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 or circular polarization is desired. As twice as many products are being created, the spectral resolution is reduced by a factor of two compared to dual polarization. Full polarization is now also available for the Stand-alone ACA (single pointing only per source) but not for the TP Array or for 7-m Array observations added to a 12-m Science Goal for purposes of imaging the largest angular scale. There is a limit on the lowest polarization that can be detected - 0.1 per cent in linear and 1.8 per cent in circular polarization.
Mosaics are now possible for continuum observations of linear source polarization only - circular polarization mosaics are not allowed. Where mosaics cannot be performed there are restrictions on the maximum source size that can be observed (different for linear and circular polarizations) and Single Continuum with the default frequencies must be used.
For projects using the VLBI system (including the Phased Array) technical issues require that full polarization be used.