Sensitivity

The very great effort going into giving the ALMA very high sensitivity for mapping of total intensity also will yield high sensitivity for polarization work so long as that sensitivity is not compromised by the instrumental design. The fact that the ALMA will have dual receivers with feeds sensitive to orthogonal polarizations is the first necessary step. But if that system is to achieve its potential for polarization work, the design must have a focus on the effect on polarization of all aspects of the system. Polarization is usually less than 5%, and over large spatial areas the percentage polarization is 1% or less. Hence, the dynamic range that can be achieved is automatically significantly lower than for intensity observations. In order not to further reduce sensitivity, one would like to be able to map polarization to the limits set by thermal noise rather than instrumental polarization.