Instrumentation for the next-generation cryogenic space-borne far-IR observatories

We present scientific rationale, concepts and technologies for far-IR (lambda=35-600 mu m) instrumentation for the cryogenic single-dish space telescopes envisioned for the next two decades. With the tremendous success of Spitzer, the stage is set for larger (3-10 meter) actively-cooled telescopes and several are under consideration including SPICA in Japan, and CALISTO/SAFIR in the US. The cold platforms offer the potential for far-IR observations limited only by the zodiacal dust emission and other diffuse astrophysical foregrounds. Optimal instrumentation for these missions includes large-format direct-detector arrays with sensitivity matched to the low photon backgrounds. This will require major improvements relative to the current state of the art, especially for wavelengths beyond the 38-micron silicon BIB cutoff, We review options and present progress with one approach: superconducting bolometers. We highlight in particular the scientific potential for moderate-resolution broadband spectroscopy. The large cold telescopes can provide line sensitivities below 10(-20) W m(-2), enabling the first routine survey spectroscopy of the redshift 0.5 to 5 galaxies that produced the cosmic far-IR backo-round. These far-IR-bright dusty galaxies account for half of the photon energy released since stars and galaxies began forming, and the new far-IR spectroscopic capability will reveal their energy sources and chart their history. We describe concepts for the background-limited IR-Submillimeter Spectrograph (BLISS) designed for this purpose. BLISS is a suite of R similar to 1000 spectrometer modules spanning the far-IR range, and is under study for SPICA; a similar but more capable instrument can be scaled for CALISTO/SAFIR.