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Polarization Products

All ALMA receivers detect two orthogonal senses of linearly-polarized radiation (X and Y). For maximum sensitivity, dual-polarization observations correlate both signals, producing XX and YY for each spectral window. For each baseband, a total of 8192 channels are available and these are divided up between the two polarization pairs. Single polarization (XX) is rarely observed, but as this can place the full 8192 channels across a single spectral window, it can be useful when the highest possible spectral resolution is required.

Full polarization (XX, YY, XY and YX) should only be selected if a measurement of the source linear polarization (magnitude and position angle) is desired and the spectral resolution is reduced by a factor of two compared to dual polarization. This can only currently be selected for sources that are no larger than one third the size of the antenna beamsize i.e. mosaics and the ACA are not available. For VLBI projects, technical issues require that full polarization be used. The detection of circular polarization is not yet officially supported.



The ALMA OT Team, 2017 Dec 18