The Fabry-Pérot section uses the grating as an order sorter that suppresses unwanted F-P orders. This suppression is not complete for part of the SWS wavelength range. Neighbouring F-P orders are `leaking' through the wings of the grating instrumental profile, contributing significantly to the total measured signal (See Fig. 5.12) . For observations of a single line superposed on a smooth continuum, the only drawback of leakage is a more complicated flux calibration, since leakage will increase the observed continuum but not the line. More serious effects occur in crowded spectra, where strong lines seen in the `wrong' F-P order may significantly distort the observed spectrum, requiring observations of wider wavelength ranges in order to determine the severity of this effect and `clean' the spectrum (See Fig. 5.14).
Leakage has been quantified by analysis of the grating instrumental profile as seen by the F-P detectors in scans of solid state laser lines. Fig. 5.13 shows the leakage as a function of wavelength. It must be considered preliminary, since it is based on extrapolating the grating instrumental profile from measurements only at a few wavelengths of laser lines. The grating instrumental profile will be characterized in more detail in orbit.
The spectral response data in section 5.2.2 have not been corrected for the effects of leakage, i.e. they refer to the total signal.
Figure 5.12: Basics of leakage in F-P spectra. The instrumental profile of
the combination of F-P and grating is given by the solid line. Residual
transmission from neighbouring F-P orders occurs if the instrumental
profile of the grating alone (dashed line) is too wide