Each of these measurements was used to determine a wavelength calibration of the SWS, applicable for the instrument at the time of the calibration measurement. The wavelength calibration for a single post-He observation can be calculated by interpolating these calibration parameters in time. The spectral resolution as measured from the FWHM of unresolved lines in NGC 6543 varied throughout the post-He mission, but within the scatter of similar measurements during the nominal mission. For the major part of the ISO mission, this source was visited every two weeks for an AOT2 scan of selected lines. This allowed to continue the monitoring of wavelength calibration stability and resolution on external sources.
Crosses are the values during the nominal mission.