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The half flux diameter or HFD is a definition used by astronomers to define the star size in an astronomical image. Mainly due to the seeing, stars are not imaged as a dot but spread out like a Gaussian shape. [1] The half flux diameter defines the diameter of a circle around the bright center in which half of the star flux or energy is contained.
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).
In astronomy, the spectral index of a source is a measure of the dependence of radiative flux density (that is, radiative flux per unit of frequency) on frequency. Given frequency ν {\displaystyle \nu } in Hz and radiative flux density S ν {\displaystyle S_{\nu }} in Jy, the spectral index α {\displaystyle \alpha } is given implicitly by S ...
Then the horizontal components of flux are considered to cancel each other by symmetry, leaving only the vertical component of the flux as non-zero. In this case [4] some astrophysicists think in terms of the astrophysical flux (density), which they define as the vertical component of the flux (of the above general definition) divided by the ...
The flux to which the jansky refers can be in any form of radiant energy. It was created for and is still most frequently used in reference to electromagnetic energy, especially in the context of radio astronomy. The brightest astronomical radio sources have flux densities of the order of 1–100
After determining the flux of an object in counts, the flux is normally converted into instrumental magnitude. Then, the measurement is calibrated in some way. Which calibrations are used will depend in part on what type of photometry is being done. Typically, observations are processed for relative or differential photometry. [32]
In astronomy, the zero point in a photometric system is defined as the magnitude of an object that produces 1 count per second on the detector. [1] 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]
More specifically, it is a flux density measuring mean solar electromagnetic radiation (total solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (au) from the Sun (roughly the distance from the Sun to the Earth).