11. Fourier Spectrometers

This chapter examines the theory and applications of Fourier transform spectroscopy. A Michelson interferometer is ideally suited for work in high-resolution infrared spectroscopy of astronomical objects. In this application the available signal-to-noise ratio limits the number of observable objects and, therefore, any gain in signal-to-noise is of tremendous importance. The Michelson interferometer is a device for interfering two beams of light. Light entering the interferometer is divided into two separate beams by a beam splitter and is recombined after a controllable phase delay has been applied to one of the beams The use of digital computers in performing the Fourier transform of the interferogram, implies that the data be supplied in digital form. The effect of the slow-beat frequency is to make the contributions from spectral elements near the calculated element essentially positive rather than averaging to zero. The dispersive properties of the beam splitter can cause wave number-dependent phase shifts. The data recording and processing systems are as important as the optical and mechanical systems of the interferometer.