Examples
Write a FITS image file that uses velocity rather than frequency as the spectral axis, overwriting an existing file of the same name if it already exists.
exportfits(imagename="my.im", fitsimage="my.fits", velocity = True, overwrite=True)
Write a FITS image file that uses velocity rather than frequency as the spectral axis and where the degenerate Stokes axis is dropped (i.e. the product has only one value in the polarisation dimension).
exportfits(imagename="my.im", fitsimage="my.fits", dropstokes = True, velocity = True)