Port compensation using the Herschel/SPIRE imaging Fourier Transform Spectrometer

In Fourier Transform Spectroscopy (FTS), observations of a broad continuum source produce an interferogram which has a large dynamic range around the position of zero optical path difference (ZPD). The process of port compensation involves a broadband spectral source placed at the second, complementary input port of an FTS, reducing the dynamic range requirements of the detector system. Port compensation is particularly advantageous in cases where blackbody emission from the focusing optics of an instrument produce a radiative background which dominates a much weaker source signal, as is often found in far-infrared and submillimetre astronomy. The Herschel/SPIRE imaging FTS uses a calibration source (SCAL) to compensate for the emission of the passively cooled telescope optics. In the case of Herschel, it will not be possible to determine the temperature and emissivity of the telescope accurately until after launch; therefore SCAL must have sufficient variability to accommodate this uncertainty. Although simple in theory, port compensation of the SPIRE FTS is non-trivial since it is not possible to match precisely the spectral signature of the Herschel optics over their possible temperature and emissivity parameter space. Typically only partial spectral cancelation can be expected which causes complications in the subsequent data processing and spectral analysis. We discuss the specific challenges to processing data from the SPIRE imaging FTS when both input ports are well balanced and present respective results from the ground- based test campaigns of the SPIRE imaging FTS flight model.