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A Fabry-Pérot interferometer (FPI or Fabry-Pérot cavity) is an alternate to a grating-based interferometer that can be used as a spectrograph's disperser. Rather than a creating a source's interfering rays using diffraction due to multiple slits, it produces the interfering rays with two parallel partially-reflective surfaces (an etalon), such that a ray striking it (other than precisely perpendicular) reflects between them once, twice, three times, etc., each time displaced sideways a bit. (This pair of reflecting mirrors facing each other explains the term cavity.) Subsequently focusing the resulting rays with a lens results in converging light that interferes constructively or destructively based upon wavelength, the length set by the distance between the two partially-reflective surfaces. The resulting interference pattern reveals incoming light at a certain wavelength. Like a grating spectrograph, by nature, it produces higher order interference, so the rest of the instrument-design generally aims to eliminate or reduce this.
The spectrograph is tuned through the distance set between the parallel reflecting surfaces. I believe imaging versions of such spectrographs are accomplished through scanning.
Such spectrographs have been designed for infrared (including far infrared) and ultraviolet, as well as visible light.