Turn on the grid ( edit / preferences / guide,grid & slices).
Set the spacing so that the number of the number of grid lines is greater than about 3x or 4x the number of circles that you want. This ensures you have enough grid lines to snap to.
Make sure "snap to grid" is on (... under the view menu)
Keep hitting shift-U till you activate the ellipse (path) tool.
In the settings for this tool, select "shape", no fill, whatever type (ie, solid, dashed, etc.), width, and color of stroke that you want, hit the "gear" button and select "circle" and "from center".
Move your cursor to the intersection of the center of the canvas, and pull a circle out from the center.
Make the radius of the circle snap to successive guides. If you are having problems getting a good "snap", turn off all other "snap to's" except "snap to grid".
Repeat the last two steps till you get the number of circles that you want.
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HTH,
Tom
