Originally posted by Digitalis If they look like newton rings, which I have seen many times before, then the chances are high that that is EXACTLY what they are.
This is another theory, it is plausible. But the first problem is what we are seeing the Fabry-Perot lnterference would propagate across the image as the interference pattern isn't limited by physical contact the way newton rings are, which isn't what we are seeing here. the interference is occurring in a very small area. Secondly: that the two surfaces facing each other have to be planar to set up the conditions for Fabry-Perot effect to appear this isn't the case:
the first optical cell is dominantly spherical.
Well, first. The Newton ring explanation is obviously not right, at the effect would occure in a place where to parts are in contact and then gradually moves away from each other. Also, it you have this problem with a lens you would see it all the time, not just with Auroras. Also, remember the days of mounting slides in frames? Nice, colourful patterns moving when the slides got warm. If it walks like a duck and quacks like a duck, it could in fact be a bird you havenīt seen before
Second: if you look at the links I referred to the shape of the pattern is the same as in the Fabry-Perot descriptions. You will have the centre of the rings at the optical axis, in the middle of the picture depening a little of the quality of the optics. The size of the pattern depends on the distance between the surfaces, i.e. how thick the filter is, and the clarity depends on the reflectivity of the surfaces at the relevant wavelength.
Third: I have tried, with Auroras. Remove the filter and the pattern disappears.
To the other questions this night (it is morning now in Norway):
The internal reflectivity and the external is the same. About 4% for uncoated glass, a lot less for coated glass, but coating efficiencies may vary with the light wavelength.
Could the pattern occure inside the lens or at the sensor? I wouldnīt say never, but the interference requires essentially parallel light rays (or wavefronts). You get this when focussing on infinity, which you get when taking pictures of the aurora. Inside the lens the rays are refracted, and are no longer parallel. Especially at the sensor plane where all rays from one place are focused at one spot and obviously are not parallel.
You do, of course, get interference at the sensor, but we know that as diffraction, which is a very local phenomena. Moving air inside the lens will be in the same range, and not cover a large part of the sensor.
Should you throw away your filters? No, but in same cases you should take it off.
---------- Post added 02-11-16 at 10:04 AM ----------
Originally posted by Sagitta This is a good idea in general for optimal image quality (at least of you're a pixel peeper).
Moving air currents inside the camera and/or lens will cause a diffraction effect of one degree or another, as well as possibly result in condensation if the temperature difference is extreme enough. (ie, air conditioned car to hot muggy outside = wet glass)
I would not remove the lens during a transition in temperature. The weather sealing protects the camera and lens from condensation on the outside, but I would not want condensation inside.
Last edited by DagT; 02-11-2016 at 06:27 AM.