The NIST Fourier transform spectrometer consists of a folded Michelson interferometer with the two mirrors replaced by cat's-eye retroreflectors. Each cat's-eye runs on a rail consisting of an oil bearing and linear motor. The cat's-eyes move in opposite directions along the rails, with the zero path difference being when both cat's-eyes are in the center. Each rail is 1 m long, giving a maximum possible path difference of 2 m, which in turn gives a maximum possible resolution of 0.0025 cm-1.
The wavelength range of the instrument is 200 nm to 5.5 µm, with various detectors and beamsplitters.
Photograph of spectrometer with vacuum chamber lifted away.