4 Spectropolarimetry Data Reduction

POLPACK assumes that spectropolarimetry data is identical to single waveband polarimetry data, except that each data file has an extra pixel axis giving spectral channel. Thus, analysed intensity data is stored in a 3 dimensional NDF in which the first two pixel axes are spatial axes, and the third axis gives the spectral channel. Stokes vectors are stored in a 4 dimensional NDF in which axes 1 and 2 are spatial, axis 3 is spectral, and axis 4 corresponds to Stokes parameter (1=I, 2=Q, 3=U, etc). Vector catalogues have an extra column (“Z”) giving the spectral channel.

The data in each spectral channel is processed as if it were data from a single waveband polarimeter (i.e. the processing of each spectral channel is largely independent of the processing of the other spectral channels). The one exception to this is that the POLBIN application can bin adjacent spectral channels, as well as binning adjacent spatial pixels.

In general, and unless otherwise stated, all POLPACK applications will handle spectropolarimeter data in the same way that it handles single waveband data. However, for “historical reasons” you may find a bias towards single waveband data in the documentation. References to intensity images should be understood as meaning intensity images or cubes, and Stokes cubes should be taken as Stokes cubes or 4D hyper-cubes.