### AB system

**(AB magnitude)**
(monochromatic magnitude calibration based upon absolute values)

The **AB system** of magnitudes (or **AB magnitude**) sets the zero
point for monochromatic magnitudes, specifically setting it
so that zero represents 3631 janskys. It can be applied to
bands through a type of averaging over the frequency
range. The monochromatic magnitude is defined as:

m_{AB} = - 5/2 log_{10}( f_{ν}/3631 )

- m
_{AB} - AB magnitude at frequency ν
- f
_{ν} - monochromatic flux density at frequency ν in janskys.

A consequence of magnitudes being logarithmic is no magnitude
can represent "zero flux" since zero has no logarithm (or it
might be referred to as "minus infinity"). A brightness must be
chosen to be represented by zero magnitude, which defines the
magnitude's scale.

**AB** stands for "absolute". Other systems include the Vega system,
which defines magnitudes for passbands. Exact conversion between
the AB system and systems like the Vega system
depend upon the sensitivity function associated with the
passband.

(*measure,brightness,logarithmic,EMR,magnitude*)
http://en.wikipedia.org/wiki/AB_magnitude

http://www.astronomy.ohio-state.edu/~martini/usefuldata.html

http://dls.physics.ucdavis.edu/calib/vegaab.html

https://www.astro.umd.edu/~ssm/ASTR620/mags.html

**Referenced by:**

Vega system

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