how to make a partial difference from multivariable function?

phi = atan2(sin(psi)*sin(theta),cos(psi)+cos(theta));  %eq.(30)

i want to make a partial difference of phi(psi,theta) once into psi and once into theta. i use the "diff" funtion in form of "diff(phi,psi) and MATLAB gave me an long answer that contains of real and imaginary part. but i want a compeletly real answer.

[Walter Roberson]

You could try simplify() on it.