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A.2 Photometric Quantities

The nomenclature of photometric quantitities in use in astronomical literature is far from standard and sometimes ambiguous. Here we therefore give a brief summary of the definitions and units of measure of these quantities as they are used in this study. Astronomers, however, generally express brightness and surface brightness in logarithmic units, i.e. in magnitudes (mag) and magnitudes per square second of arc (mag/arcsec$ ^2$), respectively. To define a magnitude scale, one has to arbitrarily choose a reference brightness $ F_{zp}$, and the corresponding reference surface brightness $ \Sigma_{zp}$ of $ F_{zp}$ per square second of arc. The brightness of a source expressed in magnitudes is then

$\displaystyle m=-2.5\,\log\frac{F}{F_{zp}}~\mathrm{[mag]}~,$ (A.4)

while the surface brightness of a region of a diffuse source in magnitudes per square second of arc is

$\displaystyle \mu=-2.5\,\log\frac{\Sigma}{\Sigma_{zp}}~\mathrm{[mag/arcsec^2]}~.$ (A.5)

$ F_{zp}$ is called the zero-point of the adopted magnitude scale since $ m=0$ for $ F=F_{zp}$ (and thus $ \mu=0$ for $ \Sigma=\Sigma_{zp}$).

Note that these definitions equally apply to bolometric measurements and to measurements in a given photometric band.

Note also that the sky background is often expressed in different units, such as those described by [Leinert et al. 1998].


next up previous contents
Next: B. The Historical Development Up: A. Units of Measure, Previous: A.1 Angular Quantities   Contents
Mattia Vaccari 2000-12-05