  Dedicated spectrometers for terahertz radiation with [0.3, 30] THz
frequencies using traditional optomechanical interferometry are substantially
less common than their infrared and microwave counterparts. This paper presents
the design and initial performance measurements of a tabletop Fourier transform
spectrometer (FTS) for multi-terahertz radiation using infrared optics in a
Michelson arrangement. This is coupled to a broadband pyroelectric
photodetector designed for [0.1, 30] THz frequencies. We measure spectra of
narrowband and broadband input radiation to characterize the performance of
this instrument above 10 THz, where signal-to-noise is high. This paves the
groundwork for planned upgrades to extend below 10 THz. We also briefly discuss
potential astroparticle physics applications of such FTS instruments to
broadband axion dark matter searches, whose signature comprises low-rate
monochromatic photons with unknown frequency.
