I have a galvanometer rotating scanner. Is it possible to obtain the resonant frequency for the scanner or obtain the frequency response if I only have the step response?
Yes, if you assume a simple spring-mass system (second order differential equation). You possibly don't see additional modes, most likely above the frequency range of interest anyway
If the step response shows ringing, then the frequency of this ringing is the resonant frequency.
I also seem to recall from DSP theory that if you perform an FFT of the impulse response (not step response) of a system, that you obtain the frequency response of that system.