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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)