Weird ring effect on stacked image, and problem with flats.

Started 9 months ago | Questions thread
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betelgeuse91 Junior Member • Posts: 29
Weird ring effect on stacked image, and problem with flats.

I took a photo of big dipper several days ago, and noticed that this weird ring thing on the image became very apparent. I had this ring thingy for all the pictures I took but this is the most apparent one.


  • Sony a6000
  • Canon FD200mm F4


  • 90 x 3'' light, 30 x dark, 30 x bias, (+ 30 x flat)
  • @200mm F4, ISO 6400, 3'' exposure
  • Photos are take under heavy light pollution


  • Stacked in Pixinsight
  • Pixinsight: background extractor (both automatic and dynamic)

I firstly did this all without taking flats and saw this weird brown ring in the middle of the photo and decided to take flats to see if it can help.

Master flat I used, integration of 30 flats.

Master flat zoomed, no significant noise pattern shown.

Stacked with flat(left) and without flat(right). Only automatic background extractor, and auto-stretching was done in pixinsight.

There is a bright brown ring in the center of the images. I noticed that stacking with flat didn't help much. It flattened out the top two corners, but it unflattened the bottom two corners. Also it made the top of the brown right even brighter. There is another problem with the flat.

Zoomed image of the above.

The problem with the image stacked with flat(left) is that it has the longitudinal noise pattern. This noise pattern was not seen in the master flat frame above. The two images stacked with/without flats were stacked with exactly same parameters other than of course, the masterflat. The below is the same image with dynamic background extractor, instead of automatic.

With dynamic background extractor, instead of automatic. Stacked with flat(left) and without(right).

Zoomed image of above.

The bright top of the image with flat appears the same, and the longitudinal noise pattern still appears. The ring is almost gone though, with the image with dynamic background extractor and flat. But the bright top still remains.

I am wondering what is causing this ring effect. Also for the flats, I am wondering why there is the bright top, and the noise pattern. How do I fix this problem?

Thank you for reading my post, and I will greatly appreciate any comments or advises.

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Sony a6000
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