A type of optical interferometer that, when placed in front of the diffraction grating in a spectrograph, can reveal fine details in the spectrum of a galaxy, nebula, or other extended object that are beyond the scope of a grating alone. Light from an object is passed through an etalon – a pair of partially-silvered, parallel glass plates separated by an adjustable air space – that uses interference to transmit only a narrow range of wavelengths. By changing in steps the gap between the plates, it is possible to scan the spectral region of interest and produce an image of the object at each chosen wavelength. The design was conceived by French physicists Charles Fabry and Alfred Pérot (1863-1925) in the late nineteenth century.
