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polbin stsdas.hst_calib.fos.spec_polar


NAME · USAGE · DESCRIPTION_ · PARAMETERS_ · EXAMPLES · BUGS
REFERENCES · HELP · SEE_ALSO

NAME

polbin -- Rebin FOS polarization spectra.

USAGE

polbin input output bin_mode perror bins

DESCRIPTION

This routine rebins the spectra in an FOS polarimetry data (c3h) file using either uniform or variance weighting when computing rebinned values. Three binning modes are available: one that rebins the data into equal-sized wavelength bins, one that rebins the data so that the linear polarization error per bin is equal, and one that allows the user to specify the exact beginning and end locations (in wavelength space) for each bin. The constant-error method results in output data with bins of different width in wavelength space, depending on the character of the spectrum and the variation in the local signal-to-noise ratio. In all modes a binned wavelength file is produced to accompany the spectra during subsequent analysis.

The algorithms used to compute polarization quantities from the Stokes IQUV parameters are identical to those used in the calfos, calpolar, polave, and polcalc tasks (see below). Note that these algorithms assume that the Stokes QUV spectra have NOT been previously normalized by Stokes I.

The output polarization (c3h) file has the same group format as the input file, but the lengths of the spectra are shortened to be equal to the number of output bins. Likewise the output wavelength (c0h) file has the same number of bins as the output polarization file, with the value in each bin being the average of the wavelengths of all input pixels covered by that bin. The output wavelength file contains only 4 data groups, with the wavelengths in each of the 4 groups corresponding to the binned wavelengths of each of the 4 sets of spectral data contained in the output polarization file; wavelength file group 1 matches spectral (c3h) groups 1-14, wavelength group 2 matches spectral groups 15-28, wavelength group 3 matches spectral groups 29-42, and wavelength group 4 matches spectral groups 43-56.

BINNING MODES

If binning to equal linear polarization error per bin (bin_mode = cerror), the user must choose one of the 4 sets of spectra in the input polarization (c3h) file to serve as a reference for computing the bin sizes and locations (via the parameter refset). The spectra in the reference set are binned first so that the linear polarization error per bin for that set matches the value chosen by the user (via the parameter perror). The same bin sizes and locations are then used to rebin the remaining 3 sets of polarization spectra.

When binning in constant error mode the user may also choose a wavelength at which to begin the binning. This is useful if, for example, there are spectral features of interest, so that polarization effects in line wings can be shown to best effect. If the chosen starting wavelength occurs within the range of the data, the polarization is calculated from the channel closest to this wavelength, binning to the end of the data and then to the beginning. A null value for beginning wavelength or a value not within the range of the data causes binning to commence from the beginning of the data (i.e. first channel).

When binning in constant wavelength mode (bin_mode = cwave), the user has the choice of whether the bin sizes should be constant in wavelength or constant in inverse wavelength (1/lambda) via the parameter invlam. Regardless of the setting of invlam the output binned wavelength spectra are always in normal wavelength space, i.e. setting invlam = yes only causes the calculation of the bin locations to be performed in inverse-wavelength space while the wavelength vectors themselves remain unchanged.

The user may specify exact bin widths and locations by using the bin_mode = list option. In this case, the task reads beginning and ending wavelength values for each bin from a text table supplied by the user. This offers the user complete flexibility in controlling the bin locations and sizes. The default name for this file is "bins.list". This file can be created using any text file editor. Each line of the file is assumed to contain two wavelength values specifying the beginning and end of each desired bin. Bins are allowed to overlap one another, or you may leave dead space between bins.

BINNING AND WEIGHTING ALGORITHMS

The polbin task operates by computing the average values of the Stokes IQUV parameters (and their associated errors) within the output bins, and then recomputing the corresponding values of linear and circular polarization and polarization position angle (and their errors) from the rebinned Stokes parameters. In constant error per bin mode, where the bin sizes are not known a priori, the task computes running means of the Stokes parameters, constantly reevaluating the linear polarization error as each new pixel is added to a bin. When the linear polarization error reaches the value specified by the parameter perror, all polarization values are computed from the current mean Stokes parameter values and recorded in the output spectra. The running means are then reset to zero and the computations are restarted for the next bin.

The mean Stokes parameter values can be calculated using either uniform or variance weighting. In uniform weighting mode, each pixel value receives equal weight, while in variance weighting each input Stokes pixel value is weighted by the inverse square of its error value. In either weighting mode, input Stokes values that have an associated error value of zero are not used in the computations (i.e. they are assigned a weight of zero). If all of the input pixels for a given output bin are rejected for this reason, all output values for that bin (Stokes parameters, polarization values, and their errors) are set to zero. In mathematical form, the mean Stokes parameters for a given output bin are calculated as follows:


             Uniform Weighting                    Variance Weighting

wgt(i)     = 1 or 0 (if S_err(i)=0)               1 / S_err(i)**2

wsum       = sum{ wgt(i) }                        sum{ wgt(i) }

ave. S     = sum{ S(i)*wgt(i) } / wsum            sum{ S(i)*wgt(i) } / wsum

ave. S_err = sqrt{ sum{ S_err(i)**2 }} / wsum     sqrt{ 1/wsum }

where the sums are over the number of pixels contained within the bin, S(i) and S_err(i) are the values of one of the Stokes IQUV parameters and its error, and wgt(i) is the assigned weight value for input pixel number "i".

Linear polarization (PL), circular polarization (PC), and polarization position angle (THETA), and their errors, are calculated from the rebinned Stokes parameters as follows:


PL        = sqrt { (Q/I)**2 + (U/I)**2 }

                 {  Qerr**2  Q**2    Uerr**2  U**2   Q**2+U**2 Ierr**2 }
PL_err    = sqrt { ---------*---- + ---------*---- + ---------*------- }
                 { Q**2+U**2 I**2   Q**2+U**2 I**2     I**2     I**2   }

PC        = V/I

                 { Verr**2   Ierr**2   V**2 }
PC_err    = sqrt { ------- + ------- * -----}
                 {  I**2      I**2     I**2 }

THETA     = O.5 * ARCTAN(U/Q)

                       {  Q**2*Uerr**2     U**2*Qerr**2  }
THETA_err = 0.5 * sqrt { -------------- + -------------- }
                       { (Q**2+U**2)**2   (Q**2+U**2)**2 }

PARAMETERS

input [file name]
Input polarimetry (c3h) file. A file name extension of c3h is assumed if not given.
output [file name]
Output root file name for the rebinned polarimetry and wavelength files. The output polarimetry file name extension will be the same as that of the input polarimetry file. The output wavelength file name extension will be c0h.
bin_mode [string, allowed values: cerror | cwave | list]
The binning mode. The choices are "cerror" for constant linear polarization error per bin, "cwave" for constant wavelength interval per bin, or "list" to specify the bin locations via a list file.
perror [real, min=0, max=100]
Desired linear polarization error (in percent) for the output data. This parameter is ignored if bin_mode = cwave or "list".
bins [integer, min=1, max=npts]
Number of output bins if bin_mode = cwave. This parameter is ignored if bin_mode = cerror or "list".
(listfile = "bins.list") [file name]
Name of the file containing beginning and ending wavelength positions for the desired bins when using bin_mode = list. This parameter is ignored if bin_mode = cerror or "cwave".
(refset = 4) [integer, min=1, max=4]
The reference set of spectra to use when bin_mode = cerror. The bin locations will be calculated for this set of spectra so as to give the desired level of linear polarization error per bin. The same bin locations will then be applied to the remaining three sets of spectra. The default value of 4 corresponds to the set of spectra in data groups 43-56 in the polarization (c3h) file. This parameter is ignored if bin_mode = cwave or "list".
(weight = "uniform") [string, allowed values: uniform | variance]
The type of weighting to use when computing the average Stokes IQUV values in each output bin. Uniform weighting gives equal weight to all input pixels, while variance weighting applies weights equal to the inverse square of the error associated with each Stokes value. Input pixels having errors equal to zero are assigned zero weights in either weighting mode.
(wavcen = 0) [real]
Wavelength at which to center the binning process when bin_mode = cerror. The binning starts at this location and proceeds to the last channel, then runs backwards from wavcen to the first channel of the spectra. If the value of wavcen is outside the wavelength limits of the data, then binning starts from the first channel and proceeds to the last channel. This parameter is ignored if bin_mode = cwave or "list".
(invlam = no) [boolean]
Type of wavelength interval for bin_mode = cwave. If "yes", then bins are made to be equal width in units of 1/lambda; if "no", then bins are in constant wavelength increments. This parameter is ignored if bin_mode = cerror or "list".
(wave = "") [file name]
Name of the input wavelength file. If null, the root name of the input polarization data (c3h) file is used, with a file name extension of c0h. If the wavelength file name extension is not given, c0h is assumed.
(verbose = yes) [boolean]
Print operations as they are performed?

EXAMPLES

1. Rebin the polarization data file y15v0403t.c3h so that the linear polarization error per output bin in the 4th set of spectral data is 0.5%. Use uniform weighting and start the binning at a wavelength of 1550. The input wavelength file is y15v0403t.c0h and the output polarization and wavelength files are y15v0403t_bin.c3h and y15v0403t_bin.c0h.


   sp> polbin y15v0403t y15v0403t_bin cerror 0.1 wavcen=1550

2. Rebin the polarization data file y15v0403t.c3h into 258 equal-sized output bins. Use variance weighting for computing the binned averages. The input wavelength file is y15v0403t.c0h and the output files will be y15v0403b.c3h and y15v0403b.c0h.

   sp> polbin y15v0403t y15v0403b cwave bins=258 weight=variance

3. Rebin the file y15v0403t.c3h into 4 (variable size) bins where the locations of the bins are specified in the text file "bins.list". Use variance weighting for computing the binned values. The file "bins.list" contains the following lines:

	1565 1800
	1800 1900
	1900 2050
	2050 2310

Polbin is executed as follows:

   sp> polbin y15v0403t y15bin4 list weight=variance

BUGS

REFERENCES

Author: Howard Bushouse, STSDAS

HELP

For assistance using this or any other tasks, please contact help@stsci.edu or call the help desk at 410-338-1082.

SEE ALSO

plbias, polave, polnorm

Type "help spec_polar opt=sys" for a tutorial on FOS polarimetry datasets and the use of the spec_polar tasks in a coordinated fashion.


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