Concept Study of the Heterodyne Spectroscopy Instrument (HSI) for the Proposed Far-IR Spectroscopy Space Telescope (FIRSST)

Abstract

The heterodyne spectroscopy instrument (HSI) is a focal plane array receiver (FPA) for the Far-IR Spectroscopy Space Telescope (FIRSST), a proposal submitted to NASA’s astrophysics probe explorer call. Though FIRSST was not selected for further study, the receiver design allowed us to quantitatively demonstrate that small (few to few tens of pixels) heterodyne FPAs are now feasible for space missions, that they can be built with high technology readiness level components, are reliable and fit within the mass/volume/power constraints of medium to large missions. The design also identified the challenges and critical design considerations, in particular great care needs to be taken to minimize the cryogenic heat load of the cryogenic IF amplifiers and to keep the power consumption of the spectrometer backends low. The HSI was designed primarily to study the trail of water from the interstellar medium to planets and to observe the important low-lying transitions of water and its isotopes between 500 and 2000 GHz undetectable from the ground due to Earth’s atmosphere. HSI has a spectral resolving power of up to 10⁷ (0.03 km/s), ideal for kinematic studies or line tomography. HSI has six 5-pixel-arrays covering 3 frequency bands and 2 linear polarizations. The concept study showed that heterodyne array receivers such as HSI can be built today even for fast missions and that they are powerful and versatile instruments, with low-risk technology capable of opening up the THz sky at very high spectral resolving power for new discoveries.