An optical instrument used for high-resolution spectroscopy of extended objects such as galaxies and nebulae. Light from an object is passed through an etalon, which transmits only a narrow range of wavelengths. The spacing (or gap) between the plates of the etalon can be adjusted in steps, making it possible to scan the spectral region of interest, for example a particular group of spectral lines, and producing an image of the object at each chosen wavelength. In combination with sensitive detectors such as CCDs, Fabry–Perot interferometers can achieve a typical resolution of 0.03 nm. The design was first constructed by the French physicists (Marie Paul Auguste) Charles Fabry (1867–1945) and (Jean-Baptiste Gaspard Gustav) Alfred Pérot (1863–1925) in the late 19th century.