I would like to suggest an alternate that allows Photoshop to create the concentric disks.
Open a file and add a blank layer
Use the eillipical marquee tool and its Options bar set to Fixed Size 200 px x 200 px
Click on the picture area to create a circle of marching ants. Fill with a color.
Deselect
Add another blank layer.
Use the eillipical marquee tool and its Options bar set to Fixed Size 150 px x 150 px
Click anywhere on the picture area to create a circle of marching ants. Fill with another color.
Deselect
Choose the move tool.
Click on Layer 1 and Layer 2 to activate them together
In the Options bar click first on the horizontal and then the vertical alignment symbols.
Done.
Flatten the image if desired
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