Understanding Magnitude Systems in Extragalactic Imaging

[ian2012]

I didn't realize there are different magnitude systems.

I have been doing an experiment on CCD extragalactic imaging and apparently I have been using values of magnitudes calibrated to the AB magnitude system. I noticed that my magnitudes were oddly smaller (numerically) that expected. Are my results wrong or have I been fooled by the AB system?

 

[mgb_phys]

The difference between AB and Johnson magnitudes is pretty small.
The filters you are using are unlikely to perfect match the response curve of either magnitude system
The magnitudes were based on photographic emulsions which don't match the response of your CCD
The atmopshere varies

Basically - be suspicous of highly accurate magnitude figures unless the experimenter describes the calibration in detail