Development of a cryogenic far-infrared post-dispersed polarising Fourier transform spectrometer

The sensitivity of state-of-the-art superconducting far-infrared (FIR) detectors is such that wideband spectroscopic observations will require techniques to reduce the spectral bandwidth of a detector to limit the photon noise from an astronomical source. The leading instrument concept features grating spectrometers to post-disperse the light that has been modulated by a polarising Fourier transform spectrometer (FTS) onto a detector array, thereby reducing the photon noise on each detector. While the principles of this method are understood, to date an integrated system has not been realized in the laboratory. We present the development of a fully cryogenic post-dispersed polarising FTS (PDPFTS). To assess the data processing challenges posed by this hybrid spectrometer realistic astronomical spectra are generated by combining line emission from a tunable THz photomixer source with continuum emission from a variable blackbody source.