Why is it difficult to determine the center of a Newton's ring pattern?

[ft92]

can anyone please explain to me why it's not possible to determine accurately the position of a centre of a Newton's ring pattern? I know that in a Newton's ring only the distance between one side of the ring to the other i.e. the diameter can be accurately determined, not the distance from the centre of the pattern i.e. the radius. why?

 

[blue_leaf77]

I would say it's because what you have in the center is not a point, instead it's an extended circle. It's not impossible to locate the exact center of this circle, it's just more difficult than locating the marker on the other side of the same ring because these rings have small thickness, hence easier to pick the center on the thickness. I hope I convey my message well.