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-rw-r--r--fg21sim/utils/convert.py63
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diff --git a/fg21sim/utils/convert.py b/fg21sim/utils/convert.py
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+# Copyright (c) 2016 Weitian LI <liweitianux@live.com>
+# MIT license
+
+"""
+Utilities for conversion among common astronomical quantities.
+"""
+
+import astropy.units as au
+
+
+def Fnu_to_Tb(Fnu, omega, freq):
+ """Convert flux density to brightness temperature, using the
+ Rayleigh-Jeans law, in the Rayleigh-Jeans limit.
+
+ Parameters
+ ----------
+ Fnu : `~astropy.units.Quantity`
+ Input flux density, e.g., `1.0*au.Jy`
+ omega : `~astropy.units.Quantity`
+ Source angular size/area, e.g., `1.0*au.sr`
+ freq : `~astropy.units.Quantity`
+ Frequency where the flux density measured, e.g., `1.0*au.MHz`
+
+ Returns
+ -------
+ Tb : `~astropy.units.Quantity`
+ Brightness temperature, with default unit `au.K`
+
+ References
+ ----------
+ - Brightness and Flux
+ http://www.cv.nrao.edu/course/astr534/Brightness.html
+ - Wikipedia: Brightness Temperature
+ https://en.wikipedia.org/wiki/Brightness_temperature
+ - NJIT: Physics 728: Introduction to Radio Astronomy: Lecture #1
+ https://web.njit.edu/~gary/728/Lecture1.html
+ - Astropy: Equivalencies: Brightness Temperature / Flux Density
+ http://docs.astropy.org/en/stable/units/equivalencies.html
+ """
+ equiv = au.brightness_temperature(omega, freq)
+ Tb = Fnu.to(au.K, equivalencies=equiv)
+ return Tb
+
+
+def Sb_to_Tb(Sb, freq):
+ """Convert surface brightness to brightness temperature, using the
+ Rayleigh-Jeans law, in the Rayleigh-Jeans limit.
+
+ Parameters
+ ----------
+ Sb : `~astropy.units.Quantity`
+ Input surface brightness, e.g., `1.0*au.Jy/au.sr`
+ freq : `~astropy.units.Quantity`
+ Frequency where the flux density measured, e.g., `1.0*au.MHz`
+
+ Returns
+ -------
+ Tb : `~astropy.units.Quantity`
+ Brightness temperature, with default unit `au.K`
+ """
+ omega = 1.0 * au.sr
+ Fnu = Sb * omega
+ return Fnu_to_Tb(Fnu, omega, freq)