3.4 Polarization Observations

The continuum radiation of Quasars (QSO) and the line radiation of SiO masers is polarized, in single dish and interferometer observations from a few percent to $\sim$10$\%$ for QSO's, and up to $\sim$70 - 80$\%$ for the individual components of SiO masers. VLBI observations are normally made at Left Hand-Circular Polarization (LCP = $\cal L$) in order to eliminate amplitude modulations of a polarized source due to hour angle variations. For polarization observations, measurements are made at LCP and RCP (right hand-circular, $\cal R$) polarization. The Stokes parameters I,Q,U,V, describing the polarization, and the complex correlations $\cal RR$, $\cal LL$, $\cal RL$, $\cal LR$ are related by $\cal RR$ = I + V; $\cal LL$ = I - V; $\cal RL$ = Q + iU; and $\cal LR$ = Q - iU ([Kemball et al. 1995]). cm Table 3.5 summarizes the possibility of VLBI polarization observations (status 2000) at 86GHz. The BIMA (Hat Creek) interferometer can be phased and split into 3 antennas LCP and 3 antennas RCP; this will eventually also be possible with the PdB interferometer. cm Polarization observations at 86GHz of QSO's are so far without convincing success, mainly because of their low degree of polarization; VLBI polarization observations at 86GHz of SiO masers are in progress.

Table 3.5: Polarization Possibilities (LCP, RCP)

Telescope	Polarization	Bandwidth
	(number of channels, 4MHz)
Effelsberg	14 LCP	122 MHz LCP
Onsala	14 LCP	122 MHz LCP
+ Pico Veleta	7 LCP / 7 RCP	56 MHz LCP / 56 MHz RCP
+ Haystack	7 LCP / 7 RCP	56 MHz LCP / 56 MHz RCP
Amherst	14 LCP	122 MHz LCP
+ Pietown	4 LCP / 4 RCP	28 MHz LCP / 28 MHz RCP
+ BIMA	7 LCP / 7 RCP	56 MHz LCP / 56 MHz RCP