2.11. Transverse velocities
The transverse velocities of clusters can be extracted from the comparison of the amplitude and polarization of their thermal SZ effects. Even for the brightest SZ cluster, the amplitude of the polarization signal is expected to be under 10 µK, and this polarization (with a uniform direction across the cluster) must be separated from the circumferential pattern of polarization induced by multiple scatterings (Sec. 1.1).
Since many of the radio sources contributing to the confusing background are themselves polarized, a first requirement for this observation to be made is that the confusion be well controlled (Sec. 2.3.2).
2.11.1. Polarization calibration
It is clear that excellent polarization calibration of the equipment used will be essential. Attempting to detect sub-µJy polarization signals in the presence of mJy signals from the thermal SZ effect and similar or larger signals from confusing radio sources will require the polarization behaviour of the telescope to be well controlled and the cross-polarization calibrations to be unusually precise. No telescope currently existing is capable of making this measurement on the interesting angular scales: an exploration of the possibilities of polarization measurements may be possible with the next generation of SZ effect telescopes (Sec. 4.3).