3D Gui Sphere having distorted edges

I’m trying to make a make switch board with circular button.

For it i’m using Quad as a base and then overlaying 3D Spheres for the buttons.

For the texture of sphere’s, I have made a Material to which a rectangular image is set.

in preview it looks good with all nice edges, however, when I look closely at the sphere’s their edges are distorted.

Is there any restriction which should be adhered to make the sphere’s have perfect edges.

or I must be missing something.

Still waiting for ideas to resolve the issue