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Opencv has great tools for this problem.

The 1st image from the left below is the blue channel extracted.

The 2nd image is after a threshold of 200.

The 3rd image is after one iteration of erosion with a 3x3 rect kernel.

That 4th image shows the single contour of the circle after contours have been filtered by area (< 1500).

Take the average of all the points in this remaining contour to get a center x,y about 348, 319