The absolute calibration of the Johnson System of magnitudes
|Filter Band||Central Wavelength||Absolute flux density for mag=0.00|
|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|
|Ha||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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