[Aavso-photometry] Differential vs. absolute magnitudes

[Michael Koppelman]

A couple of other thoughts on this topic

Besides the confusion with the other definition of absolute magnitude  
it is meaningless to talk about "absolute magnitudes" without also  
talking about the bandpass and system in question. There is the  
Johnson/Cousins system, the SDSS system and a whole bunch of others.  
So transformations are, in general, meant to convert one system to  
another. Most times one of the "systems" is your own instrumental  
magnitudes. Thus, even if the term "absolute magnitude" was available,  
it's still not appropriate.

Also note that if you use Johnson/Cousins comparison stars without  
transformation coefficients, you are still reporting standard  
magnitudes. It's just that they are "first order" and are not  
corrected with a color term and thus the zeropoint error is larger. If  
you add the color term, you have a "second order" estimate of the  
magnitude and your zeropoint error is smaller.

M.