STScI Logo

plbias stsdas.hst_calib.fos.spec_polar



plbias -- Correct linear polarization (PL) for bias in Q and U.


plbias input output


The plbias task takes an FOS polarization (c3h) file and corrects the linear polarization spectrum (PL) in each of the 4 sets of polarization data (pass direction 1, pass direction 2, 1+2 combined, 1+2 combined and corrected) for bias introduced by the fact that the values of Q**2 and U**2 are always positive when used to compute PL. The output file is a copy of the input c3h file, with only the four data groups containing PL spectra (9, 23, 37, 51) replaced by the corrected PL spectra.

Linear polarization is calculated by the calfos, calpolar, polave, polcalc, and polbin tasks as follows:

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

The bias-corrected value of PL is calculated as:

      PL_corr = sqrt { (Q/I)**2 + (U/I)**2 - PL_err**2 }

              = sqrt { PL**2 - PL_err**2 }

where PL_err is the linear polarization error from the c3h data groups 12, 26, 40, and 54.

In the event that the quantity (PL**2 - PL_err**2) for a given pixel is less than zero, the user has several options available as to how to compute the resulting value of PL_corr, using different settings of the parameter wrap. If wrap = zero, the value of PL_corr is set to zero. If wrap = positive, PL_corr is calculated from the absolute value of (PL**2 - PL_err**2), thus resulting in a positive value for PL_corr. If wrap = negative, PL_corr is again calculated from the absolute value of (PL**2 - PL_err**2), but the resulting value is assigned a negative sign.


input [file name]
Input image(s). May be a list or a file name template. If the input image name extension is not given, an extension of c3h is assumed.
output [file name]
Output image(s). May be a list, but must contain the same number of elements as the input image list. If the output image name extension is not given, an extension of c3h is assumed.
(wrap = "zero") [string, allowed values: negative | zero | positive]
Controls the handling of output PL values when the quantity (PL**2 - PL_err**2) is negative (see the description above).
(verbose = yes) [boolean]
Print operations as they are performed?


1. Correct for the bias in linear polarization in the data files y0nb0203t.c3h and y0nb0205t.c3h. Set output values of PL to zero in cases where the corrected value would go negative. Store the results in y0nb0203c.c3h and y0nb0205c.c3h.

   sp> plbias y0nb0203t,y0nb0205t y0nb0203c,y0nb0205c

2. Correct for bias in PL in the dataset y15v0403t. Let negative values of PL wrap around to equivalent positive values. Store the result in y15v0403t_corr.c3h.

   sp> plbias y15v0403t.c3h y15v0403t_corr.c3h wrap=positive



Author: Howard Bushouse, STSDAS


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


polcalc, polbin

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.

Package Help · Search Form · STSDAS