Dual-beam Facilities

• It is assumed that the polarimeter or spectropolarimeter contains a fixed analyser and a rotating half or quarter wave-plate which is stepped in units of 45 degrees.

• Both linear and circular polarization can be measured.

• Different target exposures must not be rotated with respect to one another. That is, the linear mapping between corresponding positions in any two target exposures can only include magnification and a shift of origin (rotation and shear are not allowed).

• The analyser is assumed to be perfect (i.e. the transmission and efficiency of the analyser are both assumed to be $1.0$).

• Variances can only be calculated for the reduced quantities (Stokes vectors, polarization vectors, etc.) if the observed intensity images have usable variances associates with them (i.e. these input variances cannot be estimated for dual-beam data).

• Corrections can be applied when calculating the Stokes vectors which take account of any differences in the exposure times between raw frames, and any difference in the sensitivity of the two channels of the dual-beam polarimeter. They rely on redundancy in the supplied data, and require a specific set of analyser positions to be used (described here) when obtaining the data.