Grating Wavelength Calibration

The wavelength transmitted to each detector by the grating was determined by its angle, which could be scanned by up to $\pm 7\hbox {$^\circ $}$. The grating angle was recorded digitally by the on-board electronics in terms of grating encoder engineering units, which had values between 0 and 4095. These units were converted to wavelength by fitting a third order polynomial to the centroid of many observations of emission lines plotted against their expected wavelengths. The calibration targets studied were mainly planetary nebulae and HII regions, chosen to provide many different lines that were unresolved by the grating and that were on view throughout the mission. The calibration was found to have a jump at ISO revolution 346 but apart from this there was only a very slow drift with time through the mission. The overall accuracy of the grating wavelength calibration was found to be $\pm0.07 \mu$m for the SW detectors (grating second order) and $\pm0.15 \mu$m for the LW detectors (grating first order). The grating wavelength calibration is discussed in more detail in the LWS Handbook (Gry et al., 2002).