Beware Newton's Rings!
And you thought the most expensive part of photography was the camera!
Just wait till you start buying lenses that really perform well in low light for the aurora - wide viewing angles and fast apertures all mean more glass, and more money - in the same ballpark as your camera.
Protection for an optical price
The conventional wisdom is to protect this investment - one scratch on the large exposed 'eyeball' of the lens and it will never be quite the same.
So you put a UV/Haze filter over that big front element as a bit of insurance against dust, impact, dirt and abrasion. It's a great idea and it works, especially if the filter is high quality.
Shooting aurora with a front element filter in place can cause a surprising optical artifact right in the middle of every one of your photos - they are called Newton's Rings and once you see it, you can't un-see it - have a look below:
They are not obvious at all on the rear display of the camera. Not till you are looking at your images on your computer later on that you notice the circular ripples in the exact middle of every single image!
What are they?
For a long time I wondered where these came from and why I only saw them in my Aurora images - why not during the day?
They are an interference pattern, like ripples on water, except in this case it is caused by light bouncing between glass surfaces and interfering with itself.
Specifically, they show up when one glass surface is curved differently than another, which is exactly the case between a UV filter and the front curved element of a lens.
Newton's rings are not all bad - they are used to measure the curve of mirrors and lenses for astronomy and shape them perfectly - an imperfect surface will make the rings asymmetrical or spaced incorrectly.
Why do they only show up in aurora images?
During daytime photography these are not visible because there are many wavelengths of light bouncing around the camera optics. The rings are still there, but are smeared over many many wavelength and practically invisible.
Aurora photography has a very different light source - the aurora are emissive with only a few dominant wavelengths from very narrow emission lines - mainly the green at 558nm from atomic (single) Oxygen ions.
One wavelength means one set of dominant Newton's rings, and so they show up especially in aurora photography, where the aurora are (hopefully) the dominant light source.
The proof (in my case)
The internet is nothing if not explain-y. But the photography community seemed divided as to where the rings came from - internal reflection within the UV filter, or between the lens front element and the filter. Math[s] might find the answer for us.
I measured the number of pixels between the rings in the image and plugged them into a radius of curvature equation and matched them up to the lens that was used in the photos. The first result was 2cm which seemed low, but then I noticed I had measured using a downsized image. The more correct value is 24cm which seems about right for the 17-55mm front element.
The risky fix
The following is not really advice - it is what I ended up doing. Your equipment may not have this problem at all, and you may wish to keep a UV filter on for safety.
To reduce or eliminate Newton's rings in aurora photography, I don't use any filters...and that can cause quite a bit of anxiety. On the Sigma 14mm, the front element is the size of a crystal softball, and suddenly snowflakes, condensation, sea spray all have access.
The lens protection checklist
So, what is the checklist for taking care of that big exposed front element?
✅ Treat it like it's a thousand dollar egg - don't rush any of your setup, takedown, or movement of your camera and tripod setup
✅ Have the right cloth and blower to carefully clean the front element. Read the directions about lens cleaning that come with the lens (or are on the manufacturer website) Remember that the glass is also has a delicate anti-reflective coating on it, and that hard rubbing or solvents can ruin an anti-reflective coating
✅ Keep the lens cap at the ready - when the camera isn't shooting, put the lens cap on immediately. Never move the camera and tripod without the lens cap on
✅ When moving the camera and tripod, always put the neck strap over your neck before picking up anything. It is not uncommon for the camera to fall off the tripod!
✅ When taking the camera on and off your tripod have the lens cap on and the neck strap around your neck
✅ Near waterfalls and shorelines, hold a spare UV filter up for a few seconds to see if mist or condensation will build up. If you are not comfortable with amount of build up, don't take the lens cap off the lens - find a more sheltered place
Newton's rings are a pretty unique problem to aurora photography. They are really hard to reliably remove in Photoshop, so try not to get them in the first place.
If your lens does not exhibit this effect in a few aurora photos, then maybe you can just leave the filter on for your setup, lucky you!