Fourier sampling

A large fraction of polarization mapping with the ALMA will be of extended objects. Hence, procedures for obtaining short and zero spacing polarization data that will not degrade the quality of the interferometric data are essential. Single-dish polarization observations have traditionally been done by rotating a polarizer and detecting the total intensity of the time-modulated signal. Because this involves subtracting two big numbers (intensities in two different polarization states) to determine a small number (a Stokes Q, U, or V), it is very difficult to achieve calibrated instrumental sensitivities of 0.1%. New methods of single-dish polarization mapping must be developed for the ALMA.