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Flat causing ring  

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(@thibautflt)
Brown Dwarf Customer
Joined: 2 months ago
Posts: 6
January 18, 2021 15:07  

Hi everybody !

First of all, I would like to precise I'm a beginner in all of this : stack, DOF, etc. But I've read a lot, and I think I'm rather aware of basic concepts. I'm shooting with a D850 unmodded, with a Tamron 35mm f1.4 @ f2 ; 2min ; ISO 400 for this example.

I'm facing a problem, and I cannot find its origin. After Light pollution removal, I have a kind of ring in my picture. And I cannot remove it. It's "subtle" but increasing with further post processing. Here with a huge stretch.

M42 region mod

 

It may come from the flat, so I did a loooot of tests : expose to the left, to the right, centered, on the sky, using a teeshirt, high ISO, low ISO, fast flat, low flats... I used 1, 15 and 30 flats to calibrate. With other post-processing software, the same occurs. I try to stack uncalibrated raw files, and the ring seems to be here as well.

I've read this : https://www.cloudynights.com/topic/557931-concentric-coloured-rings-caused-by-flats/ ; and this https://www.cloudynights.com/topic/746131-nikon-coloured-concentric-rings/ but not sure if related.

Or, maybe I'm not good at using light pollution removal tools. Or I failed, each time, doing proper flats. I'm currently stack an other object, with another lens (90mm), from another place... I will see if I observe the same, to check if it comes from my camera.

Do you have other ideas of what I could check ? Here is a flat, a bias and a light (no dark as I was at ISO 400 and my sensor's camera is really powerfull) : https://drive.google.com/drive/folders/18fy2jwC3ae0Q8flq5YwJShjmV2Pe8crl?usp=sharing
It's the first time I ask on a forum a solution, normally I always find it by myself. 😓 

PS : just an idea by writing, the focal could be the problem ? 35mm on a full frame sensor is quite wide.


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(@wvreeven)
Galaxy Admin
Joined: 2 years ago
Posts: 593
January 18, 2021 16:01  

@thibautflt That is quite a challenge, yes. Could you post the unprocessed stacked file as well? Also, what exposure time do you use for the flats?


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(@thibautflt)
Brown Dwarf Customer
Joined: 2 months ago
Posts: 6
January 19, 2021 14:21  

Thank you for your answer. I've added some fits files from my tests in the previous folder, you can look this one : https://drive.google.com/file/d/1Q6OdIKus0oWtDiYEntLXfNXTswPbl_U2/view?usp=sharing

 

Forgot to mention that I've tried both electronic and mecanic shutter to do my flats, no changes. And I have flats with 3s exposure, others with 1/8000s. They produce more or less the same result.


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(@wvreeven)
Galaxy Admin
Joined: 2 years ago
Posts: 593
January 19, 2021 15:34  

@thibautflt Many thanks for that. Indeed the ring is difficult to get rid of. I started with saturating the colors to an extreme and then with placing a few small boxes along the edges of the image and then slowly added more and more boxes in both the darkest and the lightest areas with no emission nebulae and managed to suppress the ring quite a bit.

Screenshot 2021 01 19 at 15.27.58

Then I cropped the image and reset the saturation. Then I upped the saturation again a bit to bring out the colors of the nebulae and this is what I get:

Screenshot 2021 01 19 at 15.30.27

What happens if you integrate the lights without flats and only use darks?


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(@thibautflt)
Brown Dwarf Customer
Joined: 2 months ago
Posts: 6
January 20, 2021 07:58  

@wvreeven your result is far better than mine !

In the folder here : https://drive.google.com/drive/folders/18fy2jwC3ae0Q8flq5YwJShjmV2Pe8crl there is a .fits file "M42_region_noDOF.fits" I obtained stacking light frames without calibration. It is not clear for me but the ring seems to be here... what do you think ?

I'm going to redo the integration, using only darks as you suggested.


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