POWERS only operates on fourier transforms generated with VISTA command FFT. If you want to generate the power spectrum from a more general complex image (p.e. an image synthesized with command CMPLX) which is supposed to be the fourier transform of a real image, use command CABS to get the modulus of the transform and multiply it by itself (with MULT not CMULT since the modulus of a complex image is real). The periodogram estimate and the modulus-squared are basically identical (they may only differ in the 0th and Nyquist frequencies).