The correct etalon to use for a given project is one which gives the required spectral resolution, whilst at the same time providing a free spectral range which is larger than the velocity range of the emission-line region to be observed. Since the spectral resolution is directly dependent on the free spectral range, and there is in any case only a limited choice of etalons, it may not always be possible to satisfy both of these criteria.
It should be emphasised that if the free spectral range of the etalon being used is smaller than the velocity range of the emission-line region being observed, then emission from more than one interference order will be detected simultaneously, making interpretation of the date close to impossible. It is not possible to increase the observable velocity range by combining multiple observations obtained at slightly different wavelengths.
It is worth noting that the 86 
m etalon gives a similar wavelength
range and resolution at [OII]
3727 as the 125 
m etalon at
[OIII]
5007 and H
. One reason for using the former etalon
is that by observing the [OII] doublet it should be possible to map
the density distribution of the emission line region as well as the
velocity field; however, at the time of writing, this has not yet been tried
in practice.