In scanning Fourier spectrometer optical and electric modulation of signal is used to avoid this problem, for static Fourier spectrometer there are no such ways. Estimation of the Schottky noise level gives limit for threshold light intensity 80 Rayleighs for spectral range 420-700 nm in the case of limb observations and 1.5 Rayleighs for nadir. Block - scheme of static Fourier spectrometers is shown in Fig. Gregory telescope is used as entry optic in limb channel. Interferometer is built as a single unit on the base of 90 degree prisms with secondary mirror for increasing mechanical stability of device.
FOV diaphragm is placed onto surface of prism as a hole in mirror film.
Optical filter is glued onto the entry surface of first prism. Such scanning is useful for restoring by tomography methods the emission intensity altitude distributions. Flat scanning mirror is used to measure spatial distributions of emissions intensity in direction perpendicular to the vehicle orbit track. The scheme of scanning is shown in Fig. 4. The view geometry of the FS nadir channel.