Hi guys. How do astrophysicists measure the redshift of electromagnetic waves from galaxies due to gravity without the use of General Relativity? If I can be more specific, how do astrophysicists know that the gravitational redshift of light emitted from some part of a galaxy or galaxy cluster is small relative to kinematic redshifts (if these light emitting components of a galaxy or galaxy cluster are moving away from us of course) without using General Relativity to prove that such a redshift is small? For example, when creating the rotation curves for galaxies, it is often claimed that the redshifts measured are significantly due to kinematic effects rather than due to gravitational redshifts. How do astrophysicists know this without using General Relativity to show that this is true?