Circle of confusion estimation

Hey folks, does anyone have any good resources on figuring out what a distant point source (ie a star) will look like on a camera when it is in and out of focus? I know the f number of my lens system, the focal length, and the pixel pitch, but I need to figure out accurately what the star will look like on the camera at different levels of focus.

I started with just using a sombrero function based airy disk, but there are so many other factors I feel like I am on the wrong track with only basing it on that. Please help!