The SAFARI far-infrared instrument for the SPICA space telescope

The far-infrared spectrometer SAFARI is one of the three scientific instruments on the SPICA mission, a joint European-Japanese project, which was recently selected as one of the three mission candidates for further study in ESA's M5 call. SPICA employs a 2.5m large telescope deeply cooled to below 8K, improving sensitivities by more than two orders of magnitude with respect to Herschel and Spitzer, filling in the spectral gap between JWST, ELT and ALMA. SAFARI will provide unprecedented spectroscopic observing capabilities in the far-infrared targeting the physical processes governing the formation and evolution of galaxies over cosmic time, and of planetary systems.SAFARI will provide limiting line sensitivities of order of a few times 10--20 W/m2 (5$-10hr) instantaneously covering the 34-230 textmum wavelength range. The extremely high sensitivity of the instrument is realized by utilization of TES detector arrays distributed over four grating modules, offering a native spectral resolving power of 300. The high-resolution spectroscopy mode of the instrument is carefully designed around a post-dispersed Martin-Puplett polarizing interferometer yielding R up to 11000 at the short wavelength limit. The cryogenic translation mechanism in the heart of the FTS spectrometer layout, presents challenging development goals within the context of SAFARI, and is provided by a Canadian consortium sponsored by the CSA.In this paper we provide a comprehensive overview of the instrument architecture and key technologies currently baselined for the subsystems, units and components of SAFARI. We will present the rationale of the FTS architecture employing a MP interferometer as the best way to meet the high-resolution spectroscopic and sensitivity requirements of SAFARI, and we will discuss the different spectroscopic modes in which the instrument can be configured. We conclude the paper by discussing the projected instrument performance and spectroscopic characteristics in view of the scientific goals.