Understanding Magnitude Systems in Extragalactic Imaging

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SUMMARY

This discussion centers on the differences between the AB and Johnson magnitude systems in the context of CCD extragalactic imaging. The participant discovered that their results, calibrated to the AB magnitude system, yielded numerically smaller values than expected. Key factors affecting magnitude accuracy include the response curves of filters, the use of photographic emulsions versus CCDs, and atmospheric variations. Participants concluded that one should be cautious of highly accurate magnitude figures unless detailed calibration methods are provided.

PREREQUISITES
  • Understanding of CCD extragalactic imaging techniques
  • Familiarity with AB and Johnson magnitude systems
  • Knowledge of filter response curves
  • Awareness of atmospheric effects on astronomical observations
NEXT STEPS
  • Research the differences between AB and Johnson magnitude systems
  • Learn about CCD calibration techniques for astronomical imaging
  • Investigate the impact of atmospheric conditions on photometric measurements
  • Explore filter response curves and their implications for magnitude accuracy
USEFUL FOR

Astronomers, astrophysics researchers, and anyone involved in CCD imaging and photometric analysis will benefit from this discussion.

ian2012
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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?
 
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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
 

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