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