We present an overview of the facilities provided by POLPACK an imaging polarimetry reduction package written by the UK Starlink project

Berry, D.S.

Gledhill, T.

Greaves, J.S.

Jenness, T.

English

University of Hertfordshire Research Archive

Astronomical Polarimetry: Current Status and Future DirectionsASP Conference Series, Vol. 343, 2005Adamson, Aspin, Davis, and FujiyoshiPOLPACK—An Imaging Polarimetry Reduction PackageD. S. Berry,1 T. M. Gledhill,2 J. S. Greaves,3 and T. Jenness41Starlink Project, Rutherford Appleton Laboratory, Chilton, Didcot,Oxon, OX11 0QX, United Kingdom2University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK3UK Astronomy Technology Centre, Edinburgh EH9 3HJ, UK4Joint Astronomy Centre, 660 N. A’oho¯ku¯ Place, Hilo, HI 96720,U.S.A.Abstract. We present an overview of the facilities provided by POLPACK—an imaging polarimetry reduction package written by the UK Starlink project.1. Introduction to POLPACKPOLPACK is a package of applications for mapping the linear or circular polar-ization of extended astronomical objects, either in a single waveband, or in mul-tiple wavebands (spectro-polarimetry). Data from both single and dual beampolarimeters can be processed. It is part of the Starlink Software Collection(SSC) which is currently available for Linux, Solaris and Tru64 Unix (moresystems will be added to this list in the near future). Facilities 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 with errors.• binning of Stokes parameters.• creation of catalogues of polarization vectors with errors.• graphical display of vector maps.• creation of simulated intensity images from a Stokes cube.2. Single and Dual Beam FacilitiesData can be processed from single-beam polarimeters or spectro-polarimeterscontaining either a fixed analyser and a rotating half-wave plate, or multiplefixed analysers, or a single rotating analyser (only linear polarization is sup-ported). The transmission and efficiency of non-perfect analysers can be taken7172 Berry et al.into account, if known values for these quantities are available. Estimates ofthe variance in the observed intensity images can be made if necessary. This isuseful if your data does not have usable variance information associated with it.In dual-beam mode, it is assumed that the polarimeter or spectro-polarimetercontains a fixed analyser and a rotating half or quarter wave-plate which isstepped in units of 45◦. Both linear and circular polarization can be measured.Variances can be calculated for the reduced quantities (Stokes vectors, polariza-tion vectors, etc.) if the observed intensity images have usable variances asso-ciates with them (i.e. these input variances cannot be estimated for dual-beamdata).Corrections can be applied when calculating the Stokes vectors which takeaccount of any differences in the exposure times between raw frames, or anydifference in the sensitivity of the two channels of the dual-beam polarimeter(such as may be produced by an imperfect analyser). The level of correctionrequired is found automatically and relies on redundancy in the supplied data,requiring a specific set of analyser positions to be used when obtaining the data.3. Integration with GAIAGAIA1 is a popular graphical image browser and analysis tool produced anddistributed by Starlink. It includes a toolbox for displaying and analysing po-larization maps. Facilities include:• Overlaying of vector maps on top of images and/or contour maps withcomplete control over the appearance of vectors (colour, width, scale, etc.)• Simultaneous display of a table showing all parameters for all displayedvectors. Statistics for each column in the table can be displayed.• Selection of vectors either by clicking and dragging on the vector map,clicking and dragging on the vector table, or by giving a Boolean expressionbased on algebraic functions of the vector parameters (e.g.“dp < 5 && dang < 20”). Selected vectors are highlighted in both thevector map and the vector table, and the selected vectors can be binned,erased, copied, saved to a new file, etc.• FITS binary tables can be read which use arbitrary (e.g. non-POLPACK)column names.4. Further InformationFor further information, including download instructions, see the POLPACKweb page2, or contact David Berry (dsb@ast.man.ac.uk).1http://www.starlink.ac.uk/gaia/2http://www.starlink.ac.uk/polpack

POLPACK An Imaging Polarimetry Reduction Package

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