rotate polar angle by an arbitrary angle

I got a polar angle map THETA0 from running the snippet below.

N = 1024;  
x0 = linspace(-N/2, N/2, N); 
y0 = x0;
[X0,Y0] = meshgrid(x0,y0);
[THETA0,R0] = cart2pol(X0,Y0);

But cart2pol returns polar angle map starting from the positive x-axis. I want to get a polar angle map starting at an arbitrary angle of beta. How can I realize this?

Answer 1

[Matt J]

THETA0=THETA0-beta;

[Jingtao]

Thanks! THETA0=mod(THETA0-beta,2*pi) works right!