Bins a catalogue containing Stokes parameters
The bins used form a grid of equally sized rectangular cells, the dimensions of each cell being specified by the parameter BOX in terms of the X and Y columns in the catalogue. Spectropolarimetry data can also be binned in the frequency axis (see parameter ZBOX). The grid contains sufficient cells to include all the vector positions included in the input catalogue. Each position in the output catalogue corresponds to one of these cells. The Stokes parameters for the cell are formed by combining together the Stokes parameters of all input positions which fall within the cell, using the method specified by the parameter METHOD. The degree of polarization, angle of polarization, and polarized intensity are then derived from these combined Stokes parameters. The vector position in the output catalogue is the position at the centre of the corresponding cell.
"
X"
and "
Y"
and will usually be in units of pixels. This parameter is not accessed if
parameter INTEGRATE is TRUE. Parameter ZBOX specifies binning along the frequency
axis when dealing with spectropolarimeter data. "
Polarization
measurements analysis II. Best estimators of polarization fraction and angle"
(A&A, 2018):
"
AS"
: The asymptotic estimator. See section 2.3 of Montier et al. This estimator produces bad P and
PI values if the squared PI value is less than the variance in PI.
"
MAS"
: The modified asymptotic estimator. See section 2.5 of Montier et al. This estimator does not
produces bad P and PI values, even if the squared PI value is less than the variance in PI.
This parameter was introduced at version 3.6.2 of POLPACK. Earlier versions always used the "
AS"
estimator if de-biasing was requested. The dynamic default is the current value, which is initially "
AS"
. []
MEAN – Mean of the input data values
MEDIAN – Median of the input data values
SIGMA – A sigma clipped mean
In all cases, if variances are available for the input Stokes parameters, then the reciprocals of these variances will be used to weight the input Stokes parameters when forming the output Stokes parameters using the selected method. [MEDIAN]
"
SIGMA"
. [4.0] "
intab.FIT"
and produces catalogue "
outtab.FIT"
containing binned Stokes parameters and
corresponding polarization parameters. Each bin measures 4 pixels along both the X and Y axes,
and has a value based on the median of the corresponding input Stokes values. The reference direction for the Stokes vectors and polarization vectors in the output catalogue will be north if the input catalogue has a celestial co-ordinate Frame within its WCS information. Otherwise, the reference direction will be the second pixel axis. The POLANAL Frame in the WCS information of the output catalogue is updated to describe the new reference direction.
The bottom left corner of each bin is chosen so that the origin of the (X,Y) Frame (or (X,Y,Z) Frame if the data is 3D) would correspond to a bin corner.