Upper panel: SPD of a B-type star with the added Hα emission line (black line). The projection of this SPD onto the basis of is shown by the red line. Middle panel: simulated RP observations (black dots) and the forward calibration result (red line). The Hα line is the peak centred at u = 13.6. Bottom panel: coefficients in in a reference system in which the covariance matrix diagonalises (black dots). The true coefficients are shown as green circles. The orange dots show the coefficients in the absence of the Hα line.