Fighting the Nikon Color Rings

20 December 2021

While it was cloudy for about a month the two days of clear skies just before and at solstice was a welcome change to the depressing weather we had since my last image. I planned to shoot some standard images I had not captured with my new scope before: M33 the Triangulum Galaxy, M42 the Great Nebula in Orion, and the Flame Nebula respective the Horse Head Nebula.

As I was thinking about getting an astromod for my D5300 I wanted to test it again without modification to come to a final verdict if the modification is the next step to go.

Since I had no time processing the images of the first night before taking the images of the second night, there was no chance for me to see the problem and act accordingly by switching to another camera.

What problem am I talking about you may ask: I am referring to the quite obvious colored rings one can see in the images above. Up until now I sometimes noticed these rings in my pictures but thought the cause for them was the missing flat frames - as the equipment required to take them is relatively new for me I never had propper flat frames until now. But this time I had flat frames - I had a whole library of them with different exposure times so I can be sure there were some frames with the right exposure. But tragically the rings stayed. A spot on the sensor (or mirror or coma corrector) was removed as expected - the rings were not.

Reaching out to friends and some own research showed that I am not alone with my problem (Nikon Coloured Concentric Rings, Colored Ring in Stacked Image).

The cause for the colored rings seems to be rooted in the lossy compression of the raw images taken with the D5300 which can't be turned off. Sadly there seems to be no workaround or hack to get rid of those rings and the only thing that will lead to less obvious rings will be to expose the lights less and the flats more (without clipping).

Lucky for me my D7500 offers to save RAWs lossless compressed - an option which I had already turned on. So my next images will be taken with the D7500 and perhaps I bring out the Star Adventurer to explore the viability of the aforementioned workaround to get less obvious rings. However, this might take a while as currently, I want to spend as little time outside in the cold as possible.

Despite the problems caused by the colored rings, I did my best to process the data I had and I will post the resulting pictures in the coming days.

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