  The principles and practice of astronomical imaging Fourier transform
spectroscopy (FTS) at far-infrared wavelengths are described. The Mach-Zehnder
interferometer design has been widely adopted for current and future imaging
FTS instruments; we compare this design with two other common interferometer
formats. Examples of three instruments based on the Mach-Zehnder design are
presented. The techniques for retrieving astrophysical parameters from the
measured spectra are discussed using calibration data obtained with the
Herschel SPIRE instrument. The paper concludes with an example of imaging
spectroscopy obtained with the SPIRE FTS instrument.

Instrumentation and Methods for Astrophysics