Starlink Project
Starlink User Note 223.7

D.S. Berry & T.M. Gledhill

14th December 2020

An Imaging Polarimetry Reduction Package

Version 3.6-3

Users’ Manual



POLPACK is a package of applications for reducing imaging polarimetry or spectropolarimetry data. They cover registration, sky subtraction, calculation of Stokes parameters, and display of vector maps.

The polarization map on the front cover shows 850 μm dust grain emission in the cloud core W3. The map shows large scale magnetic fields in this region of high-mass star-formation. The data were obtained with the SCUBA polarimeter on the James Clerk Maxwell Telescope (operated by the Joint Astronomy Centre in Hawaii), and reduced using SURF and POLPACK.


1 Introduction
 1.1 Single-beam Facilities
 1.2 Dual-beam Facilities
2 Dual-beam Polarimetry
 2.1 The Polarimeter
 2.2 The Observational Procedure
 2.3 The Data Reduction
  2.3.1 Corrections Related to the Detector
  2.3.2 Extraction of the O and E Ray Images
  2.3.3 Image Alignment
  2.3.4 Sky Subtraction
  2.3.5 Estimation of the Stokes Parameters
  2.3.6 Binning of the Stokes Parameters
  2.3.7 Calculation of the Degree and Orientation of the Polarization
  2.3.8 Display of the Final Polarization Data
3 Single-beam Polarimetry
 3.1 The Polarimeter
 3.2 The Observational Procedure
 3.3 The Data Reduction
4 Spectropolarimetry Data Reduction
5 Data Reduction Using POLPACK
 5.1 Format Conversion
 5.2 Corrections for Instrumental Effects
 5.3 Starting up POLPACK
 5.4 Importing Header Information
 5.5 Reference Directions
 5.6 Creation of Stokes Parameters
 5.7 Displaying the Results
  5.7.1 Displaying Spectropolarimetry Data
 5.8 Getting Help
 5.9 Using RA/DEC Information
  5.9.1 Adding an RA/DEC Calibration to your Data
  5.9.2 Annotating Axes with RA/DEC Values
  5.9.3 Aligning Vector Maps with Other Images
  5.9.4 Re-instating Lost WCS Information in a Catalogue
6 Running from the C-shell
7 Group Expressions
 7.1 Indirection
 7.2 Modification
 7.3 Groups of Data Files
 7.4 Examples
8 Trouble-shooting
9 Acknowledging this Software
A Description of the POLPACK applications
 A.1 Alphabetic list of POLPACK routines.
 A.2 Complete routine descriptions
POLBIN – Bins a catalogue containing Stokes parameters
POLCAL – Converts a set of analysed intensity images into a cube holding Stokes vectors
POLCONCAT – Concatenate two or more vector catalogues
POLEDIT – Change the values in a vector catalogue
POLEXP – Copies information from the POLPACK extension to named FITS keywords
POLEXT – Sets explicit values in the POLPACK extension
POLHELP – Display information about POLPACK
POLIMAGE – Converts a catalogue into an NDF
POLIMP – Copies FITS keyword values into the POLPACK extension
POLKA – Creates Stokes vectors from a set of 2-dimensional intensity frames
POLPLOT – Plots a 2-dimensional vector map
POLROTREF – Rotate the reference direction in a pair of Q and U images
POLSELECT – Select vectors from a catalogue
POLSIM – Produces intensity data corresponding to given Stokes vectors
POLSTACK – Stack a set of intensity images
POLVEC – Calculates polarization vectors from supplied Stokes parameters
POLVERSION – Checks the package version number
POLWCSCOPY – Copies WCS from one catalogue to another

B Calculating the Polarization for Dual-beam Data
C Dual-beam Flat-field Corrections, and the E and F-Factors
D Using GAIA to Create an RA/DEC Calibration
E Calculation of Stokes Vectors in Single-beam Mode
 E.1 The Noise Level in Individual Input Images
 E.2 Using POLSIM to Investigate Noise Characteristics
F The POLPACK NDF Extension
G History
 G.1 V3.6-2
 G.2 V3.6-1
 G.3 V3.6-0
 G.4 V3.5-0
 G.5 V3.4-0
 G.6 V3.3-0
 G.7 V3.2-0
 G.8 V3.1-5
 G.9 V3.1-3
 G.10 V3.1
 G.11 V3.0
 G.12 V2.1-7
 G.13 V2.1-5
 G.14 V2.0-14
 G.15 V2.0
 G.16 V1.1
H Backpage alphabetic listing