Design and end-to-end validation of a cryogenic far-infrared post-dispersed polarizing Fourier transform spectrometer

The PRobe far-Infrared Mission for Astrophysics (PRIMA) will feature a 1.8 m cryogenically cooled telescope that will enable astronomical spectroscopy at both low-resolution and high-resolution over a broad spectral range. In the absence of telescope self-emission and with access to ultra-sensitive detectors, the point source sensitivity of a broad spectral observation is limited by photon noise. The only meaningful way to reduce photon noise is by restricting the instantaneous spectral bandwidth observed by a single detector, typically to a fraction of one percent. In the case of the Far-InfraRed Enhanced Survey Spectrometer (FIRESS) instrument, low-resolution spectroscopy is achieved using several reflection diffraction gratings to restrict the spectral bandwidth. For high-resolution spectroscopy, a Fourier transform spectrometer is placed in front of and in series with the grating spectrometer, which serves as a post-dispersing element to achieve the same goal. The polarization encoding properties of a Martin–Puplett interferometer can exploit the strong polarization dependence of the low-resolution diffraction grating, by ensuring the interferometer output presents the transverse magnetic polarization mode to the grating. We term this hybrid instrument the post-dispersed polarizing Fourier transform spectrometer (PDPFTS). A fully cryogenic far-infrared PDPFTS has been developed in our laboratory to gain a better understanding of the challenges presented by this instrument. The results we present will help to guide the development of FIRESS.