Characterisation of Herschel-SPIRE flight model optical performances

The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments on ESA's Herschel Space Observatory. This long wavelength instrument covers 200 to 670μm with a three band photometric camera and a two band imaging Fourier Transform Spectrometer (IFTS). Following first results reported in a previous paper, we discuss the in-band optical performances of the flight model as measured extensively during several dedicated test campaigns. Complementary to the experimentally probed spectral characteristics of the instrument detailed in an accompanying paper (see L.D. Spencer et al., in these proceedings), attention is focused here on a set of standard but key tests aimed at measuring the spatial response of the Photometer and Spectrometer end-to-end optical chain, including detector. Effects of defocus as well as source size extent, in-band wavelength, and polarization are also investigated over respective Photometer and Spectrometer field-of-views. Comparison with optical modelling, based on instrument design knowledge and some of the internal component measured characteristics, is performed. Beyond the specific characterisation of each effect, this allows estimating in each band where optical behaviour and detector behaviour respectively dominates and also reconstructing some of the contributors to the instrument throughput. Based on this analysis, retrieved optical performances are finally assessed against the related science-driven instrument requirements.