How can i achieve the blue concentric circles?

[Alejandro Luna]

I'm looking to recreate the background effect in this image. The concentric circles with the faded edges. i can think of a couple of ways but I'm wondering what would be the quickest. Also it's hard to see on the preview but the circles each have a different opacity.

 

[sPECtre]

Welcome to PsG, Alejandro!

I'd create a new layer, filled with blue, add a layer mask, filled with radial white to black gradient.
I'd then make sure that the mask is selected, and use image>adjustments>posterize, then use levels to get to the desired effect.
Hope this helps!