Flux calibration. Left panel: we fit the baseline from the data shown in green dots with the function A * sin(k x + b)+m x + n to account for the stand wave. Right panel: we correct the Doppler velocity and convert the antenna temperatures to flux intensities in mJy. H I emission from galaxies should have no polarisation. As a consistency check, we also reduce the spectrum including only one polarisation, and show the results in blue and red.