Although the built differences between the two RGS units are small, the energy resolutions of the two instruments are slightly different due to their slightly different focusing as shown in Fig. 81.
The shape of the LSF core largely determines the ability of the spectrometer to separate closely spaced emission lines. The various components scale differently with wavelength, giving rise to a composite line shape which cannot easily be characterised in terms of a simple analytical function. The telescope blur contributes a constant term to the spectrometer line width, while misalignments and flatness errors of the gratings contribute a term which slowly increases with increasing wavelength. The broadening due to pointing instability, which affects the resolving power through variations of the angle of incidence on the gratings has proved to be negligible. The scattering component is most significant at the shortest wavelengths and the highest diffraction orders. This is illustrated in Fig. 81 where the predicted resolution (the FWHM of the LSF) for the two RGSs is shown in the second and fourth panels. The steep rise below 7 Å is due to the scattering component of the gratings. Also shown are the measurements, which are deduced from narrow, bright emission lines in HR 1099 (Ly lines of Ne, O, N and C). Fig. 81 demonstrates that the instrument resolving power is close to expectations.
The ability to detect weak emission lines above the background or the continuum is better measured by the half energy width (HEW) of the profile. This width, especially at short wavelengths, is more dependent on the amplitude and width of the scattering wings. Again, the data agree with the predictions and, in the HEW sense, the resolving power goes to 800 at the longest wavelengths. The first and third panels of Fig. 81 display the HEW as a function of wavelength, for both RGS units separately.