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Emission or absorption from the Milky Way contributes a foreground when observing sources beyond it. For cosmic microwave background measurements, important components include thermal interstellar dust, synchrotron radiation and free-free emission. Their spectral and spatial behavior helps distinguish them from a near-isotropic blackbody background.

James J. Condon coauthored Essential Radio Astronomy by James J. Condon and Scott M. Ransom with Scott M. Ransom. The text develops the connection between physical radiation mechanisms and the measurements made by radio telescopes.

Its treatment of free-free emission connects the electron temperature, emission measure and optical depth to the observed radio signal. This is useful because an astronomical radio image records radiation after both its production and its passage through intervening material; interpreting brightness requires both parts of that model.^[ref]