The coordination between Fabry-Pérot scans and the movement of the grating that tracks the F-P scan is a source of signature (or, loosely, `noise') that ultimately limits the quality of F-P spectra on bright sources. For all but very short F-P scans, the wavelength sampled by the F-P would soon be no longer tuned to the transmission peak of the grating if there were no tracking. The discrete steps of the grating's tracking movement result in the final F-P scan of a pure continuum source looking like a repetition of a small section of the grating instrumental profile.
The amplitude of this `noise' depends on various factors like quality of grating wavelength calibration, ratio between grating stepsize and line profile FWHM, pointing stability. It is hence difficult to predict for an in-orbit observation at a certain wavelength. Instrument level tests with optimized grating wavelength calibration showed `signal-to-noise' around 200. In the case of an observation of a single line on a smooth continuum, part of the `noise' could be calibrated out by observing a larger region of continuum around the line, provided the non-systematic contribution of pointing jitter is not too large.