The astronomical product from SWS is a spectrum - an array of flux as a function of wavelength. Hence, a relationship must be established between each instrument position (either grating position or FP gap width) and its corresponding wavelength.
The general approach behind the wavelength calibration and its actual application is described primarily in two documents: Valentijn et al. 1996, [37]; and Kester, Beintema & Valentijn 1998, [18]. For the Fabry-Pérot there is a separate document: Feuchtgruber 1998a, [6]. This section is a summary of those papers, which should be read for a further understanding of the wavelength calibration of the SWS.
The gratings and FPs were characterised in the laboratory during a series of Instrument Level Tests (ILT), and a first approximation to the wavelength calibration was achieved. After launch this calibration was checked and updated. Throughout the mission the wavelength calibration was checked and refined.