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Correct the spectrum for elevation gain losses as a function of frequen-
cy and elevation. This is valid for point sources. For extended sources,
the correction is less, depending on the size of the source.
The rest frequency and elevation are taken from the spectrum header, as-
suming that the gain stays constant over the entire bandpass of the
This correction is only implemented for 30M data. The formulae for the
gains are documented in Juan Penalver memo 16-Apr-2012. The gain is the
ratio of aperture efficiency at the elevation of the observation, divid-
ed by the maximum aperture efficiency near 50 degrees.