Added a note about adopted definition of redshift

Author: astrofrog

astropy/coordinates/spectral_coordinate.py

@@ -688,7 +688,7 @@ def _project_velocity_and_shift(self, init_vel, fin_vel, line_of_sight_unit_vec)
     def with_observer_stationary_relative_to(self, frame, velocity=None, preserve_observer_frame=False):
         """
         A new  `SpectralCoord` with the velocity of the observer altered,
-       but not the position.
+        but not the position.
 
         If a coordinate frame is specified, the observer velocities will be
         modified to be stationary in the specified frame. If a coordinate

docs/coordinates/spectralcoord.rst

@@ -159,6 +159,9 @@ velocity equivalent to that redshift, and it is indeed large enough that we don'
 to worry about the rotation of the Earth on itself around the Sun (which would be
 at most a ~30km/s contribution).
 
+.. note:: In the context of |SpectralCoord|, we use the full relativistic relation
+          between redshift and velocity, i.e. :math:`1 + z = \sqrt{(1 + v/c)/(1 - v/c)}`
+
 We now want to shift the wavelengths so that they would be in the rest frame of
 the galaxy. We can do this using the
 :meth:`~astropy.coordinates.SpectralCoord.to_rest` method::