SPECPR Users’ Manual Page 8-f43.1 F43: Fast Fourier Transform (FFT) and Inverse FFT Alias = fft This routine performs a Fast Fourier Transform (FFT) on a spectrum. The spectrum can be any number of channels; the algorithm pads the spectrum with zeros to a power or 2 number of channels in order to do the FFT. For example, if you have 931 channels, the algorithm pads the data to 1024 channels. The resulting FFT is NOT folded: zero frequency occurs at channel 1 and the maximum channel, and the maximum frequency occurs in the middle channel. The f43 routine does not save both the real and imaginary components at this time. Thus the user must run f42 twice: once to save the real component and the second time to save the imaginary component. The inverse FFT requires both real and imaginary components. If you have only the real, you could use a data set of zeros for the imaginary components (but this is not strictly valid).