The SWS has built-in calibration sources to tie the flux calibration observations on external sources to frequent internal photometric calibration checks during every observation. During the Post-Helium Phase the signal from these photometric checks increased with the increase of the focal plane temperature. However, the system response to the external calibration sources decreases with time (Figure 6.7). This discrepancy can be understood as a combination of different effects. The increase of temperature of the calibration source could result in a higher flux. Also temperature changes of the telescope could result in a slight defocussing, which could explain (part of) the lower response to the external source. This second effect is likely to be marginal, since no significant change in the wavelength resolution was seen (Section 6.3). Defocussing the telescope, i.e. broadening the profile of the beam entering the instrument, would result in a broader instrumental profile.
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The exact reasons for the system responsivity changes cannot be disentangled, but the strategy for an empiric flux calibration is straightforward: for each spectral band signal/flux ratios per detector are determined from the daily observations of standard calibration stars. The calibration sources and reference spectra used are listed in Section 6.7. A polynomial fit (n=2) to those values against time is the flux calibration function. Evaluation of this polynome in the time of a post-helium observation gives the signal/flux ratio to apply on the observation.
Also here the method averages out uncertainties in the accuracy of spectral features in the individual reference spectra and calibration observations.