In every post-helium revolution with SWS observations, a full spectrum of at least one calibration star was measured. Section 6.7 gives an overview of the calibration sources used for the flux calibration (both relative and absolute) of the Post-Helium Phase. Comparison of the observed calibration source spectra to a reference spectral energy distribution (SED) shows an increasing tilt of the observed spectrum as the temperatures increases. As can be seen in Figure 6.5, if the spectrum of $\gamma$ Dra observed in revolution 880 and revolution 907 are divided by the model SED, the linear fit to the residues shows a steeper slope in the later revolutions.

**Figure 6.5:** Division of the observed spectrum of $\gamma$ Dra by a synthetic spectrum in the beginning and the end of the Post-Helium Phase. The Relative Spectral Response of the SWS tilts more as the temperature increases.
$\resizebox {10cm}{!}{\includegraphics{posthe_rsrfchange.eps}}$

This effect was characterised for every spectral band by fitting a polynome $R(\lambda)$ (n=1) to the division of each calibration star spectrum by its reference SED. The change of the coefficients of these fits in time was then characterised by fitting 2 polynomes

to the coefficients of the polynomes $R(\lambda)$ . The tilt to apply on a spectrum observed at time t is then $P_1(T) + \lambda P_2(t)$ . Figure 6.6 shows the coefficients of the fits to the RFRF residues and the fits to the change of these coefficients.

**Figure 6.6:** Characterisation of the changing slope of the RSRF for detector 7 in band 1A. The triangles are the polynomial coefficients of the fits to the slope difference between calibration observations and model spectra. The solid line is a linear fit to the coefficients against time.
$\resizebox {\hsize}{!}{\includegraphics{posthe_rsrfslopefit.ps}}$

The characterisation described above minimizes the risk of biasing the correcting tilt to a noisy observation or a specific spectral feature not accurately represented in the synthetic spectra. The different calibration stars cover a wide range in spectral types. The deficiencies in the synthetic spectra are of a different nature and in different spectral regions (Section 6.7, Decin 2000, [4], Decin et al. 2000, [5]). The double fitting process described above results in some sort of 'averaging out' of the uncertainties in the synthetic spectra and the observations.

6.5 Relative Spectral Response Change