ALMA Memo #289

Atmospheric Noise in Single Dish Observations

Atmospheric noise and pointing fluctuations will severely impact the signal-to-noise and image fidelity of ALMA observations. Single dish observations are required for extended sources. For wideband total power observations, the maximum integration time ~ 0.1 s in order to keep atmospheric fluctuations less than the thermal noise. A gain stability ~ 10^-4 in 0.1 s is required. For OTF mapping, the noise is limited by the atmospheric fluctuations between the ON and OFF positions. For sources which are only a few beamwidths across, OTF mapping is inefficient. The single dish capability of ALMA could be improved by instrumenting a few antennas to scan the beam pattern, either with x-y nutating subreflectors, or better, with array receivers. A number of experiments are possible to check the conclusions of atmospheric modeling. These could be done on existing submillimeter telescopes such as the JCMT, and might impact the ALMA design.

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Last modified: March 15, 2000