Coherent Systems for the detection of the CMB Polarization

Polarization of the Cosmic Microwave Background (CMB) is a very important tool of present day Observational Cosmology. Because the expected amplitude of the CMB polarization is very tiny (of the order of a part over 10^6 or even less), observations carried on using standard radiotelescopes are practically impossible. Ad hoc systems and methods of observations have to be studied and implemented. To confirm and extend the few positive detections so far obtained, it is essential to reach a very deep understanding of the instrumentation used for these studies and of all the sources of spurious effects which can obscure the true signal or, even worst, mimic nonexistent signals. In the following, we first look at what we can expect and has been so far detected. We then analyze the structures common to many sorts of polarimeters dedicated to CMB studies and concentrate our attention on coherent systems, which take advantage of the correlation methods to recognize the presence of a polarized signal in a much stronger sea of unpolarized radiation and polarized foregrounds. We then discuss various sorts of spurious effects and briefly compare coherent with (a) incoherent (bolometric) systems and (b) systems based on interferometric methods