Hi, I've been trying to figure out how to convert a spectrum from wavelength space into velocity space. I thought it would be a simple relation, but I can't see it - maybe I'm missing something? I've got Flux v Wavelength at the moment, so I converted into frequency space. I've played around with [itex]h\nu=1/2 mv^2[/itex], but unsure what to use for mass. I also looked at a Doppler relation, but I'm sure that's for individual lines rather than for an entire spectrum. If someone could put me on the right track, I'd appreciate it!