Imaging FTS for Herschel SPIRE

The design of the Fourier Transform Spectrometer for the Herschel sub-millimetre Spectral and Photometric Imaging Receiver (SPIRE) is described. This is an innovative design for a sub-millimetre spectrometer as it uses intensity beam splitters in a Mach-Zehnder configuration rather than the traditional polarising beam splitters. The instrument is required to have a resolution of 0.04 cm-1; have a relatively large field of view (2.6 arcmin circular) and cover a large wavelength range - 200 to 670 microns. These performance requirements lay stringent requirements on all aspects of the design. The details of the optical; mechanical and electrical implementation of the instrument are discussed in the light of the science and engineering requirements and laboratory testing on development models of the mechanism and control system are reported.