Introduction

In planning far infrared astronomy missions, it is important to understand the types of noise they will encounter. Confusion by faint galaxies and by infrared cirrus emission can limit the signal to noise achievable in the far infrared. We calculate the magnitude of these effects where the telescope beam is limited purely by diffraction and the confusing galaxies appear as unresolved point sources. These assumptions are appropriate for the majority of envisioned far infrared telescopes, which are designed with image quality and pointing stability required for near infrared operation and which are therefore virtually perfect optical systems in the far infrared.

A number of previous works have argued that natural-background-limited telescopes will be severely limited by confusion noise in a few seconds of integration. These authors have neglected the improvements in performance that will be possible with imaging arrays in the far infrared. When used with arrays in an optimum manner, a telescope cold enough to be photon noise limited from celestial backgrounds has far superior performance to warmer telescopes throughout the far infrared. Telescopes sufficiently cold to be natural background limited can be essentially photon noise limited at wavelengths of 100m and shorter for long integrations (e.g., as long as 10,000 sec at 60m).