Design of a novel cryogenic stiffness-compensated reactionless scan mechanism for the Fourier transform spectrometer of SPICA SAFARI instrument

The high spectral resolution mode of the SpicA FAR-infrared Instrument (SAFARI) is enabled by inserting a Fourier Transform Spectrometer (FTS), based on a Martin-Puplett interferometer, into the signal path of the instrument. The cryogenic mechanism (FTSM) enables linear scans of two back-to-back rooftop mirrors sharing a common apex. ABB Inc. is under contract with the Canadian Space Agency to develop and test at 4 K an FTSM Engineering Demonstration Unit (EDU) for TRL-5 demonstration. The main SAFARI FTSM performance drivers are the stringent mechatronic demands (position stability of roof-top mirrors <10 nm RMS, <34 mm linear stroke), severely constrained by a tight thermal budget (heat dissipation <1.5 mW) under a specific micro-vibrations environment (30 μg/√Hz external), all at cryogenic temperatures (4 K). In this paper, we describe a novel cryogenic FTSM design using a reactionless and longstroke flexure-based 4-bar linkage with stiffness compensation. This 1-DOF mechanism passively controls the guiding of the roof-top mirrors with flex pivots while the axial scanning is actuated and controlled with a custom moving magnet actuator (MMA). Static and dynamic balancing of the FTSM ensures that low vibration levels are transferred to/from the FTSM baseplate, and compensation of the mechanism stiffness reduces the force and drive current required from the MMA by a factor <10. Both features lead to MMA power consumption/dissipation <1.5 mW. Results from an engineering analysis of a dynamic model developed for the FTSM EDU are discussed to assess the compliance of this design to the challenging cryogenic SAFARI FTSM performance requirements.