Fundamental Limits of Detection in the Far Infrared

We study the fundamental limits of detection for astrophysical observations in the far infrared. Understanding these fundamental limits is critical to the planning and analysis of experiments in this region. We specifically characterize the difficulties associated with observing in the 0.1-10 THz (30-3000 \mu m) regime including extraterrestrial, atmospheric, and optical emission. We present signal, noise, and integration time models for selected terrestrial, aircraft, balloon, and space missions. While ground based telescopes offer the great advantage of aperture size, and hence angular resolution, they suffer from the relatively low transmission and high radiance of the atmosphere, particularly for wavelengths less than 500 \mu m. Space telescopes are the inverse; they are limited by a small aperture, while an airborne telescope is constrained by both. Balloon-borne telescopes provide an option over much of the band. A quantitative understanding of this is critical in comparing the sensitivity of various experiments and in planning the next generation of missions. As representative sources we use the luminous far-IR dusty galaxies NGC 958 and Mrk 231, but the same formalism can be applied to any source. In this paper we focus on continuum emission but a future paper will focus on line emission.