6.3 Wavelength Calibration and Spectral Resolution

Figure 6.2: As temperatures changed, geometry changes in the grating scanner mechanism induced a shift in the position of spectral feature like the Hydrogen Pfund-$\beta$ line in the spectrum of $\alpha$ Lyr.

As the focal plane temperatures of ISO increased, a distinct shift in the position of spectral features could be seen. Fig 6.2 shows how the position of the Hydrogen Pfund-$\beta$ line shifts in the uncalibrated spectrum of $\alpha$ Lyr.

To facilitate the wavelength calibration of the SWS, a fixed Fabry-Pérot is mounted in front of the internal calibration source. During the post-helium operations, a spectrum of the fringes produced by this system was observed at the beginning of every revolution when the SWS was switched on.

Each of these measurements was used to determine a wavelength calibration of the SWS, applicable to the instrument at the time of the calibration measurement. The wavelength calibration for a single post-helium observation can be calculated by interpolating these calibration parameters in time. This results in a wavelength calibration better than 1 resolution element ($\lambda$ / $\delta \lambda \sim 2200$) and comparable to the grating wavelength calibration of an observation during the nominal part of the mission (Valentijn et al. 1996, [37]).

The spectral resolution as measured from the FWHM of unresolved lines in NGC 6543 varied throughout the post-helium 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 SWS02 scan of selected lines. A Calibration Uplink System (CUS) observation was engineered to perform linescans similar to the SWS02 scanning mode. This allowed to continue the monitoring of wavelength calibration stability and resolution on external sources. Figure 6.3 depicts the variation of the spectral resolution of the unresolved H95 line at 3.297 $\mu$m in the planetary nebula NGC 6543. One can see that the resolution during the Post-Helium Phase is not drastically different from the values during the nominal mission (Valentijn et al. 1996, [37]).

Figure 6.3: The variation of the spectral resolution as measured from the width of the H95 line [3.297 $\mu$m] in NGC 6543. Crosses are the values during the nominal mission. the squares are the values measured during the Post-Helium Phase.