Bad Times with Bad Filters it takes just the wrong combination.

Sorry, I can't see the picture in the first link, but yes, please try the next time you have a chance. If you find a high quality filter (which would have nice, plane surfaces) without coating (which would secure lots of internal reflections) the chances should be good. In any case, let us know