Fundamental and Practical Limits to the Sensitivity of Submillimeter Astronomical Observations

Unwanted radiation emited by sources in the foreground and immediate environment of the detectors is much more important than what astronomers want to observe. The success or the failure of submillimeter observations depends mainly on the techniques used to remove this parasitic radiation and its fluctuations such as photon noise. We analyse with some detail the formation of photon noise in multimode instruments and show that the usual simplified equations may give significantly wrong results in practical cases. Other sources of fluctuations, such as the atmospheric noise, and the way used to partly remove them are also described.