The ALMA Band 9 (600–720 GHz) receiver is a dual channel heterodyne system capable of detecting orthogonally polarized signals using a wire grid. The light entering the front end is refocused with a compact arrangement of mirrors, which is fully contained within the cartridge. The cross–polar performance of existing Band 9 receivers does not meet the requirements specified for the ALMA channels, i.e. a cross-polar level lower than -23 dB. Moreover, this channel shows a relatively large beam squint, which makes this channel not suitable for extended-source polarimetry. Within the framework of ESO “Advanced Study for Upgrades of the Atacama Large Millimeter/sub-millimeter Array (ALMA)”, we analyze the possibility of improving cross-polarization performance by removing the grid and using an OMT to separate the two orthogonal polarizations. We set up the electromagnetic model of the optics to simulate its performance with and without the grid. The results of the simulations are compared with measurements. Another limiting factor for polarimetric observations in ALMA is the beam squint, which translates directly into a gradient of Stokes Q error across the field of view. The OMT-based ALMA bands show a beam squint within 2% of the FWHM, whereas band with wire grids show a much larger scatter. We investigate what causes the scatter in Band 9, trying to understand if it could be improved by reducing tolerances of the grid mounting or if it is a limiting factor related to aberrations in the current optics.
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