The effective gap 
of the Fabry-Pérot units obtained
from the
relation 
is not the same for all wavelengths. Short
wavelengths penetrate slightly deeper into the mesh reflectors.
The amplitude of this effect has been calibrated using water and 
absorption lines as wavelength references (Fig. 
).
It amounts to about one F-P resolution element in the worst case and may be
neglected for planning of observations. It will however be considered
in the off-line data reduction.
Figure: SWS Fabry-Pérot leakage. Leakage is defined here as the fraction
of the total detected signal that originates in unwanted Fabry-Pérot
orders. Preliminary data shown here are based on extrapolation from
measurements at a few wavelengths. A better characterization is expected
from in-orbit calibration
Figure: Effects of leakage on a crowded spectrum. A wavelength range of
high leakage has been selected for demonstration. The upper line shows the
assumed true source spectrum at the SWS F-P spectral resolution, the lower
line how it will appear in the presence of leakage.
Figure: SWS Fabry-Pérot effective gap as a function of wavelength. To
obtain the effective gap width for a given wavelength, the gap correction
has to be added to the gap width for the reference wavelength of 25
m.
Both F-P units have gap widths of about 12.500 m.