An instrument used for high-resolution spectroscopy, usually based on the principle of the Michelson interferometer. A Michelson interferometer splits the incoming starlight and sends it to two mirrors, one stationary, the other movable. When the light recombines, an interference pattern is produced. Since the incoming light consists of a range of wavelengths (the star’s spectrum), the interference pattern becomes very complicated as the mirror is moved. The input spectrum can, however, be recovered by taking the Fourier transform of the output signal. The Fourier transform can be thought of as breaking down the output signal into a sum of sine waves of different amplitudes. This technique is particularly useful in the infrared, where conventional (dispersive) spectroscopy is difficult. A similar technique is also used in radio astronomy in the analysis of the 21-cm hydrogen line and other radio spectral lines. In this case the Fourier transformation splits the band of received frequencies into thousands of narrow channels, revealing the features of the spectral lines.