If we are considering a problem involving generic photon emitters and detectors that are moving at various relativistic velocities with respect to each other, do we need to move outside of non-relativistic quantum optics? I'd like to stress that this question is not about any definite species or flavor of massive particles moving at relativistic velocities. (That would, IIUC, take us into QFT). But I'm asking here about generic photon emission events and photon detection events where the rest frames of the emitters and detectors are different. The detectors interact only with propagating free photons, and no massive entities interact directly with each other.