The absolute calibration of the Johnson System of magnitudes

Filter Band | Central Wavelength | Absolute flux density for mag=0.00 | |
---|---|---|---|

µm | W.cm^{-2}.µm | W.m^{-2}.Hz | |

U | 0.36 | 4.35 10^{-12} | 1.88 10^{-23} |

B | 0.44 | 7.20 10^{-12} | 4.44 10^{-23} |

V | 0.55 | 3.92 10^{-12} | 3.81 10^{-23} |

R | 0.70 | 1.76 10^{-12} | 3.01 10^{-23} |

I | 0.90 | 8.30 10^{-13} | 2.43 10^{-23} |

J | 1.25 | 3.40 10^{-13} | 1.77 10^{-23} |

H^{a} | 1.65 | 1.18 10^{-13} | 1.14 10^{-23} |

K | 2.2 | 3.90 10^{-14} | 6.30 10^{-24} |

L | 3.4 | 8.10 10^{-15} | 3.10 10^{-24} |

M | 5.0 | 2.20 10^{-15} | 1.80 10^{-24} |

N | 10.2 | 1.23 10^{-16} | 4.30 10^{-25} |

__The definition of the calibration system__

Here I explain my understanding of how the flux densities corresponding to m=0 are established. A main calibrator, a star, is chosen and assumed to have m=0 at __all wavelengths__. This "star" is also chosen such that it has a black-body emission which peak is located at a much shorter wavelength than that of interest.
This assumption corresponds to .

As a result the spectral density of the calibrator is:

This way the zero-magnitude fluxes at two different frequencies are linked by:

__What is red and what is blue?__

A color is the difference of two magnitudes of an object. Given that magnitudes include the log function and a minus sign, it is not always obvious to remember what is **red** and what is **blue**. The graph below summarizes that:

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