Introduction

POLPACK is a package of applications for mapping the linear or circular polarization of extended astronomical objects, either in a single waveband, or in multiple wavebands (spectropolarimetry). Data from both single and dual beam polarimeters can be processed¹.

POLPACK processes data in NDF format. This is the standard data format used by most Starlink software, and is described fully in SUN/33. However, other astronomical data formats may also be processed using transparent on-the-fly data conversion facilities provided by the NDF subroutine library, and the CONVERT package. The use of these facilities is described here, and more fully in SUN/55.

The facilities provided by POLPACK include:

• alignment of images on the sky.
• extraction of $O$ and $E$ images from individual frames of dual-beam data.
• sky subtraction.
• calculation of Stokes parameters.
• binning of Stokes parameters.
• creation of catalogues of polarization vectors.
• graphical display of vector maps.

POLPACK does not provide facilities for performing instrumental corrections such as flat-fielding, de-biassing, etc. Such corrections should be applied to the data before using POLPACK, so that POLPACK can assume that pixel values are proportional to the combined intensity of sky and object. Further comments on these corrections can be found here.

Slightly different facilities are available when using single-beam or dual-beam data, as described in the following sections².