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The monochromatic AB magnitude is defined as the logarithm of a spectral flux density with the usual scaling of astronomical magnitudes and a zero-point of about 3 631 janskys (symbol Jy), [1] where 1 Jy = 10 −26 W Hz −1 m −2 = 10 −23 erg s −1 Hz −1 cm −2 ("about" because the true definition of the zero point is based on magnitudes as shown below).
The zero point is used to calibrate a system to the standard magnitude system, as the flux detected from stars will vary from detector to detector. [2] Traditionally, Vega is used as the calibration star for the zero point magnitude in specific pass bands (U, B, and V), although often, an average of multiple stars is used for higher accuracy. [3]
The vector approach defines flux density as a vector at a point of space and time prescribed by the investigator. To distinguish this approach, one might speak of the 'full spherical flux density'. In this case, nature tells the investigator what is the magnitude, direction, and sense of the flux density at the prescribed point.
The flux density of astronomical sources is many orders of magnitude below 1 W·m −2 ·Hz −1, so the result is multiplied by 10 26 to get a more appropriate unit for natural astrophysical phenomena. [7] The millijansky, mJy, was sometimes referred to as a milli-flux unit (mfu) in older astronomical literature. [8]
Therefore, the magnitude m, in the spectral band x, would be given by = (,), which is more commonly expressed in terms of common (base-10) logarithms as = (,), where F x is the observed irradiance using spectral filter x, and F x,0 is the reference flux (zero-point) for that photometric filter.
For objects within the immediate neighborhood of the Sun, the absolute magnitude M and apparent magnitude m from any distance d (in parsecs, with 1 pc = 3.2616 light-years) are related by = = (), where F is the radiant flux measured at distance d (in parsecs), F 10 the radiant flux measured at distance 10 pc.
The tesla (symbol: T) is the unit of magnetic flux density (also called magnetic B-field strength) in the International System of Units (SI).. One tesla is equal to one weber per square metre.
In optics the noise-equivalent flux density (NEFD) or noise-equivalent irradiance (NEI) of a system is the level of flux density required to be equivalent to the noise present in the system. [1] It is a measure used by astronomers in determining the accuracy of observations.