Re: Evenly-spaced concentric circles using a grid and subdivisions
This isn't scientifically accurate, but it may be useful where artistic rather than absolute microscopic accuracy is adequate. I used a grid and drew the initial circle so the top, bottom and extreme sides cover the respective subdivision borders of the grid; copy/paste, expand the concentric copy to the next square of subdivisions, etc.
I've tried other methods (e.g., building progressively larger circles on adjacent layers of the Layers panel, but that "grew old" pretty quickly).
So this will work for what little concentric circle work I do. Of course saving your best construction for future use would avoid having to "re-invent the wheel" each time.