Michelson Interferometer, circle fringes

[Gab]

Hello guys!

Lets say we have a laser beam and we send it to a michelson interferometer.
Why the beam pattern at the screen gives circles and not lines or something else?

Thanks

P.S.
see for instance
http://techtv.mit.edu/collections/physicsdemos/videos/9823-michelson-interferometer

 

[Andy Resnick]

If the beam is not well-collimated, you will get fringes similar to what is displayed.