Thursday, November 26, 2015

Magnitude system

The magnitude system has been with astronomy since the 'nomy'. It is a logical extension of the sensitivity curve of our own eyes and therefore makes some historic (if not logical) sense. Calibration of said magnitude system (the zeropoints) initially made sense to calibrate off a star that was almost always available: Vega. When moving to space observations, most of these considerations were moot so astronomy introduced the AB magnitude system.

And now for my rant...

Astronomers working in stellar physics unconsciously assume everyone works in Vega magnitudes. Everyone else...mostly AB. So when someone uses the word "magnitudes" it really should be prefaced. Or clearly marked in the header of their table. or something. Otherwise tacit assumptions are going to bite someone. In this case: me.

I needed J-band absolute magnitudes for M-dwarf subtypes. A stellar value. I compared those to HST photometry (AB)...and in the near-infrared, AB and Vega don't differ by huge numbers +0.89 if anyone is interested...

Just had a helpful referee point this out but it is so very frustrating. In combination with a library glitch in coordinates, this warrants a Erratum on a paper of mine. So I am going through the whole paper submission process again juuust because Astronomy doesn't set single standards but insists on using different ones and relies on a "everyone knows". gah.

1 comment:

  1. Yeah, frustrating, but something that I try to make every grad student do an archive search exercise do, so that they are aware of the problem.

    And redshifts in UV (high z people) are in vacuum, while in optical (nearby galaxies including our own) in air...I wonder how many errata are hidden in papers there in survey papers...