I need a physicist to explain this red-shift stuff.
I believe etalon filters use multiple beam interferometry to precisely control the exact wavelength of light permitted through the filter.
At this level of precision, the refractive index of the air in the gap between the two elements becomes significant so changes in barometric pressure need to be compensated for.
"the etalon is doppler shifted to the RED side of the hydrogen-alpha line" means (in RC speak):
The etalon's band pass frequency is set to be slightly BELOW the frequency of the hydrogen-alpha line. The Hydrogen alpha line means a specific photon energy level corresponding to the difference in potential energy between an electron in the third and second quantum number.
In air on earth at sea level, photons at this energy level correspond to a wavelength of 656.28nm, which is dark red. This wavelength varies based on altitude, and hence barometric pressure. This means it must be compensated for based on altitude of the observer.